src/HOL/Analysis/Polytope.thy
 author haftmann Sun Nov 18 18:07:51 2018 +0000 (8 months ago) changeset 69313 b021008c5397 parent 69260 0a9688695a1b child 69508 2a4c8a2a3f8e permissions -rw-r--r--
removed legacy input syntax
```     1 section \<open>Faces, Extreme Points, Polytopes, Polyhedra etc\<close>
```
```     2
```
```     3 text\<open>Ported from HOL Light by L C Paulson\<close>
```
```     4
```
```     5 theory Polytope
```
```     6 imports Cartesian_Euclidean_Space
```
```     7 begin
```
```     8
```
```     9 subsection%important \<open>Faces of a (usually convex) set\<close>
```
```    10
```
```    11 definition%important face_of :: "['a::real_vector set, 'a set] \<Rightarrow> bool" (infixr "(face'_of)" 50)
```
```    12   where
```
```    13   "T face_of S \<longleftrightarrow>
```
```    14         T \<subseteq> S \<and> convex T \<and>
```
```    15         (\<forall>a \<in> S. \<forall>b \<in> S. \<forall>x \<in> T. x \<in> open_segment a b \<longrightarrow> a \<in> T \<and> b \<in> T)"
```
```    16
```
```    17 lemma%unimportant face_ofD: "\<lbrakk>T face_of S; x \<in> open_segment a b; a \<in> S; b \<in> S; x \<in> T\<rbrakk> \<Longrightarrow> a \<in> T \<and> b \<in> T"
```
```    18   unfolding face_of_def by blast
```
```    19
```
```    20 lemma%unimportant face_of_translation_eq [simp]:
```
```    21     "((+) a ` T face_of (+) a ` S) \<longleftrightarrow> T face_of S"
```
```    22 proof -
```
```    23   have *: "\<And>a T S. T face_of S \<Longrightarrow> ((+) a ` T face_of (+) a ` S)"
```
```    24     apply (simp add: face_of_def Ball_def, clarify)
```
```    25     apply (drule open_segment_translation_eq [THEN iffD1])
```
```    26     using inj_image_mem_iff inj_add_left apply metis
```
```    27     done
```
```    28   show ?thesis
```
```    29     apply (rule iffI)
```
```    30     apply (force simp: image_comp o_def dest: * [where a = "-a"])
```
```    31     apply (blast intro: *)
```
```    32     done
```
```    33 qed
```
```    34
```
```    35 lemma%unimportant face_of_linear_image:
```
```    36   assumes "linear f" "inj f"
```
```    37     shows "(f ` c face_of f ` S) \<longleftrightarrow> c face_of S"
```
```    38 by (simp add: face_of_def inj_image_subset_iff inj_image_mem_iff open_segment_linear_image assms)
```
```    39
```
```    40 lemma%unimportant face_of_refl: "convex S \<Longrightarrow> S face_of S"
```
```    41   by (auto simp: face_of_def)
```
```    42
```
```    43 lemma%unimportant face_of_refl_eq: "S face_of S \<longleftrightarrow> convex S"
```
```    44   by (auto simp: face_of_def)
```
```    45
```
```    46 lemma%unimportant empty_face_of [iff]: "{} face_of S"
```
```    47   by (simp add: face_of_def)
```
```    48
```
```    49 lemma%unimportant face_of_empty [simp]: "S face_of {} \<longleftrightarrow> S = {}"
```
```    50   by (meson empty_face_of face_of_def subset_empty)
```
```    51
```
```    52 lemma%unimportant face_of_trans [trans]: "\<lbrakk>S face_of T; T face_of u\<rbrakk> \<Longrightarrow> S face_of u"
```
```    53   unfolding face_of_def by (safe; blast)
```
```    54
```
```    55 lemma%unimportant face_of_face: "T face_of S \<Longrightarrow> (f face_of T \<longleftrightarrow> f face_of S \<and> f \<subseteq> T)"
```
```    56   unfolding face_of_def by (safe; blast)
```
```    57
```
```    58 lemma%unimportant face_of_subset: "\<lbrakk>F face_of S; F \<subseteq> T; T \<subseteq> S\<rbrakk> \<Longrightarrow> F face_of T"
```
```    59   unfolding face_of_def by (safe; blast)
```
```    60
```
```    61 lemma%unimportant face_of_slice: "\<lbrakk>F face_of S; convex T\<rbrakk> \<Longrightarrow> (F \<inter> T) face_of (S \<inter> T)"
```
```    62   unfolding face_of_def by (blast intro: convex_Int)
```
```    63
```
```    64 lemma%unimportant face_of_Int: "\<lbrakk>t1 face_of S; t2 face_of S\<rbrakk> \<Longrightarrow> (t1 \<inter> t2) face_of S"
```
```    65   unfolding face_of_def by (blast intro: convex_Int)
```
```    66
```
```    67 lemma%unimportant face_of_Inter: "\<lbrakk>A \<noteq> {}; \<And>T. T \<in> A \<Longrightarrow> T face_of S\<rbrakk> \<Longrightarrow> (\<Inter> A) face_of S"
```
```    68   unfolding face_of_def by (blast intro: convex_Inter)
```
```    69
```
```    70 lemma%unimportant face_of_Int_Int: "\<lbrakk>F face_of T; F' face_of t'\<rbrakk> \<Longrightarrow> (F \<inter> F') face_of (T \<inter> t')"
```
```    71   unfolding face_of_def by (blast intro: convex_Int)
```
```    72
```
```    73 lemma%unimportant face_of_imp_subset: "T face_of S \<Longrightarrow> T \<subseteq> S"
```
```    74   unfolding face_of_def by blast
```
```    75
```
```    76 lemma%important face_of_imp_eq_affine_Int:
```
```    77   fixes S :: "'a::euclidean_space set"
```
```    78   assumes S: "convex S"  and T: "T face_of S"
```
```    79   shows "T = (affine hull T) \<inter> S"
```
```    80 proof%unimportant -
```
```    81   have "convex T" using T by (simp add: face_of_def)
```
```    82   have *: False if x: "x \<in> affine hull T" and "x \<in> S" "x \<notin> T" and y: "y \<in> rel_interior T" for x y
```
```    83   proof -
```
```    84     obtain e where "e>0" and e: "cball y e \<inter> affine hull T \<subseteq> T"
```
```    85       using y by (auto simp: rel_interior_cball)
```
```    86     have "y \<noteq> x" "y \<in> S" "y \<in> T"
```
```    87       using face_of_imp_subset rel_interior_subset T that by blast+
```
```    88     then have zne: "\<And>u. \<lbrakk>u \<in> {0<..<1}; (1 - u) *\<^sub>R y + u *\<^sub>R x \<in> T\<rbrakk> \<Longrightarrow>  False"
```
```    89       using \<open>x \<in> S\<close> \<open>x \<notin> T\<close> \<open>T face_of S\<close> unfolding face_of_def
```
```    90       apply clarify
```
```    91       apply (drule_tac x=x in bspec, assumption)
```
```    92       apply (drule_tac x=y in bspec, assumption)
```
```    93       apply (subst (asm) open_segment_commute)
```
```    94       apply (force simp: open_segment_image_interval image_def)
```
```    95       done
```
```    96     have in01: "min (1/2) (e / norm (x - y)) \<in> {0<..<1}"
```
```    97       using \<open>y \<noteq> x\<close> \<open>e > 0\<close> by simp
```
```    98     show ?thesis
```
```    99       apply (rule zne [OF in01])
```
```   100       apply (rule e [THEN subsetD])
```
```   101       apply (rule IntI)
```
```   102         using \<open>y \<noteq> x\<close> \<open>e > 0\<close>
```
```   103         apply (simp add: cball_def dist_norm algebra_simps)
```
```   104         apply (simp add: Real_Vector_Spaces.scaleR_diff_right [symmetric] norm_minus_commute min_mult_distrib_right)
```
```   105       apply (rule mem_affine [OF affine_affine_hull _ x])
```
```   106       using \<open>y \<in> T\<close>  apply (auto simp: hull_inc)
```
```   107       done
```
```   108   qed
```
```   109   show ?thesis
```
```   110     apply (rule subset_antisym)
```
```   111     using assms apply (simp add: hull_subset face_of_imp_subset)
```
```   112     apply (cases "T={}", simp)
```
```   113     apply (force simp: rel_interior_eq_empty [symmetric] \<open>convex T\<close> intro: *)
```
```   114     done
```
```   115 qed
```
```   116
```
```   117 lemma%unimportant face_of_imp_closed:
```
```   118      fixes S :: "'a::euclidean_space set"
```
```   119      assumes "convex S" "closed S" "T face_of S" shows "closed T"
```
```   120   by (metis affine_affine_hull affine_closed closed_Int face_of_imp_eq_affine_Int assms)
```
```   121
```
```   122 lemma%important face_of_Int_supporting_hyperplane_le_strong:
```
```   123     assumes "convex(S \<inter> {x. a \<bullet> x = b})" and aleb: "\<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<le> b"
```
```   124       shows "(S \<inter> {x. a \<bullet> x = b}) face_of S"
```
```   125 proof%unimportant -
```
```   126   have *: "a \<bullet> u = a \<bullet> x" if "x \<in> open_segment u v" "u \<in> S" "v \<in> S" and b: "b = a \<bullet> x"
```
```   127           for u v x
```
```   128   proof (rule antisym)
```
```   129     show "a \<bullet> u \<le> a \<bullet> x"
```
```   130       using aleb \<open>u \<in> S\<close> \<open>b = a \<bullet> x\<close> by blast
```
```   131   next
```
```   132     obtain \<xi> where "b = a \<bullet> ((1 - \<xi>) *\<^sub>R u + \<xi> *\<^sub>R v)" "0 < \<xi>" "\<xi> < 1"
```
```   133       using \<open>b = a \<bullet> x\<close> \<open>x \<in> open_segment u v\<close> in_segment
```
```   134       by (auto simp: open_segment_image_interval split: if_split_asm)
```
```   135     then have "b + \<xi> * (a \<bullet> u) \<le> a \<bullet> u + \<xi> * b"
```
```   136       using aleb [OF \<open>v \<in> S\<close>] by (simp add: algebra_simps)
```
```   137     then have "(1 - \<xi>) * b \<le> (1 - \<xi>) * (a \<bullet> u)"
```
```   138       by (simp add: algebra_simps)
```
```   139     then have "b \<le> a \<bullet> u"
```
```   140       using \<open>\<xi> < 1\<close> by auto
```
```   141     with b show "a \<bullet> x \<le> a \<bullet> u" by simp
```
```   142   qed
```
```   143   show ?thesis
```
```   144     apply (simp add: face_of_def assms)
```
```   145     using "*" open_segment_commute by blast
```
```   146 qed
```
```   147
```
```   148 lemma%unimportant face_of_Int_supporting_hyperplane_ge_strong:
```
```   149    "\<lbrakk>convex(S \<inter> {x. a \<bullet> x = b}); \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<ge> b\<rbrakk>
```
```   150     \<Longrightarrow> (S \<inter> {x. a \<bullet> x = b}) face_of S"
```
```   151   using face_of_Int_supporting_hyperplane_le_strong [of S "-a" "-b"] by simp
```
```   152
```
```   153 lemma%unimportant face_of_Int_supporting_hyperplane_le:
```
```   154     "\<lbrakk>convex S; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<le> b\<rbrakk> \<Longrightarrow> (S \<inter> {x. a \<bullet> x = b}) face_of S"
```
```   155   by (simp add: convex_Int convex_hyperplane face_of_Int_supporting_hyperplane_le_strong)
```
```   156
```
```   157 lemma%unimportant face_of_Int_supporting_hyperplane_ge:
```
```   158     "\<lbrakk>convex S; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<ge> b\<rbrakk> \<Longrightarrow> (S \<inter> {x. a \<bullet> x = b}) face_of S"
```
```   159   by (simp add: convex_Int convex_hyperplane face_of_Int_supporting_hyperplane_ge_strong)
```
```   160
```
```   161 lemma%unimportant face_of_imp_convex: "T face_of S \<Longrightarrow> convex T"
```
```   162   using face_of_def by blast
```
```   163
```
```   164 lemma%unimportant face_of_imp_compact:
```
```   165     fixes S :: "'a::euclidean_space set"
```
```   166     shows "\<lbrakk>convex S; compact S; T face_of S\<rbrakk> \<Longrightarrow> compact T"
```
```   167   by (meson bounded_subset compact_eq_bounded_closed face_of_imp_closed face_of_imp_subset)
```
```   168
```
```   169 lemma%unimportant face_of_Int_subface:
```
```   170      "\<lbrakk>A \<inter> B face_of A; A \<inter> B face_of B; C face_of A; D face_of B\<rbrakk>
```
```   171       \<Longrightarrow> (C \<inter> D) face_of C \<and> (C \<inter> D) face_of D"
```
```   172   by (meson face_of_Int_Int face_of_face inf_le1 inf_le2)
```
```   173
```
```   174 lemma%unimportant subset_of_face_of:
```
```   175     fixes S :: "'a::real_normed_vector set"
```
```   176     assumes "T face_of S" "u \<subseteq> S" "T \<inter> (rel_interior u) \<noteq> {}"
```
```   177       shows "u \<subseteq> T"
```
```   178 proof
```
```   179   fix c
```
```   180   assume "c \<in> u"
```
```   181   obtain b where "b \<in> T" "b \<in> rel_interior u" using assms by auto
```
```   182   then obtain e where "e>0" "b \<in> u" and e: "cball b e \<inter> affine hull u \<subseteq> u"
```
```   183     by (auto simp: rel_interior_cball)
```
```   184   show "c \<in> T"
```
```   185   proof (cases "b=c")
```
```   186     case True with \<open>b \<in> T\<close> show ?thesis by blast
```
```   187   next
```
```   188     case False
```
```   189     define d where "d = b + (e / norm(b - c)) *\<^sub>R (b - c)"
```
```   190     have "d \<in> cball b e \<inter> affine hull u"
```
```   191       using \<open>e > 0\<close> \<open>b \<in> u\<close> \<open>c \<in> u\<close>
```
```   192       by (simp add: d_def dist_norm hull_inc mem_affine_3_minus False)
```
```   193     with e have "d \<in> u" by blast
```
```   194     have nbc: "norm (b - c) + e > 0" using \<open>e > 0\<close>
```
```   195       by (metis add.commute le_less_trans less_add_same_cancel2 norm_ge_zero)
```
```   196     then have [simp]: "d \<noteq> c" using False scaleR_cancel_left [of "1 + (e / norm (b - c))" b c]
```
```   197       by (simp add: algebra_simps d_def) (simp add: divide_simps)
```
```   198     have [simp]: "((e - e * e / (e + norm (b - c))) / norm (b - c)) = (e / (e + norm (b - c)))"
```
```   199       using False nbc
```
```   200       by (simp add: divide_simps) (simp add: algebra_simps)
```
```   201     have "b \<in> open_segment d c"
```
```   202       apply (simp add: open_segment_image_interval)
```
```   203       apply (simp add: d_def algebra_simps image_def)
```
```   204       apply (rule_tac x="e / (e + norm (b - c))" in bexI)
```
```   205       using False nbc \<open>0 < e\<close>
```
```   206       apply (auto simp: algebra_simps)
```
```   207       done
```
```   208     then have "d \<in> T \<and> c \<in> T"
```
```   209       apply (rule face_ofD [OF \<open>T face_of S\<close>])
```
```   210       using \<open>d \<in> u\<close>  \<open>c \<in> u\<close> \<open>u \<subseteq> S\<close>  \<open>b \<in> T\<close>  apply auto
```
```   211       done
```
```   212     then show ?thesis ..
```
```   213   qed
```
```   214 qed
```
```   215
```
```   216 lemma%unimportant face_of_eq:
```
```   217     fixes S :: "'a::real_normed_vector set"
```
```   218     assumes "T face_of S" "u face_of S" "(rel_interior T) \<inter> (rel_interior u) \<noteq> {}"
```
```   219       shows "T = u"
```
```   220   apply (rule subset_antisym)
```
```   221   apply (metis assms disjoint_iff_not_equal face_of_imp_subset rel_interior_subset subsetCE subset_of_face_of)
```
```   222   by (metis assms disjoint_iff_not_equal face_of_imp_subset rel_interior_subset subset_iff subset_of_face_of)
```
```   223
```
```   224 lemma%unimportant face_of_disjoint_rel_interior:
```
```   225       fixes S :: "'a::real_normed_vector set"
```
```   226       assumes "T face_of S" "T \<noteq> S"
```
```   227         shows "T \<inter> rel_interior S = {}"
```
```   228   by (meson assms subset_of_face_of face_of_imp_subset order_refl subset_antisym)
```
```   229
```
```   230 lemma%unimportant face_of_disjoint_interior:
```
```   231       fixes S :: "'a::real_normed_vector set"
```
```   232       assumes "T face_of S" "T \<noteq> S"
```
```   233         shows "T \<inter> interior S = {}"
```
```   234 proof -
```
```   235   have "T \<inter> interior S \<subseteq> rel_interior S"
```
```   236     by (meson inf_sup_ord(2) interior_subset_rel_interior order.trans)
```
```   237   thus ?thesis
```
```   238     by (metis (no_types) Int_greatest assms face_of_disjoint_rel_interior inf_sup_ord(1) subset_empty)
```
```   239 qed
```
```   240
```
```   241 lemma%unimportant face_of_subset_rel_boundary:
```
```   242   fixes S :: "'a::real_normed_vector set"
```
```   243   assumes "T face_of S" "T \<noteq> S"
```
```   244     shows "T \<subseteq> (S - rel_interior S)"
```
```   245 by (meson DiffI assms disjoint_iff_not_equal face_of_disjoint_rel_interior face_of_imp_subset rev_subsetD subsetI)
```
```   246
```
```   247 lemma%unimportant face_of_subset_rel_frontier:
```
```   248     fixes S :: "'a::real_normed_vector set"
```
```   249     assumes "T face_of S" "T \<noteq> S"
```
```   250       shows "T \<subseteq> rel_frontier S"
```
```   251   using assms closure_subset face_of_disjoint_rel_interior face_of_imp_subset rel_frontier_def by fastforce
```
```   252
```
```   253 lemma%unimportant face_of_aff_dim_lt:
```
```   254   fixes S :: "'a::euclidean_space set"
```
```   255   assumes "convex S" "T face_of S" "T \<noteq> S"
```
```   256     shows "aff_dim T < aff_dim S"
```
```   257 proof -
```
```   258   have "aff_dim T \<le> aff_dim S"
```
```   259     by (simp add: face_of_imp_subset aff_dim_subset assms)
```
```   260   moreover have "aff_dim T \<noteq> aff_dim S"
```
```   261   proof (cases "T = {}")
```
```   262     case True then show ?thesis
```
```   263       by (metis aff_dim_empty \<open>T \<noteq> S\<close>)
```
```   264   next case False then show ?thesis
```
```   265     by (metis Set.set_insert assms convex_rel_frontier_aff_dim dual_order.irrefl face_of_imp_convex face_of_subset_rel_frontier insert_not_empty subsetI)
```
```   266   qed
```
```   267   ultimately show ?thesis
```
```   268     by simp
```
```   269 qed
```
```   270
```
```   271 lemma%unimportant subset_of_face_of_affine_hull:
```
```   272     fixes S :: "'a::euclidean_space set"
```
```   273   assumes T: "T face_of S" and "convex S" "U \<subseteq> S" and dis: "~disjnt (affine hull T) (rel_interior U)"
```
```   274   shows "U \<subseteq> T"
```
```   275   apply (rule subset_of_face_of [OF T \<open>U \<subseteq> S\<close>])
```
```   276   using face_of_imp_eq_affine_Int [OF \<open>convex S\<close> T]
```
```   277   using rel_interior_subset [of U] dis
```
```   278   using \<open>U \<subseteq> S\<close> disjnt_def by fastforce
```
```   279
```
```   280 lemma%unimportant affine_hull_face_of_disjoint_rel_interior:
```
```   281     fixes S :: "'a::euclidean_space set"
```
```   282   assumes "convex S" "F face_of S" "F \<noteq> S"
```
```   283   shows "affine hull F \<inter> rel_interior S = {}"
```
```   284   by (metis assms disjnt_def face_of_imp_subset order_refl subset_antisym subset_of_face_of_affine_hull)
```
```   285
```
```   286 lemma%unimportant affine_diff_divide:
```
```   287     assumes "affine S" "k \<noteq> 0" "k \<noteq> 1" and xy: "x \<in> S" "y /\<^sub>R (1 - k) \<in> S"
```
```   288       shows "(x - y) /\<^sub>R k \<in> S"
```
```   289 proof -
```
```   290   have "inverse(k) *\<^sub>R (x - y) = (1 - inverse k) *\<^sub>R inverse(1 - k) *\<^sub>R y + inverse(k) *\<^sub>R x"
```
```   291     using assms
```
```   292     by (simp add: algebra_simps) (simp add: scaleR_left_distrib [symmetric] divide_simps)
```
```   293   then show ?thesis
```
```   294     using \<open>affine S\<close> xy by (auto simp: affine_alt)
```
```   295 qed
```
```   296
```
```   297 lemma%important face_of_convex_hulls:
```
```   298       assumes S: "finite S" "T \<subseteq> S" and disj: "affine hull T \<inter> convex hull (S - T) = {}"
```
```   299       shows  "(convex hull T) face_of (convex hull S)"
```
```   300 proof%unimportant -
```
```   301   have fin: "finite T" "finite (S - T)" using assms
```
```   302     by (auto simp: finite_subset)
```
```   303   have *: "x \<in> convex hull T"
```
```   304           if x: "x \<in> convex hull S" and y: "y \<in> convex hull S" and w: "w \<in> convex hull T" "w \<in> open_segment x y"
```
```   305           for x y w
```
```   306   proof -
```
```   307     have waff: "w \<in> affine hull T"
```
```   308       using convex_hull_subset_affine_hull w by blast
```
```   309     obtain a b where a: "\<And>i. i \<in> S \<Longrightarrow> 0 \<le> a i" and asum: "sum a S = 1" and aeqx: "(\<Sum>i\<in>S. a i *\<^sub>R i) = x"
```
```   310                  and b: "\<And>i. i \<in> S \<Longrightarrow> 0 \<le> b i" and bsum: "sum b S = 1" and beqy: "(\<Sum>i\<in>S. b i *\<^sub>R i) = y"
```
```   311       using x y by (auto simp: assms convex_hull_finite)
```
```   312     obtain u where "(1 - u) *\<^sub>R x + u *\<^sub>R y \<in> convex hull T" "x \<noteq> y" and weq: "w = (1 - u) *\<^sub>R x + u *\<^sub>R y"
```
```   313                and u01: "0 < u" "u < 1"
```
```   314       using w by (auto simp: open_segment_image_interval split: if_split_asm)
```
```   315     define c where "c i = (1 - u) * a i + u * b i" for i
```
```   316     have cge0: "\<And>i. i \<in> S \<Longrightarrow> 0 \<le> c i"
```
```   317       using a b u01 by (simp add: c_def)
```
```   318     have sumc1: "sum c S = 1"
```
```   319       by (simp add: c_def sum.distrib sum_distrib_left [symmetric] asum bsum)
```
```   320     have sumci_xy: "(\<Sum>i\<in>S. c i *\<^sub>R i) = (1 - u) *\<^sub>R x + u *\<^sub>R y"
```
```   321       apply (simp add: c_def sum.distrib scaleR_left_distrib)
```
```   322       by (simp only: scaleR_scaleR [symmetric] Real_Vector_Spaces.scaleR_right.sum [symmetric] aeqx beqy)
```
```   323     show ?thesis
```
```   324     proof (cases "sum c (S - T) = 0")
```
```   325       case True
```
```   326       have ci0: "\<And>i. i \<in> (S - T) \<Longrightarrow> c i = 0"
```
```   327         using True cge0 fin(2) sum_nonneg_eq_0_iff by auto
```
```   328       have a0: "a i = 0" if "i \<in> (S - T)" for i
```
```   329         using ci0 [OF that] u01 a [of i] b [of i] that
```
```   330         by (simp add: c_def Groups.ordered_comm_monoid_add_class.add_nonneg_eq_0_iff)
```
```   331       have [simp]: "sum a T = 1"
```
```   332         using assms by (metis sum.mono_neutral_cong_right a0 asum)
```
```   333       show ?thesis
```
```   334         apply (simp add: convex_hull_finite \<open>finite T\<close>)
```
```   335         apply (rule_tac x=a in exI)
```
```   336         using a0 assms
```
```   337         apply (auto simp: cge0 a aeqx [symmetric] sum.mono_neutral_right)
```
```   338         done
```
```   339     next
```
```   340       case False
```
```   341       define k where "k = sum c (S - T)"
```
```   342       have "k > 0" using False
```
```   343         unfolding k_def by (metis DiffD1 antisym_conv cge0 sum_nonneg not_less)
```
```   344       have weq_sumsum: "w = sum (\<lambda>x. c x *\<^sub>R x) T + sum (\<lambda>x. c x *\<^sub>R x) (S - T)"
```
```   345         by (metis (no_types) add.commute S(1) S(2) sum.subset_diff sumci_xy weq)
```
```   346       show ?thesis
```
```   347       proof (cases "k = 1")
```
```   348         case True
```
```   349         then have "sum c T = 0"
```
```   350           by (simp add: S k_def sum_diff sumc1)
```
```   351         then have [simp]: "sum c (S - T) = 1"
```
```   352           by (simp add: S sum_diff sumc1)
```
```   353         have ci0: "\<And>i. i \<in> T \<Longrightarrow> c i = 0"
```
```   354           by (meson \<open>finite T\<close> \<open>sum c T = 0\<close> \<open>T \<subseteq> S\<close> cge0 sum_nonneg_eq_0_iff subsetCE)
```
```   355         then have [simp]: "(\<Sum>i\<in>S-T. c i *\<^sub>R i) = w"
```
```   356           by (simp add: weq_sumsum)
```
```   357         have "w \<in> convex hull (S - T)"
```
```   358           apply (simp add: convex_hull_finite fin)
```
```   359           apply (rule_tac x=c in exI)
```
```   360           apply (auto simp: cge0 weq True k_def)
```
```   361           done
```
```   362         then show ?thesis
```
```   363           using disj waff by blast
```
```   364       next
```
```   365         case False
```
```   366         then have sumcf: "sum c T = 1 - k"
```
```   367           by (simp add: S k_def sum_diff sumc1)
```
```   368         have "(\<Sum>i\<in>T. c i *\<^sub>R i) /\<^sub>R (1 - k) \<in> convex hull T"
```
```   369           apply (simp add: convex_hull_finite fin)
```
```   370           apply (rule_tac x="\<lambda>i. inverse (1-k) * c i" in exI)
```
```   371           apply auto
```
```   372           apply (metis sumcf cge0 inverse_nonnegative_iff_nonnegative mult_nonneg_nonneg S(2) sum_nonneg subsetCE)
```
```   373           apply (metis False mult.commute right_inverse right_minus_eq sum_distrib_left sumcf)
```
```   374           by (metis (mono_tags, lifting) scaleR_right.sum scaleR_scaleR sum.cong)
```
```   375         with \<open>0 < k\<close>  have "inverse(k) *\<^sub>R (w - sum (\<lambda>i. c i *\<^sub>R i) T) \<in> affine hull T"
```
```   376           by (simp add: affine_diff_divide [OF affine_affine_hull] False waff convex_hull_subset_affine_hull [THEN subsetD])
```
```   377         moreover have "inverse(k) *\<^sub>R (w - sum (\<lambda>x. c x *\<^sub>R x) T) \<in> convex hull (S - T)"
```
```   378           apply (simp add: weq_sumsum convex_hull_finite fin)
```
```   379           apply (rule_tac x="\<lambda>i. inverse k * c i" in exI)
```
```   380           using \<open>k > 0\<close> cge0
```
```   381           apply (auto simp: scaleR_right.sum sum_distrib_left [symmetric] k_def [symmetric])
```
```   382           done
```
```   383         ultimately show ?thesis
```
```   384           using disj by blast
```
```   385       qed
```
```   386     qed
```
```   387   qed
```
```   388   have [simp]: "convex hull T \<subseteq> convex hull S"
```
```   389     by (simp add: \<open>T \<subseteq> S\<close> hull_mono)
```
```   390   show ?thesis
```
```   391     using open_segment_commute by (auto simp: face_of_def intro: *)
```
```   392 qed
```
```   393
```
```   394 proposition%important face_of_convex_hull_insert:
```
```   395    "\<lbrakk>finite S; a \<notin> affine hull S; T face_of convex hull S\<rbrakk> \<Longrightarrow> T face_of convex hull insert a S"
```
```   396   apply (rule face_of_trans, blast)
```
```   397   apply (rule face_of_convex_hulls; force simp: insert_Diff_if)
```
```   398   done
```
```   399
```
```   400 proposition%important face_of_affine_trivial:
```
```   401     assumes "affine S" "T face_of S"
```
```   402     shows "T = {} \<or> T = S"
```
```   403 proof%unimportant (rule ccontr, clarsimp)
```
```   404   assume "T \<noteq> {}" "T \<noteq> S"
```
```   405   then obtain a where "a \<in> T" by auto
```
```   406   then have "a \<in> S"
```
```   407     using \<open>T face_of S\<close> face_of_imp_subset by blast
```
```   408   have "S \<subseteq> T"
```
```   409   proof
```
```   410     fix b  assume "b \<in> S"
```
```   411     show "b \<in> T"
```
```   412     proof (cases "a = b")
```
```   413       case True with \<open>a \<in> T\<close> show ?thesis by auto
```
```   414     next
```
```   415       case False
```
```   416       then have "a \<in> open_segment (2 *\<^sub>R a - b) b"
```
```   417         apply (auto simp: open_segment_def closed_segment_def)
```
```   418         apply (rule_tac x="1/2" in exI)
```
```   419         apply (simp add: algebra_simps)
```
```   420         by (simp add: scaleR_2)
```
```   421       moreover have "2 *\<^sub>R a - b \<in> S"
```
```   422         by (rule mem_affine [OF \<open>affine S\<close> \<open>a \<in> S\<close> \<open>b \<in> S\<close>, of 2 "-1", simplified])
```
```   423       moreover note \<open>b \<in> S\<close> \<open>a \<in> T\<close>
```
```   424       ultimately show ?thesis
```
```   425         by (rule face_ofD [OF \<open>T face_of S\<close>, THEN conjunct2])
```
```   426     qed
```
```   427   qed
```
```   428   then show False
```
```   429     using \<open>T \<noteq> S\<close> \<open>T face_of S\<close> face_of_imp_subset by blast
```
```   430 qed
```
```   431
```
```   432
```
```   433 lemma%unimportant face_of_affine_eq:
```
```   434    "affine S \<Longrightarrow> (T face_of S \<longleftrightarrow> T = {} \<or> T = S)"
```
```   435 using affine_imp_convex face_of_affine_trivial face_of_refl by auto
```
```   436
```
```   437
```
```   438 lemma%important Inter_faces_finite_altbound:
```
```   439     fixes T :: "'a::euclidean_space set set"
```
```   440     assumes cfaI: "\<And>c. c \<in> T \<Longrightarrow> c face_of S"
```
```   441     shows "\<exists>F'. finite F' \<and> F' \<subseteq> T \<and> card F' \<le> DIM('a) + 2 \<and> \<Inter>F' = \<Inter>T"
```
```   442 proof%unimportant (cases "\<forall>F'. finite F' \<and> F' \<subseteq> T \<and> card F' \<le> DIM('a) + 2 \<longrightarrow> (\<exists>c. c \<in> T \<and> c \<inter> (\<Inter>F') \<subset> (\<Inter>F'))")
```
```   443   case True
```
```   444   then obtain c where c:
```
```   445        "\<And>F'. \<lbrakk>finite F'; F' \<subseteq> T; card F' \<le> DIM('a) + 2\<rbrakk> \<Longrightarrow> c F' \<in> T \<and> c F' \<inter> (\<Inter>F') \<subset> (\<Inter>F')"
```
```   446     by metis
```
```   447   define d where "d = rec_nat {c{}} (\<lambda>n r. insert (c r) r)"
```
```   448   have [simp]: "d 0 = {c {}}"
```
```   449     by (simp add: d_def)
```
```   450   have dSuc [simp]: "\<And>n. d (Suc n) = insert (c (d n)) (d n)"
```
```   451     by (simp add: d_def)
```
```   452   have dn_notempty: "d n \<noteq> {}" for n
```
```   453     by (induction n) auto
```
```   454   have dn_le_Suc: "d n \<subseteq> T \<and> finite(d n) \<and> card(d n) \<le> Suc n" if "n \<le> DIM('a) + 2" for n
```
```   455   using that
```
```   456   proof (induction n)
```
```   457     case 0
```
```   458     then show ?case by (simp add: c)
```
```   459   next
```
```   460     case (Suc n)
```
```   461     then show ?case by (auto simp: c card_insert_if)
```
```   462   qed
```
```   463   have aff_dim_le: "aff_dim(\<Inter>(d n)) \<le> DIM('a) - int n" if "n \<le> DIM('a) + 2" for n
```
```   464   using that
```
```   465   proof (induction n)
```
```   466     case 0
```
```   467     then show ?case
```
```   468       by (simp add: aff_dim_le_DIM)
```
```   469   next
```
```   470     case (Suc n)
```
```   471     have fs: "\<Inter>d (Suc n) face_of S"
```
```   472       by (meson Suc.prems cfaI dn_le_Suc dn_notempty face_of_Inter subsetCE)
```
```   473     have condn: "convex (\<Inter>d n)"
```
```   474       using Suc.prems nat_le_linear not_less_eq_eq
```
```   475       by (blast intro: face_of_imp_convex cfaI convex_Inter dest: dn_le_Suc)
```
```   476     have fdn: "\<Inter>d (Suc n) face_of \<Inter>d n"
```
```   477       by (metis (no_types, lifting) Inter_anti_mono Suc.prems dSuc cfaI dn_le_Suc dn_notempty face_of_Inter face_of_imp_subset face_of_subset subset_iff subset_insertI)
```
```   478     have ne: "\<Inter>d (Suc n) \<noteq> \<Inter>d n"
```
```   479       by (metis (no_types, lifting) Suc.prems Suc_leD c complete_lattice_class.Inf_insert dSuc dn_le_Suc less_irrefl order.trans)
```
```   480     have *: "\<And>m::int. \<And>d. \<And>d'::int. d < d' \<and> d' \<le> m - n \<Longrightarrow> d \<le> m - of_nat(n+1)"
```
```   481       by arith
```
```   482     have "aff_dim (\<Inter>d (Suc n)) < aff_dim (\<Inter>d n)"
```
```   483       by (rule face_of_aff_dim_lt [OF condn fdn ne])
```
```   484     moreover have "aff_dim (\<Inter>d n) \<le> int (DIM('a)) - int n"
```
```   485       using Suc by auto
```
```   486     ultimately
```
```   487     have "aff_dim (\<Inter>d (Suc n)) \<le> int (DIM('a)) - (n+1)" by arith
```
```   488     then show ?case by linarith
```
```   489   qed
```
```   490   have "aff_dim (\<Inter>d (DIM('a) + 2)) \<le> -2"
```
```   491       using aff_dim_le [OF order_refl] by simp
```
```   492   with aff_dim_geq [of "\<Inter>d (DIM('a) + 2)"] show ?thesis
```
```   493     using order.trans by fastforce
```
```   494 next
```
```   495   case False
```
```   496   then show ?thesis
```
```   497     apply simp
```
```   498     apply (erule ex_forward)
```
```   499     by blast
```
```   500 qed
```
```   501
```
```   502 lemma%unimportant faces_of_translation:
```
```   503    "{F. F face_of image (\<lambda>x. a + x) S} = image (image (\<lambda>x. a + x)) {F. F face_of S}"
```
```   504 apply (rule subset_antisym, clarify)
```
```   505 apply (auto simp: image_iff)
```
```   506 apply (metis face_of_imp_subset face_of_translation_eq subset_imageE)
```
```   507 done
```
```   508
```
```   509 proposition%important face_of_Times:
```
```   510   assumes "F face_of S" and "F' face_of S'"
```
```   511     shows "(F \<times> F') face_of (S \<times> S')"
```
```   512 proof%unimportant -
```
```   513   have "F \<times> F' \<subseteq> S \<times> S'"
```
```   514     using assms [unfolded face_of_def] by blast
```
```   515   moreover
```
```   516   have "convex (F \<times> F')"
```
```   517     using assms [unfolded face_of_def] by (blast intro: convex_Times)
```
```   518   moreover
```
```   519     have "a \<in> F \<and> a' \<in> F' \<and> b \<in> F \<and> b' \<in> F'"
```
```   520        if "a \<in> S" "b \<in> S" "a' \<in> S'" "b' \<in> S'" "x \<in> F \<times> F'" "x \<in> open_segment (a,a') (b,b')"
```
```   521        for a b a' b' x
```
```   522   proof (cases "b=a \<or> b'=a'")
```
```   523     case True with that show ?thesis
```
```   524       using assms
```
```   525       by (force simp: in_segment dest: face_ofD)
```
```   526   next
```
```   527     case False with assms [unfolded face_of_def] that show ?thesis
```
```   528       by (blast dest!: open_segment_PairD)
```
```   529   qed
```
```   530   ultimately show ?thesis
```
```   531     unfolding face_of_def by blast
```
```   532 qed
```
```   533
```
```   534 corollary%important face_of_Times_decomp:
```
```   535     fixes S :: "'a::euclidean_space set" and S' :: "'b::euclidean_space set"
```
```   536     shows "c face_of (S \<times> S') \<longleftrightarrow> (\<exists>F F'. F face_of S \<and> F' face_of S' \<and> c = F \<times> F')"
```
```   537      (is "?lhs = ?rhs")
```
```   538 proof%unimportant
```
```   539   assume c: ?lhs
```
```   540   show ?rhs
```
```   541   proof (cases "c = {}")
```
```   542     case True then show ?thesis by auto
```
```   543   next
```
```   544     case False
```
```   545     have 1: "fst ` c \<subseteq> S" "snd ` c \<subseteq> S'"
```
```   546       using c face_of_imp_subset by fastforce+
```
```   547     have "convex c"
```
```   548       using c by (metis face_of_imp_convex)
```
```   549     have conv: "convex (fst ` c)" "convex (snd ` c)"
```
```   550       by (simp_all add: \<open>convex c\<close> convex_linear_image fst_linear snd_linear)
```
```   551     have fstab: "a \<in> fst ` c \<and> b \<in> fst ` c"
```
```   552             if "a \<in> S" "b \<in> S" "x \<in> open_segment a b" "(x,x') \<in> c" for a b x x'
```
```   553     proof -
```
```   554       have *: "(x,x') \<in> open_segment (a,x') (b,x')"
```
```   555         using that by (auto simp: in_segment)
```
```   556       show ?thesis
```
```   557         using face_ofD [OF c *] that face_of_imp_subset [OF c] by force
```
```   558     qed
```
```   559     have fst: "fst ` c face_of S"
```
```   560       by (force simp: face_of_def 1 conv fstab)
```
```   561     have sndab: "a' \<in> snd ` c \<and> b' \<in> snd ` c"
```
```   562             if "a' \<in> S'" "b' \<in> S'" "x' \<in> open_segment a' b'" "(x,x') \<in> c" for a' b' x x'
```
```   563     proof -
```
```   564       have *: "(x,x') \<in> open_segment (x,a') (x,b')"
```
```   565         using that by (auto simp: in_segment)
```
```   566       show ?thesis
```
```   567         using face_ofD [OF c *] that face_of_imp_subset [OF c] by force
```
```   568     qed
```
```   569     have snd: "snd ` c face_of S'"
```
```   570       by (force simp: face_of_def 1 conv sndab)
```
```   571     have cc: "rel_interior c \<subseteq> rel_interior (fst ` c) \<times> rel_interior (snd ` c)"
```
```   572       by (force simp: face_of_Times rel_interior_Times conv fst snd \<open>convex c\<close> fst_linear snd_linear rel_interior_convex_linear_image [symmetric])
```
```   573     have "c = fst ` c \<times> snd ` c"
```
```   574       apply (rule face_of_eq [OF c])
```
```   575       apply (simp_all add: face_of_Times rel_interior_Times conv fst snd)
```
```   576       using False rel_interior_eq_empty \<open>convex c\<close> cc
```
```   577       apply blast
```
```   578       done
```
```   579     with fst snd show ?thesis by metis
```
```   580   qed
```
```   581 next
```
```   582   assume ?rhs with face_of_Times show ?lhs by auto
```
```   583 qed
```
```   584
```
```   585 lemma%unimportant face_of_Times_eq:
```
```   586     fixes S :: "'a::euclidean_space set" and S' :: "'b::euclidean_space set"
```
```   587     shows "(F \<times> F') face_of (S \<times> S') \<longleftrightarrow>
```
```   588            F = {} \<or> F' = {} \<or> F face_of S \<and> F' face_of S'"
```
```   589 by (auto simp: face_of_Times_decomp times_eq_iff)
```
```   590
```
```   591 lemma%unimportant hyperplane_face_of_halfspace_le: "{x. a \<bullet> x = b} face_of {x. a \<bullet> x \<le> b}"
```
```   592 proof -
```
```   593   have "{x. a \<bullet> x \<le> b} \<inter> {x. a \<bullet> x = b} = {x. a \<bullet> x = b}"
```
```   594     by auto
```
```   595   with face_of_Int_supporting_hyperplane_le [OF convex_halfspace_le [of a b], of a b]
```
```   596   show ?thesis by auto
```
```   597 qed
```
```   598
```
```   599 lemma%unimportant hyperplane_face_of_halfspace_ge: "{x. a \<bullet> x = b} face_of {x. a \<bullet> x \<ge> b}"
```
```   600 proof -
```
```   601   have "{x. a \<bullet> x \<ge> b} \<inter> {x. a \<bullet> x = b} = {x. a \<bullet> x = b}"
```
```   602     by auto
```
```   603   with face_of_Int_supporting_hyperplane_ge [OF convex_halfspace_ge [of b a], of b a]
```
```   604   show ?thesis by auto
```
```   605 qed
```
```   606
```
```   607 lemma%important face_of_halfspace_le:
```
```   608   fixes a :: "'n::euclidean_space"
```
```   609   shows "F face_of {x. a \<bullet> x \<le> b} \<longleftrightarrow>
```
```   610          F = {} \<or> F = {x. a \<bullet> x = b} \<or> F = {x. a \<bullet> x \<le> b}"
```
```   611      (is "?lhs = ?rhs")
```
```   612 proof%unimportant (cases "a = 0")
```
```   613   case True then show ?thesis
```
```   614     using face_of_affine_eq affine_UNIV by auto
```
```   615 next
```
```   616   case False
```
```   617   then have ine: "interior {x. a \<bullet> x \<le> b} \<noteq> {}"
```
```   618     using halfspace_eq_empty_lt interior_halfspace_le by blast
```
```   619   show ?thesis
```
```   620   proof
```
```   621     assume L: ?lhs
```
```   622     have "F \<noteq> {x. a \<bullet> x \<le> b} \<Longrightarrow> F face_of {x. a \<bullet> x = b}"
```
```   623       using False
```
```   624       apply (simp add: frontier_halfspace_le [symmetric] rel_frontier_nonempty_interior [OF ine, symmetric])
```
```   625       apply (rule face_of_subset [OF L])
```
```   626       apply (simp add: face_of_subset_rel_frontier [OF L])
```
```   627       apply (force simp: rel_frontier_def closed_halfspace_le)
```
```   628       done
```
```   629     with L show ?rhs
```
```   630       using affine_hyperplane face_of_affine_eq by blast
```
```   631   next
```
```   632     assume ?rhs
```
```   633     then show ?lhs
```
```   634       by (metis convex_halfspace_le empty_face_of face_of_refl hyperplane_face_of_halfspace_le)
```
```   635   qed
```
```   636 qed
```
```   637
```
```   638 lemma%unimportant face_of_halfspace_ge:
```
```   639   fixes a :: "'n::euclidean_space"
```
```   640   shows "F face_of {x. a \<bullet> x \<ge> b} \<longleftrightarrow>
```
```   641          F = {} \<or> F = {x. a \<bullet> x = b} \<or> F = {x. a \<bullet> x \<ge> b}"
```
```   642 using face_of_halfspace_le [of F "-a" "-b"] by simp
```
```   643
```
```   644 subsection%important\<open>Exposed faces\<close>
```
```   645
```
```   646 text\<open>That is, faces that are intersection with supporting hyperplane\<close>
```
```   647
```
```   648 definition%important exposed_face_of :: "['a::euclidean_space set, 'a set] \<Rightarrow> bool"
```
```   649                                (infixr "(exposed'_face'_of)" 50)
```
```   650   where "T exposed_face_of S \<longleftrightarrow>
```
```   651          T face_of S \<and> (\<exists>a b. S \<subseteq> {x. a \<bullet> x \<le> b} \<and> T = S \<inter> {x. a \<bullet> x = b})"
```
```   652
```
```   653 lemma%unimportant empty_exposed_face_of [iff]: "{} exposed_face_of S"
```
```   654   apply (simp add: exposed_face_of_def)
```
```   655   apply (rule_tac x=0 in exI)
```
```   656   apply (rule_tac x=1 in exI, force)
```
```   657   done
```
```   658
```
```   659 lemma%unimportant exposed_face_of_refl_eq [simp]: "S exposed_face_of S \<longleftrightarrow> convex S"
```
```   660   apply (simp add: exposed_face_of_def face_of_refl_eq, auto)
```
```   661   apply (rule_tac x=0 in exI)+
```
```   662   apply force
```
```   663   done
```
```   664
```
```   665 lemma%unimportant exposed_face_of_refl: "convex S \<Longrightarrow> S exposed_face_of S"
```
```   666   by simp
```
```   667
```
```   668 lemma%unimportant exposed_face_of:
```
```   669     "T exposed_face_of S \<longleftrightarrow>
```
```   670      T face_of S \<and>
```
```   671      (T = {} \<or> T = S \<or>
```
```   672       (\<exists>a b. a \<noteq> 0 \<and> S \<subseteq> {x. a \<bullet> x \<le> b} \<and> T = S \<inter> {x. a \<bullet> x = b}))"
```
```   673 proof (cases "T = {}")
```
```   674   case True then show ?thesis
```
```   675     by simp
```
```   676 next
```
```   677   case False
```
```   678   show ?thesis
```
```   679   proof (cases "T = S")
```
```   680     case True then show ?thesis
```
```   681       by (simp add: face_of_refl_eq)
```
```   682   next
```
```   683     case False
```
```   684     with \<open>T \<noteq> {}\<close> show ?thesis
```
```   685       apply (auto simp: exposed_face_of_def)
```
```   686       apply (metis inner_zero_left)
```
```   687       done
```
```   688   qed
```
```   689 qed
```
```   690
```
```   691 lemma%unimportant exposed_face_of_Int_supporting_hyperplane_le:
```
```   692    "\<lbrakk>convex S; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<le> b\<rbrakk> \<Longrightarrow> (S \<inter> {x. a \<bullet> x = b}) exposed_face_of S"
```
```   693 by (force simp: exposed_face_of_def face_of_Int_supporting_hyperplane_le)
```
```   694
```
```   695 lemma%unimportant exposed_face_of_Int_supporting_hyperplane_ge:
```
```   696    "\<lbrakk>convex S; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<ge> b\<rbrakk> \<Longrightarrow> (S \<inter> {x. a \<bullet> x = b}) exposed_face_of S"
```
```   697 using exposed_face_of_Int_supporting_hyperplane_le [of S "-a" "-b"] by simp
```
```   698
```
```   699 proposition%important exposed_face_of_Int:
```
```   700   assumes "T exposed_face_of S"
```
```   701       and "u exposed_face_of S"
```
```   702     shows "(T \<inter> u) exposed_face_of S"
```
```   703 proof%unimportant -
```
```   704   obtain a b where T: "S \<inter> {x. a \<bullet> x = b} face_of S"
```
```   705                and S: "S \<subseteq> {x. a \<bullet> x \<le> b}"
```
```   706                and teq: "T = S \<inter> {x. a \<bullet> x = b}"
```
```   707     using assms by (auto simp: exposed_face_of_def)
```
```   708   obtain a' b' where u: "S \<inter> {x. a' \<bullet> x = b'} face_of S"
```
```   709                  and s': "S \<subseteq> {x. a' \<bullet> x \<le> b'}"
```
```   710                  and ueq: "u = S \<inter> {x. a' \<bullet> x = b'}"
```
```   711     using assms by (auto simp: exposed_face_of_def)
```
```   712   have tu: "T \<inter> u face_of S"
```
```   713     using T teq u ueq by (simp add: face_of_Int)
```
```   714   have ss: "S \<subseteq> {x. (a + a') \<bullet> x \<le> b + b'}"
```
```   715     using S s' by (force simp: inner_left_distrib)
```
```   716   show ?thesis
```
```   717     apply (simp add: exposed_face_of_def tu)
```
```   718     apply (rule_tac x="a+a'" in exI)
```
```   719     apply (rule_tac x="b+b'" in exI)
```
```   720     using S s'
```
```   721     apply (fastforce simp: ss inner_left_distrib teq ueq)
```
```   722     done
```
```   723 qed
```
```   724
```
```   725 proposition%important exposed_face_of_Inter:
```
```   726     fixes P :: "'a::euclidean_space set set"
```
```   727   assumes "P \<noteq> {}"
```
```   728       and "\<And>T. T \<in> P \<Longrightarrow> T exposed_face_of S"
```
```   729     shows "\<Inter>P exposed_face_of S"
```
```   730 proof%unimportant -
```
```   731   obtain Q where "finite Q" and QsubP: "Q \<subseteq> P" "card Q \<le> DIM('a) + 2" and IntQ: "\<Inter>Q = \<Inter>P"
```
```   732     using Inter_faces_finite_altbound [of P S] assms [unfolded exposed_face_of]
```
```   733     by force
```
```   734   show ?thesis
```
```   735   proof (cases "Q = {}")
```
```   736     case True then show ?thesis
```
```   737       by (metis IntQ Inter_UNIV_conv(2) assms(1) assms(2) ex_in_conv)
```
```   738   next
```
```   739     case False
```
```   740     have "Q \<subseteq> {T. T exposed_face_of S}"
```
```   741       using QsubP assms by blast
```
```   742     moreover have "Q \<subseteq> {T. T exposed_face_of S} \<Longrightarrow> \<Inter>Q exposed_face_of S"
```
```   743       using \<open>finite Q\<close> False
```
```   744       apply (induction Q rule: finite_induct)
```
```   745       using exposed_face_of_Int apply fastforce+
```
```   746       done
```
```   747     ultimately show ?thesis
```
```   748       by (simp add: IntQ)
```
```   749   qed
```
```   750 qed
```
```   751
```
```   752 proposition%important exposed_face_of_sums:
```
```   753   assumes "convex S" and "convex T"
```
```   754       and "F exposed_face_of {x + y | x y. x \<in> S \<and> y \<in> T}"
```
```   755           (is "F exposed_face_of ?ST")
```
```   756   obtains k l
```
```   757     where "k exposed_face_of S" "l exposed_face_of T"
```
```   758           "F = {x + y | x y. x \<in> k \<and> y \<in> l}"
```
```   759 proof%unimportant (cases "F = {}")
```
```   760   case True then show ?thesis
```
```   761     using that by blast
```
```   762 next
```
```   763   case False
```
```   764   show ?thesis
```
```   765   proof (cases "F = ?ST")
```
```   766     case True then show ?thesis
```
```   767       using assms exposed_face_of_refl_eq that by blast
```
```   768   next
```
```   769     case False
```
```   770     obtain p where "p \<in> F" using \<open>F \<noteq> {}\<close> by blast
```
```   771     moreover
```
```   772     obtain u z where T: "?ST \<inter> {x. u \<bullet> x = z} face_of ?ST"
```
```   773                  and S: "?ST \<subseteq> {x. u \<bullet> x \<le> z}"
```
```   774                  and feq: "F = ?ST \<inter> {x. u \<bullet> x = z}"
```
```   775       using assms by (auto simp: exposed_face_of_def)
```
```   776     ultimately obtain a0 b0
```
```   777             where p: "p = a0 + b0" and "a0 \<in> S" "b0 \<in> T" and z: "u \<bullet> p = z"
```
```   778       by auto
```
```   779     have lez: "u \<bullet> (x + y) \<le> z" if "x \<in> S" "y \<in> T" for x y
```
```   780       using S that by auto
```
```   781     have sef: "S \<inter> {x. u \<bullet> x = u \<bullet> a0} exposed_face_of S"
```
```   782       apply (rule exposed_face_of_Int_supporting_hyperplane_le [OF \<open>convex S\<close>])
```
```   783       apply (metis p z add_le_cancel_right inner_right_distrib lez [OF _ \<open>b0 \<in> T\<close>])
```
```   784       done
```
```   785     have tef: "T \<inter> {x. u \<bullet> x = u \<bullet> b0} exposed_face_of T"
```
```   786       apply (rule exposed_face_of_Int_supporting_hyperplane_le [OF \<open>convex T\<close>])
```
```   787       apply (metis p z add.commute add_le_cancel_right inner_right_distrib lez [OF \<open>a0 \<in> S\<close>])
```
```   788       done
```
```   789     have "{x + y |x y. x \<in> S \<and> u \<bullet> x = u \<bullet> a0 \<and> y \<in> T \<and> u \<bullet> y = u \<bullet> b0} \<subseteq> F"
```
```   790       by (auto simp: feq) (metis inner_right_distrib p z)
```
```   791     moreover have "F \<subseteq> {x + y |x y. x \<in> S \<and> u \<bullet> x = u \<bullet> a0 \<and> y \<in> T \<and> u \<bullet> y = u \<bullet> b0}"
```
```   792       apply (auto simp: feq)
```
```   793       apply (rename_tac x y)
```
```   794       apply (rule_tac x=x in exI)
```
```   795       apply (rule_tac x=y in exI, simp)
```
```   796       using z p \<open>a0 \<in> S\<close> \<open>b0 \<in> T\<close>
```
```   797       apply clarify
```
```   798       apply (simp add: inner_right_distrib)
```
```   799       apply (metis add_le_cancel_right antisym lez [unfolded inner_right_distrib] add.commute)
```
```   800       done
```
```   801     ultimately have "F = {x + y |x y. x \<in> S \<inter> {x. u \<bullet> x = u \<bullet> a0} \<and> y \<in> T \<inter> {x. u \<bullet> x = u \<bullet> b0}}"
```
```   802       by blast
```
```   803     then show ?thesis
```
```   804       by (rule that [OF sef tef])
```
```   805   qed
```
```   806 qed
```
```   807
```
```   808 lemma%important exposed_face_of_parallel:
```
```   809    "T exposed_face_of S \<longleftrightarrow>
```
```   810          T face_of S \<and>
```
```   811          (\<exists>a b. S \<subseteq> {x. a \<bullet> x \<le> b} \<and> T = S \<inter> {x. a \<bullet> x = b} \<and>
```
```   812                 (T \<noteq> {} \<longrightarrow> T \<noteq> S \<longrightarrow> a \<noteq> 0) \<and>
```
```   813                 (T \<noteq> S \<longrightarrow> (\<forall>w \<in> affine hull S. (w + a) \<in> affine hull S)))"
```
```   814   (is "?lhs = ?rhs")
```
```   815 proof%unimportant
```
```   816   assume ?lhs then show ?rhs
```
```   817   proof (clarsimp simp: exposed_face_of_def)
```
```   818     fix a b
```
```   819     assume faceS: "S \<inter> {x. a \<bullet> x = b} face_of S" and Ssub: "S \<subseteq> {x. a \<bullet> x \<le> b}"
```
```   820     show "\<exists>c d. S \<subseteq> {x. c \<bullet> x \<le> d} \<and>
```
```   821                 S \<inter> {x. a \<bullet> x = b} = S \<inter> {x. c \<bullet> x = d} \<and>
```
```   822                 (S \<inter> {x. a \<bullet> x = b} \<noteq> {} \<longrightarrow> S \<inter> {x. a \<bullet> x = b} \<noteq> S \<longrightarrow> c \<noteq> 0) \<and>
```
```   823                 (S \<inter> {x. a \<bullet> x = b} \<noteq> S \<longrightarrow> (\<forall>w \<in> affine hull S. w + c \<in> affine hull S))"
```
```   824     proof (cases "affine hull S \<inter> {x. -a \<bullet> x \<le> -b} = {} \<or> affine hull S \<subseteq> {x. - a \<bullet> x \<le> - b}")
```
```   825       case True
```
```   826       then show ?thesis
```
```   827       proof
```
```   828         assume "affine hull S \<inter> {x. - a \<bullet> x \<le> - b} = {}"
```
```   829        then show ?thesis
```
```   830          apply (rule_tac x="0" in exI)
```
```   831          apply (rule_tac x="1" in exI)
```
```   832          using hull_subset by fastforce
```
```   833     next
```
```   834       assume "affine hull S \<subseteq> {x. - a \<bullet> x \<le> - b}"
```
```   835       then show ?thesis
```
```   836          apply (rule_tac x="0" in exI)
```
```   837          apply (rule_tac x="0" in exI)
```
```   838         using Ssub hull_subset by fastforce
```
```   839     qed
```
```   840   next
```
```   841     case False
```
```   842     then obtain a' b' where "a' \<noteq> 0"
```
```   843       and le: "affine hull S \<inter> {x. a' \<bullet> x \<le> b'} = affine hull S \<inter> {x. - a \<bullet> x \<le> - b}"
```
```   844       and eq: "affine hull S \<inter> {x. a' \<bullet> x = b'} = affine hull S \<inter> {x. - a \<bullet> x = - b}"
```
```   845       and mem: "\<And>w. w \<in> affine hull S \<Longrightarrow> w + a' \<in> affine hull S"
```
```   846       using affine_parallel_slice affine_affine_hull by metis
```
```   847     show ?thesis
```
```   848     proof (intro conjI impI allI ballI exI)
```
```   849       have *: "S \<subseteq> - (affine hull S \<inter> {x. P x}) \<union> affine hull S \<inter> {x. Q x} \<Longrightarrow> S \<subseteq> {x. ~P x \<or> Q x}"
```
```   850         for P Q
```
```   851         using hull_subset by fastforce
```
```   852       have "S \<subseteq> {x. ~ (a' \<bullet> x \<le> b') \<or> a' \<bullet> x = b'}"
```
```   853         apply (rule *)
```
```   854         apply (simp only: le eq)
```
```   855         using Ssub by auto
```
```   856       then show "S \<subseteq> {x. - a' \<bullet> x \<le> - b'}"
```
```   857         by auto
```
```   858       show "S \<inter> {x. a \<bullet> x = b} = S \<inter> {x. - a' \<bullet> x = - b'}"
```
```   859         using eq hull_subset [of S affine] by force
```
```   860       show "\<lbrakk>S \<inter> {x. a \<bullet> x = b} \<noteq> {}; S \<inter> {x. a \<bullet> x = b} \<noteq> S\<rbrakk> \<Longrightarrow> - a' \<noteq> 0"
```
```   861         using \<open>a' \<noteq> 0\<close> by auto
```
```   862       show "w + - a' \<in> affine hull S"
```
```   863         if "S \<inter> {x. a \<bullet> x = b} \<noteq> S" "w \<in> affine hull S" for w
```
```   864       proof -
```
```   865         have "w + 1 *\<^sub>R (w - (w + a')) \<in> affine hull S"
```
```   866           using affine_affine_hull mem mem_affine_3_minus that(2) by blast
```
```   867         then show ?thesis  by simp
```
```   868       qed
```
```   869     qed
```
```   870   qed
```
```   871 qed
```
```   872 next
```
```   873   assume ?rhs then show ?lhs
```
```   874     unfolding exposed_face_of_def by blast
```
```   875 qed
```
```   876
```
```   877 subsection%important\<open>Extreme points of a set: its singleton faces\<close>
```
```   878
```
```   879 definition%important extreme_point_of :: "['a::real_vector, 'a set] \<Rightarrow> bool"
```
```   880                                (infixr "(extreme'_point'_of)" 50)
```
```   881   where "x extreme_point_of S \<longleftrightarrow>
```
```   882          x \<in> S \<and> (\<forall>a \<in> S. \<forall>b \<in> S. x \<notin> open_segment a b)"
```
```   883
```
```   884 lemma%unimportant extreme_point_of_stillconvex:
```
```   885    "convex S \<Longrightarrow> (x extreme_point_of S \<longleftrightarrow> x \<in> S \<and> convex(S - {x}))"
```
```   886   by (fastforce simp add: convex_contains_segment extreme_point_of_def open_segment_def)
```
```   887
```
```   888 lemma%unimportant face_of_singleton:
```
```   889    "{x} face_of S \<longleftrightarrow> x extreme_point_of S"
```
```   890 by (fastforce simp add: extreme_point_of_def face_of_def)
```
```   891
```
```   892 lemma%unimportant extreme_point_not_in_REL_INTERIOR:
```
```   893     fixes S :: "'a::real_normed_vector set"
```
```   894     shows "\<lbrakk>x extreme_point_of S; S \<noteq> {x}\<rbrakk> \<Longrightarrow> x \<notin> rel_interior S"
```
```   895 apply (simp add: face_of_singleton [symmetric])
```
```   896 apply (blast dest: face_of_disjoint_rel_interior)
```
```   897 done
```
```   898
```
```   899 lemma%important extreme_point_not_in_interior:
```
```   900     fixes S :: "'a::{real_normed_vector, perfect_space} set"
```
```   901     shows "x extreme_point_of S \<Longrightarrow> x \<notin> interior S"
```
```   902 apply (case_tac "S = {x}")
```
```   903 apply (simp add: empty_interior_finite)
```
```   904 by (meson contra_subsetD extreme_point_not_in_REL_INTERIOR interior_subset_rel_interior)
```
```   905
```
```   906 lemma%unimportant extreme_point_of_face:
```
```   907      "F face_of S \<Longrightarrow> v extreme_point_of F \<longleftrightarrow> v extreme_point_of S \<and> v \<in> F"
```
```   908   by (meson empty_subsetI face_of_face face_of_singleton insert_subset)
```
```   909
```
```   910 lemma%unimportant extreme_point_of_convex_hull:
```
```   911    "x extreme_point_of (convex hull S) \<Longrightarrow> x \<in> S"
```
```   912 apply (simp add: extreme_point_of_stillconvex)
```
```   913 using hull_minimal [of S "(convex hull S) - {x}" convex]
```
```   914 using hull_subset [of S convex]
```
```   915 apply blast
```
```   916 done
```
```   917
```
```   918 lemma%important extreme_points_of_convex_hull:
```
```   919    "{x. x extreme_point_of (convex hull S)} \<subseteq> S"
```
```   920 using%unimportant extreme_point_of_convex_hull by auto
```
```   921
```
```   922 lemma%unimportant extreme_point_of_empty [simp]: "~ (x extreme_point_of {})"
```
```   923   by (simp add: extreme_point_of_def)
```
```   924
```
```   925 lemma%unimportant extreme_point_of_singleton [iff]: "x extreme_point_of {a} \<longleftrightarrow> x = a"
```
```   926   using extreme_point_of_stillconvex by auto
```
```   927
```
```   928 lemma%unimportant extreme_point_of_translation_eq:
```
```   929    "(a + x) extreme_point_of (image (\<lambda>x. a + x) S) \<longleftrightarrow> x extreme_point_of S"
```
```   930 by (auto simp: extreme_point_of_def)
```
```   931
```
```   932 lemma%important extreme_points_of_translation:
```
```   933    "{x. x extreme_point_of (image (\<lambda>x. a + x) S)} =
```
```   934     (\<lambda>x. a + x) ` {x. x extreme_point_of S}"
```
```   935 using%unimportant extreme_point_of_translation_eq
```
```   936 by%unimportant auto (metis (no_types, lifting) image_iff mem_Collect_eq minus_add_cancel)
```
```   937
```
```   938 lemma%unimportant extreme_point_of_Int:
```
```   939    "\<lbrakk>x extreme_point_of S; x extreme_point_of T\<rbrakk> \<Longrightarrow> x extreme_point_of (S \<inter> T)"
```
```   940 by (simp add: extreme_point_of_def)
```
```   941
```
```   942 lemma%important extreme_point_of_Int_supporting_hyperplane_le:
```
```   943    "\<lbrakk>S \<inter> {x. a \<bullet> x = b} = {c}; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<le> b\<rbrakk> \<Longrightarrow> c extreme_point_of S"
```
```   944 apply (simp add: face_of_singleton [symmetric])
```
```   945 by (metis face_of_Int_supporting_hyperplane_le_strong convex_singleton)
```
```   946
```
```   947 lemma%unimportant extreme_point_of_Int_supporting_hyperplane_ge:
```
```   948    "\<lbrakk>S \<inter> {x. a \<bullet> x = b} = {c}; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<ge> b\<rbrakk> \<Longrightarrow> c extreme_point_of S"
```
```   949 apply (simp add: face_of_singleton [symmetric])
```
```   950 by (metis face_of_Int_supporting_hyperplane_ge_strong convex_singleton)
```
```   951
```
```   952 lemma%unimportant exposed_point_of_Int_supporting_hyperplane_le:
```
```   953    "\<lbrakk>S \<inter> {x. a \<bullet> x = b} = {c}; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<le> b\<rbrakk> \<Longrightarrow> {c} exposed_face_of S"
```
```   954 apply (simp add: exposed_face_of_def face_of_singleton)
```
```   955 apply (force simp: extreme_point_of_Int_supporting_hyperplane_le)
```
```   956 done
```
```   957
```
```   958 lemma%unimportant exposed_point_of_Int_supporting_hyperplane_ge:
```
```   959     "\<lbrakk>S \<inter> {x. a \<bullet> x = b} = {c}; \<And>x. x \<in> S \<Longrightarrow> a \<bullet> x \<ge> b\<rbrakk> \<Longrightarrow> {c} exposed_face_of S"
```
```   960 using exposed_point_of_Int_supporting_hyperplane_le [of S "-a" "-b" c]
```
```   961 by simp
```
```   962
```
```   963 lemma%unimportant extreme_point_of_convex_hull_insert:
```
```   964    "\<lbrakk>finite S; a \<notin> convex hull S\<rbrakk> \<Longrightarrow> a extreme_point_of (convex hull (insert a S))"
```
```   965 apply (case_tac "a \<in> S")
```
```   966 apply (simp add: hull_inc)
```
```   967 using face_of_convex_hulls [of "insert a S" "{a}"]
```
```   968 apply (auto simp: face_of_singleton hull_same)
```
```   969 done
```
```   970
```
```   971 subsection%important\<open>Facets\<close>
```
```   972
```
```   973 definition%important facet_of :: "['a::euclidean_space set, 'a set] \<Rightarrow> bool"
```
```   974                     (infixr "(facet'_of)" 50)
```
```   975   where "F facet_of S \<longleftrightarrow> F face_of S \<and> F \<noteq> {} \<and> aff_dim F = aff_dim S - 1"
```
```   976
```
```   977 lemma%unimportant facet_of_empty [simp]: "~ S facet_of {}"
```
```   978   by (simp add: facet_of_def)
```
```   979
```
```   980 lemma%unimportant facet_of_irrefl [simp]: "~ S facet_of S "
```
```   981   by (simp add: facet_of_def)
```
```   982
```
```   983 lemma%unimportant facet_of_imp_face_of: "F facet_of S \<Longrightarrow> F face_of S"
```
```   984   by (simp add: facet_of_def)
```
```   985
```
```   986 lemma%unimportant facet_of_imp_subset: "F facet_of S \<Longrightarrow> F \<subseteq> S"
```
```   987   by (simp add: face_of_imp_subset facet_of_def)
```
```   988
```
```   989 lemma%unimportant hyperplane_facet_of_halfspace_le:
```
```   990    "a \<noteq> 0 \<Longrightarrow> {x. a \<bullet> x = b} facet_of {x. a \<bullet> x \<le> b}"
```
```   991 unfolding facet_of_def hyperplane_eq_empty
```
```   992 by (auto simp: hyperplane_face_of_halfspace_ge hyperplane_face_of_halfspace_le
```
```   993            DIM_positive Suc_leI of_nat_diff aff_dim_halfspace_le)
```
```   994
```
```   995 lemma%unimportant hyperplane_facet_of_halfspace_ge:
```
```   996     "a \<noteq> 0 \<Longrightarrow> {x. a \<bullet> x = b} facet_of {x. a \<bullet> x \<ge> b}"
```
```   997 unfolding facet_of_def hyperplane_eq_empty
```
```   998 by (auto simp: hyperplane_face_of_halfspace_le hyperplane_face_of_halfspace_ge
```
```   999            DIM_positive Suc_leI of_nat_diff aff_dim_halfspace_ge)
```
```  1000
```
```  1001 lemma%important facet_of_halfspace_le:
```
```  1002     "F facet_of {x. a \<bullet> x \<le> b} \<longleftrightarrow> a \<noteq> 0 \<and> F = {x. a \<bullet> x = b}"
```
```  1003     (is "?lhs = ?rhs")
```
```  1004 proof%unimportant
```
```  1005   assume c: ?lhs
```
```  1006   with c facet_of_irrefl show ?rhs
```
```  1007     by (force simp: aff_dim_halfspace_le facet_of_def face_of_halfspace_le cong: conj_cong split: if_split_asm)
```
```  1008 next
```
```  1009   assume ?rhs then show ?lhs
```
```  1010     by (simp add: hyperplane_facet_of_halfspace_le)
```
```  1011 qed
```
```  1012
```
```  1013 lemma%unimportant facet_of_halfspace_ge:
```
```  1014     "F facet_of {x. a \<bullet> x \<ge> b} \<longleftrightarrow> a \<noteq> 0 \<and> F = {x. a \<bullet> x = b}"
```
```  1015 using facet_of_halfspace_le [of F "-a" "-b"] by simp
```
```  1016
```
```  1017 subsection%important \<open>Edges: faces of affine dimension 1\<close>
```
```  1018
```
```  1019 definition%important edge_of :: "['a::euclidean_space set, 'a set] \<Rightarrow> bool"  (infixr "(edge'_of)" 50)
```
```  1020   where "e edge_of S \<longleftrightarrow> e face_of S \<and> aff_dim e = 1"
```
```  1021
```
```  1022 lemma%unimportant edge_of_imp_subset:
```
```  1023    "S edge_of T \<Longrightarrow> S \<subseteq> T"
```
```  1024 by (simp add: edge_of_def face_of_imp_subset)
```
```  1025
```
```  1026 subsection%important\<open>Existence of extreme points\<close>
```
```  1027
```
```  1028 lemma%important different_norm_3_collinear_points:
```
```  1029   fixes a :: "'a::euclidean_space"
```
```  1030   assumes "x \<in> open_segment a b" "norm(a) = norm(b)" "norm(x) = norm(b)"
```
```  1031   shows False
```
```  1032 proof%unimportant -
```
```  1033   obtain u where "norm ((1 - u) *\<^sub>R a + u *\<^sub>R b) = norm b"
```
```  1034              and "a \<noteq> b"
```
```  1035              and u01: "0 < u" "u < 1"
```
```  1036     using assms by (auto simp: open_segment_image_interval if_splits)
```
```  1037   then have "(1 - u) *\<^sub>R a \<bullet> (1 - u) *\<^sub>R a + ((1 - u) * 2) *\<^sub>R a \<bullet> u *\<^sub>R b =
```
```  1038              (1 - u * u) *\<^sub>R (a \<bullet> a)"
```
```  1039     using assms by (simp add: norm_eq algebra_simps inner_commute)
```
```  1040   then have "(1 - u) *\<^sub>R ((1 - u) *\<^sub>R a \<bullet> a + (2 * u) *\<^sub>R  a \<bullet> b) =
```
```  1041              (1 - u) *\<^sub>R ((1 + u) *\<^sub>R (a \<bullet> a))"
```
```  1042     by (simp add: algebra_simps)
```
```  1043   then have "(1 - u) *\<^sub>R (a \<bullet> a) + (2 * u) *\<^sub>R (a \<bullet> b) = (1 + u) *\<^sub>R (a \<bullet> a)"
```
```  1044     using u01 by auto
```
```  1045   then have "a \<bullet> b = a \<bullet> a"
```
```  1046     using u01 by (simp add: algebra_simps)
```
```  1047   then have "a = b"
```
```  1048     using \<open>norm(a) = norm(b)\<close> norm_eq vector_eq by fastforce
```
```  1049   then show ?thesis
```
```  1050     using \<open>a \<noteq> b\<close> by force
```
```  1051 qed
```
```  1052
```
```  1053 proposition%important extreme_point_exists_convex:
```
```  1054   fixes S :: "'a::euclidean_space set"
```
```  1055   assumes "compact S" "convex S" "S \<noteq> {}"
```
```  1056   obtains x where "x extreme_point_of S"
```
```  1057 proof%unimportant -
```
```  1058   obtain x where "x \<in> S" and xsup: "\<And>y. y \<in> S \<Longrightarrow> norm y \<le> norm x"
```
```  1059     using distance_attains_sup [of S 0] assms by auto
```
```  1060   have False if "a \<in> S" "b \<in> S" and x: "x \<in> open_segment a b" for a b
```
```  1061   proof -
```
```  1062     have noax: "norm a \<le> norm x" and nobx: "norm b \<le> norm x" using xsup that by auto
```
```  1063     have "a \<noteq> b"
```
```  1064       using empty_iff open_segment_idem x by auto
```
```  1065     have *: "(1 - u) * na + u * nb < norm x" if "na < norm x"  "nb \<le> norm x" "0 < u" "u < 1" for na nb u
```
```  1066     proof -
```
```  1067       have "(1 - u) * na + u * nb < (1 - u) * norm x + u * nb"
```
```  1068         by (simp add: that)
```
```  1069       also have "... \<le> (1 - u) * norm x + u * norm x"
```
```  1070         by (simp add: that)
```
```  1071       finally have "(1 - u) * na + u * nb < (1 - u) * norm x + u * norm x" .
```
```  1072       then show ?thesis
```
```  1073       using scaleR_collapse [symmetric, of "norm x" u] by auto
```
```  1074     qed
```
```  1075     have "norm x < norm x" if "norm a < norm x"
```
```  1076       using x
```
```  1077       apply (clarsimp simp only: open_segment_image_interval \<open>a \<noteq> b\<close> if_False)
```
```  1078       apply (rule norm_triangle_lt)
```
```  1079       apply (simp add: norm_mult)
```
```  1080       using * [of "norm a" "norm b"] nobx that
```
```  1081         apply blast
```
```  1082       done
```
```  1083     moreover have "norm x < norm x" if "norm b < norm x"
```
```  1084       using x
```
```  1085       apply (clarsimp simp only: open_segment_image_interval \<open>a \<noteq> b\<close> if_False)
```
```  1086       apply (rule norm_triangle_lt)
```
```  1087       apply (simp add: norm_mult)
```
```  1088       using * [of "norm b" "norm a" "1-u" for u] noax that
```
```  1089         apply (simp add: add.commute)
```
```  1090       done
```
```  1091     ultimately have "~ (norm a < norm x) \<and> ~ (norm b < norm x)"
```
```  1092       by auto
```
```  1093     then show ?thesis
```
```  1094       using different_norm_3_collinear_points noax nobx that(3) by fastforce
```
```  1095   qed
```
```  1096   then show ?thesis
```
```  1097     apply (rule_tac x=x in that)
```
```  1098     apply (force simp: extreme_point_of_def \<open>x \<in> S\<close>)
```
```  1099     done
```
```  1100 qed
```
```  1101
```
```  1102 subsection%important\<open>Krein-Milman, the weaker form\<close>
```
```  1103
```
```  1104 proposition%important Krein_Milman:
```
```  1105   fixes S :: "'a::euclidean_space set"
```
```  1106   assumes "compact S" "convex S"
```
```  1107     shows "S = closure(convex hull {x. x extreme_point_of S})"
```
```  1108 proof%unimportant (cases "S = {}")
```
```  1109   case True then show ?thesis   by simp
```
```  1110 next
```
```  1111   case False
```
```  1112   have "closed S"
```
```  1113     by (simp add: \<open>compact S\<close> compact_imp_closed)
```
```  1114   have "closure (convex hull {x. x extreme_point_of S}) \<subseteq> S"
```
```  1115     apply (rule closure_minimal [OF hull_minimal \<open>closed S\<close>])
```
```  1116     using assms
```
```  1117     apply (auto simp: extreme_point_of_def)
```
```  1118     done
```
```  1119   moreover have "u \<in> closure (convex hull {x. x extreme_point_of S})"
```
```  1120                 if "u \<in> S" for u
```
```  1121   proof (rule ccontr)
```
```  1122     assume unot: "u \<notin> closure(convex hull {x. x extreme_point_of S})"
```
```  1123     then obtain a b where "a \<bullet> u < b"
```
```  1124           and ab: "\<And>x. x \<in> closure(convex hull {x. x extreme_point_of S}) \<Longrightarrow> b < a \<bullet> x"
```
```  1125       using separating_hyperplane_closed_point [of "closure(convex hull {x. x extreme_point_of S})"]
```
```  1126       by blast
```
```  1127     have "continuous_on S ((\<bullet>) a)"
```
```  1128       by (rule continuous_intros)+
```
```  1129     then obtain m where "m \<in> S" and m: "\<And>y. y \<in> S \<Longrightarrow> a \<bullet> m \<le> a \<bullet> y"
```
```  1130       using continuous_attains_inf [of S "\<lambda>x. a \<bullet> x"] \<open>compact S\<close> \<open>u \<in> S\<close>
```
```  1131       by auto
```
```  1132     define T where "T = S \<inter> {x. a \<bullet> x = a \<bullet> m}"
```
```  1133     have "m \<in> T"
```
```  1134       by (simp add: T_def \<open>m \<in> S\<close>)
```
```  1135     moreover have "compact T"
```
```  1136       by (simp add: T_def compact_Int_closed [OF \<open>compact S\<close> closed_hyperplane])
```
```  1137     moreover have "convex T"
```
```  1138       by (simp add: T_def convex_Int [OF \<open>convex S\<close> convex_hyperplane])
```
```  1139     ultimately obtain v where v: "v extreme_point_of T"
```
```  1140       using extreme_point_exists_convex [of T] by auto
```
```  1141     then have "{v} face_of T"
```
```  1142       by (simp add: face_of_singleton)
```
```  1143     also have "T face_of S"
```
```  1144       by (simp add: T_def m face_of_Int_supporting_hyperplane_ge [OF \<open>convex S\<close>])
```
```  1145     finally have "v extreme_point_of S"
```
```  1146       by (simp add: face_of_singleton)
```
```  1147     then have "b < a \<bullet> v"
```
```  1148       using closure_subset by (simp add: closure_hull hull_inc ab)
```
```  1149     then show False
```
```  1150       using \<open>a \<bullet> u < b\<close> \<open>{v} face_of T\<close> face_of_imp_subset m T_def that by fastforce
```
```  1151   qed
```
```  1152   ultimately show ?thesis
```
```  1153     by blast
```
```  1154 qed
```
```  1155
```
```  1156 text\<open>Now the sharper form.\<close>
```
```  1157
```
```  1158 lemma%important Krein_Milman_Minkowski_aux:
```
```  1159   fixes S :: "'a::euclidean_space set"
```
```  1160   assumes n: "dim S = n" and S: "compact S" "convex S" "0 \<in> S"
```
```  1161     shows "0 \<in> convex hull {x. x extreme_point_of S}"
```
```  1162 using n S
```
```  1163 proof%unimportant (induction n arbitrary: S rule: less_induct)
```
```  1164   case (less n S) show ?case
```
```  1165   proof (cases "0 \<in> rel_interior S")
```
```  1166     case True with Krein_Milman show ?thesis
```
```  1167       by (metis subsetD convex_convex_hull convex_rel_interior_closure less.prems(2) less.prems(3) rel_interior_subset)
```
```  1168   next
```
```  1169     case False
```
```  1170     have "rel_interior S \<noteq> {}"
```
```  1171       by (simp add: rel_interior_convex_nonempty_aux less)
```
```  1172     then obtain c where c: "c \<in> rel_interior S" by blast
```
```  1173     obtain a where "a \<noteq> 0"
```
```  1174               and le_ay: "\<And>y. y \<in> S \<Longrightarrow> a \<bullet> 0 \<le> a \<bullet> y"
```
```  1175               and less_ay: "\<And>y. y \<in> rel_interior S \<Longrightarrow> a \<bullet> 0 < a \<bullet> y"
```
```  1176       by (blast intro: supporting_hyperplane_rel_boundary intro!: less False)
```
```  1177     have face: "S \<inter> {x. a \<bullet> x = 0} face_of S"
```
```  1178       apply (rule face_of_Int_supporting_hyperplane_ge [OF \<open>convex S\<close>])
```
```  1179       using le_ay by auto
```
```  1180     then have co: "compact (S \<inter> {x. a \<bullet> x = 0})" "convex (S \<inter> {x. a \<bullet> x = 0})"
```
```  1181       using less.prems by (blast intro: face_of_imp_compact face_of_imp_convex)+
```
```  1182     have "a \<bullet> y = 0" if "y \<in> span (S \<inter> {x. a \<bullet> x = 0})" for y
```
```  1183     proof -
```
```  1184       have "y \<in> span {x. a \<bullet> x = 0}"
```
```  1185         by (metis inf.cobounded2 span_mono subsetCE that)
```
```  1186       then show ?thesis
```
```  1187         by (blast intro: span_induct [OF _ subspace_hyperplane])
```
```  1188     qed
```
```  1189     then have "dim (S \<inter> {x. a \<bullet> x = 0}) < n"
```
```  1190       by (metis (no_types) less_ay c subsetD dim_eq_span inf.strict_order_iff
```
```  1191            inf_le1 \<open>dim S = n\<close> not_le rel_interior_subset span_0 span_base)
```
```  1192     then have "0 \<in> convex hull {x. x extreme_point_of (S \<inter> {x. a \<bullet> x = 0})}"
```
```  1193       by (rule less.IH) (auto simp: co less.prems)
```
```  1194     then show ?thesis
```
```  1195       by (metis (mono_tags, lifting) Collect_mono_iff \<open>S \<inter> {x. a \<bullet> x = 0} face_of S\<close> extreme_point_of_face hull_mono subset_iff)
```
```  1196   qed
```
```  1197 qed
```
```  1198
```
```  1199
```
```  1200 theorem%important Krein_Milman_Minkowski:
```
```  1201   fixes S :: "'a::euclidean_space set"
```
```  1202   assumes "compact S" "convex S"
```
```  1203     shows "S = convex hull {x. x extreme_point_of S}"
```
```  1204 proof%unimportant
```
```  1205   show "S \<subseteq> convex hull {x. x extreme_point_of S}"
```
```  1206   proof
```
```  1207     fix a assume [simp]: "a \<in> S"
```
```  1208     have 1: "compact ((+) (- a) ` S)"
```
```  1209       by (simp add: \<open>compact S\<close> compact_translation)
```
```  1210     have 2: "convex ((+) (- a) ` S)"
```
```  1211       by (simp add: \<open>convex S\<close> convex_translation)
```
```  1212     show a_invex: "a \<in> convex hull {x. x extreme_point_of S}"
```
```  1213       using Krein_Milman_Minkowski_aux [OF refl 1 2]
```
```  1214             convex_hull_translation [of "-a"]
```
```  1215       by (auto simp: extreme_points_of_translation translation_assoc)
```
```  1216     qed
```
```  1217 next
```
```  1218   show "convex hull {x. x extreme_point_of S} \<subseteq> S"
```
```  1219   proof -
```
```  1220     have "{a. a extreme_point_of S} \<subseteq> S"
```
```  1221       using extreme_point_of_def by blast
```
```  1222     then show ?thesis
```
```  1223       by (simp add: \<open>convex S\<close> hull_minimal)
```
```  1224   qed
```
```  1225 qed
```
```  1226
```
```  1227
```
```  1228 subsection%important\<open>Applying it to convex hulls of explicitly indicated finite sets\<close>
```
```  1229
```
```  1230 lemma%important Krein_Milman_polytope:
```
```  1231   fixes S :: "'a::euclidean_space set"
```
```  1232   shows
```
```  1233    "finite S
```
```  1234        \<Longrightarrow> convex hull S =
```
```  1235            convex hull {x. x extreme_point_of (convex hull S)}"
```
```  1236 by%unimportant (simp add: Krein_Milman_Minkowski finite_imp_compact_convex_hull)
```
```  1237
```
```  1238 lemma%unimportant extreme_points_of_convex_hull_eq:
```
```  1239   fixes S :: "'a::euclidean_space set"
```
```  1240   shows
```
```  1241    "\<lbrakk>compact S; \<And>T. T \<subset> S \<Longrightarrow> convex hull T \<noteq> convex hull S\<rbrakk>
```
```  1242         \<Longrightarrow> {x. x extreme_point_of (convex hull S)} = S"
```
```  1243 by (metis (full_types) Krein_Milman_Minkowski compact_convex_hull convex_convex_hull extreme_points_of_convex_hull psubsetI)
```
```  1244
```
```  1245
```
```  1246 lemma%unimportant extreme_point_of_convex_hull_eq:
```
```  1247   fixes S :: "'a::euclidean_space set"
```
```  1248   shows
```
```  1249    "\<lbrakk>compact S; \<And>T. T \<subset> S \<Longrightarrow> convex hull T \<noteq> convex hull S\<rbrakk>
```
```  1250     \<Longrightarrow> (x extreme_point_of (convex hull S) \<longleftrightarrow> x \<in> S)"
```
```  1251 using extreme_points_of_convex_hull_eq by auto
```
```  1252
```
```  1253 lemma%important extreme_point_of_convex_hull_convex_independent:
```
```  1254   fixes S :: "'a::euclidean_space set"
```
```  1255   assumes "compact S" and S: "\<And>a. a \<in> S \<Longrightarrow> a \<notin> convex hull (S - {a})"
```
```  1256   shows "(x extreme_point_of (convex hull S) \<longleftrightarrow> x \<in> S)"
```
```  1257 proof%unimportant -
```
```  1258   have "convex hull T \<noteq> convex hull S" if "T \<subset> S" for T
```
```  1259   proof -
```
```  1260     obtain a where  "T \<subseteq> S" "a \<in> S" "a \<notin> T" using \<open>T \<subset> S\<close> by blast
```
```  1261     then show ?thesis
```
```  1262       by (metis (full_types) Diff_eq_empty_iff Diff_insert0 S hull_mono hull_subset insert_Diff_single subsetCE)
```
```  1263   qed
```
```  1264   then show ?thesis
```
```  1265     by (rule extreme_point_of_convex_hull_eq [OF \<open>compact S\<close>])
```
```  1266 qed
```
```  1267
```
```  1268 lemma%unimportant extreme_point_of_convex_hull_affine_independent:
```
```  1269   fixes S :: "'a::euclidean_space set"
```
```  1270   shows
```
```  1271    "~ affine_dependent S
```
```  1272          \<Longrightarrow> (x extreme_point_of (convex hull S) \<longleftrightarrow> x \<in> S)"
```
```  1273 by (metis aff_independent_finite affine_dependent_def affine_hull_convex_hull extreme_point_of_convex_hull_convex_independent finite_imp_compact hull_inc)
```
```  1274
```
```  1275 text\<open>Elementary proofs exist, not requiring Euclidean spaces and all this development\<close>
```
```  1276 lemma%unimportant extreme_point_of_convex_hull_2:
```
```  1277   fixes x :: "'a::euclidean_space"
```
```  1278   shows "x extreme_point_of (convex hull {a,b}) \<longleftrightarrow> x = a \<or> x = b"
```
```  1279 proof -
```
```  1280   have "x extreme_point_of (convex hull {a,b}) \<longleftrightarrow> x \<in> {a,b}"
```
```  1281     by (intro extreme_point_of_convex_hull_affine_independent affine_independent_2)
```
```  1282   then show ?thesis
```
```  1283     by simp
```
```  1284 qed
```
```  1285
```
```  1286 lemma%unimportant extreme_point_of_segment:
```
```  1287   fixes x :: "'a::euclidean_space"
```
```  1288   shows
```
```  1289    "x extreme_point_of closed_segment a b \<longleftrightarrow> x = a \<or> x = b"
```
```  1290 by (simp add: extreme_point_of_convex_hull_2 segment_convex_hull)
```
```  1291
```
```  1292 lemma%unimportant face_of_convex_hull_subset:
```
```  1293   fixes S :: "'a::euclidean_space set"
```
```  1294   assumes "compact S" and T: "T face_of (convex hull S)"
```
```  1295   obtains s' where "s' \<subseteq> S" "T = convex hull s'"
```
```  1296 apply (rule_tac s' = "{x. x extreme_point_of T}" in that)
```
```  1297 using T extreme_point_of_convex_hull extreme_point_of_face apply blast
```
```  1298 by (metis (no_types) Krein_Milman_Minkowski assms compact_convex_hull convex_convex_hull face_of_imp_compact face_of_imp_convex)
```
```  1299
```
```  1300
```
```  1301 lemma%important face_of_convex_hull_aux:
```
```  1302   assumes eq: "x *\<^sub>R p = u *\<^sub>R a + v *\<^sub>R b + w *\<^sub>R c"
```
```  1303     and x: "u + v + w = x" "x \<noteq> 0" and S: "affine S" "a \<in> S" "b \<in> S" "c \<in> S"
```
```  1304   shows "p \<in> S"
```
```  1305 proof%unimportant -
```
```  1306   have "p = (u *\<^sub>R a + v *\<^sub>R b + w *\<^sub>R c) /\<^sub>R x"
```
```  1307     by (metis \<open>x \<noteq> 0\<close> eq mult.commute right_inverse scaleR_one scaleR_scaleR)
```
```  1308   moreover have "affine hull {a,b,c} \<subseteq> S"
```
```  1309     by (simp add: S hull_minimal)
```
```  1310   moreover have "(u *\<^sub>R a + v *\<^sub>R b + w *\<^sub>R c) /\<^sub>R x \<in> affine hull {a,b,c}"
```
```  1311     apply (simp add: affine_hull_3)
```
```  1312     apply (rule_tac x="u/x" in exI)
```
```  1313     apply (rule_tac x="v/x" in exI)
```
```  1314     apply (rule_tac x="w/x" in exI)
```
```  1315     using x apply (auto simp: algebra_simps divide_simps)
```
```  1316     done
```
```  1317   ultimately show ?thesis by force
```
```  1318 qed
```
```  1319
```
```  1320 proposition%important face_of_convex_hull_insert_eq:
```
```  1321   fixes a :: "'a :: euclidean_space"
```
```  1322   assumes "finite S" and a: "a \<notin> affine hull S"
```
```  1323   shows "(F face_of (convex hull (insert a S)) \<longleftrightarrow>
```
```  1324           F face_of (convex hull S) \<or>
```
```  1325           (\<exists>F'. F' face_of (convex hull S) \<and> F = convex hull (insert a F')))"
```
```  1326          (is "F face_of ?CAS \<longleftrightarrow> _")
```
```  1327 proof%unimportant safe
```
```  1328   assume F: "F face_of ?CAS"
```
```  1329     and *: "\<nexists>F'. F' face_of convex hull S \<and> F = convex hull insert a F'"
```
```  1330   obtain T where T: "T \<subseteq> insert a S" and FeqT: "F = convex hull T"
```
```  1331     by (metis F \<open>finite S\<close> compact_insert finite_imp_compact face_of_convex_hull_subset)
```
```  1332   show "F face_of convex hull S"
```
```  1333   proof (cases "a \<in> T")
```
```  1334     case True
```
```  1335     have "F = convex hull insert a (convex hull T \<inter> convex hull S)"
```
```  1336     proof
```
```  1337       have "T \<subseteq> insert a (convex hull T \<inter> convex hull S)"
```
```  1338         using T hull_subset by fastforce
```
```  1339       then show "F \<subseteq> convex hull insert a (convex hull T \<inter> convex hull S)"
```
```  1340         by (simp add: FeqT hull_mono)
```
```  1341       show "convex hull insert a (convex hull T \<inter> convex hull S) \<subseteq> F"
```
```  1342         apply (rule hull_minimal)
```
```  1343         using True by (auto simp: \<open>F = convex hull T\<close> hull_inc)
```
```  1344     qed
```
```  1345     moreover have "convex hull T \<inter> convex hull S face_of convex hull S"
```
```  1346       by (metis F FeqT convex_convex_hull face_of_slice hull_mono inf.absorb_iff2 subset_insertI)
```
```  1347     ultimately show ?thesis
```
```  1348       using * by force
```
```  1349   next
```
```  1350     case False
```
```  1351     then show ?thesis
```
```  1352       by (metis FeqT F T face_of_subset hull_mono subset_insert subset_insertI)
```
```  1353   qed
```
```  1354 next
```
```  1355   assume "F face_of convex hull S"
```
```  1356   show "F face_of ?CAS"
```
```  1357     by (simp add: \<open>F face_of convex hull S\<close> a face_of_convex_hull_insert \<open>finite S\<close>)
```
```  1358 next
```
```  1359   fix F
```
```  1360   assume F: "F face_of convex hull S"
```
```  1361   show "convex hull insert a F face_of ?CAS"
```
```  1362   proof (cases "S = {}")
```
```  1363     case True
```
```  1364     then show ?thesis
```
```  1365       using F face_of_affine_eq by auto
```
```  1366   next
```
```  1367     case False
```
```  1368     have anotc: "a \<notin> convex hull S"
```
```  1369       by (metis (no_types) a affine_hull_convex_hull hull_inc)
```
```  1370     show ?thesis
```
```  1371     proof (cases "F = {}")
```
```  1372       case True show ?thesis
```
```  1373         using anotc by (simp add: \<open>F = {}\<close> \<open>finite S\<close> extreme_point_of_convex_hull_insert face_of_singleton)
```
```  1374     next
```
```  1375       case False
```
```  1376       have "convex hull insert a F \<subseteq> ?CAS"
```
```  1377         by (simp add: F a \<open>finite S\<close> convex_hull_subset face_of_convex_hull_insert face_of_imp_subset hull_inc)
```
```  1378       moreover
```
```  1379       have "(\<exists>y v. (1 - ub) *\<^sub>R a + ub *\<^sub>R b = (1 - v) *\<^sub>R a + v *\<^sub>R y \<and>
```
```  1380                    0 \<le> v \<and> v \<le> 1 \<and> y \<in> F) \<and>
```
```  1381             (\<exists>x u. (1 - uc) *\<^sub>R a + uc *\<^sub>R c = (1 - u) *\<^sub>R a + u *\<^sub>R x \<and>
```
```  1382                    0 \<le> u \<and> u \<le> 1 \<and> x \<in> F)"
```
```  1383         if *: "(1 - ux) *\<^sub>R a + ux *\<^sub>R x
```
```  1384                \<in> open_segment ((1 - ub) *\<^sub>R a + ub *\<^sub>R b) ((1 - uc) *\<^sub>R a + uc *\<^sub>R c)"
```
```  1385           and "0 \<le> ub" "ub \<le> 1" "0 \<le> uc" "uc \<le> 1" "0 \<le> ux" "ux \<le> 1"
```
```  1386           and b: "b \<in> convex hull S" and c: "c \<in> convex hull S" and "x \<in> F"
```
```  1387         for b c ub uc ux x
```
```  1388       proof -
```
```  1389         obtain v where ne: "(1 - ub) *\<^sub>R a + ub *\<^sub>R b \<noteq> (1 - uc) *\<^sub>R a + uc *\<^sub>R c"
```
```  1390           and eq: "(1 - ux) *\<^sub>R a + ux *\<^sub>R x =
```
```  1391                     (1 - v) *\<^sub>R ((1 - ub) *\<^sub>R a + ub *\<^sub>R b) + v *\<^sub>R ((1 - uc) *\<^sub>R a + uc *\<^sub>R c)"
```
```  1392           and "0 < v" "v < 1"
```
```  1393           using * by (auto simp: in_segment)
```
```  1394         then have 0: "((1 - ux) - ((1 - v) * (1 - ub) + v * (1 - uc))) *\<^sub>R a +
```
```  1395                       (ux *\<^sub>R x - (((1 - v) * ub) *\<^sub>R b + (v * uc) *\<^sub>R c)) = 0"
```
```  1396           by (auto simp: algebra_simps)
```
```  1397         then have "((1 - ux) - ((1 - v) * (1 - ub) + v * (1 - uc))) *\<^sub>R a =
```
```  1398                    ((1 - v) * ub) *\<^sub>R b + (v * uc) *\<^sub>R c + (-ux) *\<^sub>R x"
```
```  1399           by (auto simp: algebra_simps)
```
```  1400         then have "a \<in> affine hull S" if "1 - ux - ((1 - v) * (1 - ub) + v * (1 - uc)) \<noteq> 0"
```
```  1401           apply (rule face_of_convex_hull_aux)
```
```  1402           using b c that apply (auto simp: algebra_simps)
```
```  1403           using F convex_hull_subset_affine_hull face_of_imp_subset \<open>x \<in> F\<close> apply blast+
```
```  1404           done
```
```  1405         then have "1 - ux - ((1 - v) * (1 - ub) + v * (1 - uc)) = 0"
```
```  1406           using a by blast
```
```  1407         with 0 have equx: "(1 - v) * ub + v * uc = ux"
```
```  1408           and uxx: "ux *\<^sub>R x = (((1 - v) * ub) *\<^sub>R b + (v * uc) *\<^sub>R c)"
```
```  1409           by auto (auto simp: algebra_simps)
```
```  1410         show ?thesis
```
```  1411         proof (cases "uc = 0")
```
```  1412           case True
```
```  1413           then show ?thesis
```
```  1414             using equx 0 \<open>0 \<le> ub\<close> \<open>ub \<le> 1\<close> \<open>v < 1\<close> \<open>x \<in> F\<close>
```
```  1415             apply (auto simp: algebra_simps)
```
```  1416              apply (rule_tac x=x in exI, simp)
```
```  1417              apply (rule_tac x=ub in exI, auto)
```
```  1418              apply (metis add.left_neutral diff_eq_eq less_irrefl mult.commute mult_cancel_right1 real_vector.scale_cancel_left real_vector.scale_left_diff_distrib)
```
```  1419             using \<open>x \<in> F\<close> \<open>uc \<le> 1\<close> apply blast
```
```  1420             done
```
```  1421         next
```
```  1422           case False
```
```  1423           show ?thesis
```
```  1424           proof (cases "ub = 0")
```
```  1425             case True
```
```  1426             then show ?thesis
```
```  1427               using equx 0 \<open>0 \<le> uc\<close> \<open>uc \<le> 1\<close> \<open>0 < v\<close> \<open>x \<in> F\<close> \<open>uc \<noteq> 0\<close> by (force simp: algebra_simps)
```
```  1428           next
```
```  1429             case False
```
```  1430             then have "0 < ub" "0 < uc"
```
```  1431               using \<open>uc \<noteq> 0\<close> \<open>0 \<le> ub\<close> \<open>0 \<le> uc\<close> by auto
```
```  1432             then have "ux \<noteq> 0"
```
```  1433               by (metis \<open>0 < v\<close> \<open>v < 1\<close> diff_ge_0_iff_ge dual_order.strict_implies_order equx leD le_add_same_cancel2 zero_le_mult_iff zero_less_mult_iff)
```
```  1434             have "b \<in> F \<and> c \<in> F"
```
```  1435             proof (cases "b = c")
```
```  1436               case True
```
```  1437               then show ?thesis
```
```  1438                 by (metis \<open>ux \<noteq> 0\<close> equx real_vector.scale_cancel_left scaleR_add_left uxx \<open>x \<in> F\<close>)
```
```  1439             next
```
```  1440               case False
```
```  1441               have "x = (((1 - v) * ub) *\<^sub>R b + (v * uc) *\<^sub>R c) /\<^sub>R ux"
```
```  1442                 by (metis \<open>ux \<noteq> 0\<close> uxx mult.commute right_inverse scaleR_one scaleR_scaleR)
```
```  1443               also have "... = (1 - v * uc / ux) *\<^sub>R b + (v * uc / ux) *\<^sub>R c"
```
```  1444                 using \<open>ux \<noteq> 0\<close> equx apply (auto simp: algebra_simps divide_simps)
```
```  1445                 by (metis add.commute add_diff_eq add_divide_distrib diff_add_cancel scaleR_add_left)
```
```  1446               finally have "x = (1 - v * uc / ux) *\<^sub>R b + (v * uc / ux) *\<^sub>R c" .
```
```  1447               then have "x \<in> open_segment b c"
```
```  1448                 apply (simp add: in_segment \<open>b \<noteq> c\<close>)
```
```  1449                 apply (rule_tac x="(v * uc) / ux" in exI)
```
```  1450                 using \<open>0 \<le> ux\<close> \<open>ux \<noteq> 0\<close> \<open>0 < uc\<close> \<open>0 < v\<close> \<open>0 < ub\<close> \<open>v < 1\<close> equx
```
```  1451                 apply (force simp: algebra_simps divide_simps)
```
```  1452                 done
```
```  1453               then show ?thesis
```
```  1454                 by (rule face_ofD [OF F _ b c \<open>x \<in> F\<close>])
```
```  1455             qed
```
```  1456             with \<open>0 \<le> ub\<close> \<open>ub \<le> 1\<close> \<open>0 \<le> uc\<close> \<open>uc \<le> 1\<close> show ?thesis by blast
```
```  1457           qed
```
```  1458         qed
```
```  1459       qed
```
```  1460       moreover have "convex hull F = F"
```
```  1461         by (meson F convex_hull_eq face_of_imp_convex)
```
```  1462       ultimately show ?thesis
```
```  1463         unfolding face_of_def by (fastforce simp: convex_hull_insert_alt \<open>S \<noteq> {}\<close> \<open>F \<noteq> {}\<close>)
```
```  1464     qed
```
```  1465   qed
```
```  1466 qed
```
```  1467
```
```  1468 lemma%unimportant face_of_convex_hull_insert2:
```
```  1469   fixes a :: "'a :: euclidean_space"
```
```  1470   assumes S: "finite S" and a: "a \<notin> affine hull S" and F: "F face_of convex hull S"
```
```  1471   shows "convex hull (insert a F) face_of convex hull (insert a S)"
```
```  1472   by (metis F face_of_convex_hull_insert_eq [OF S a])
```
```  1473
```
```  1474 proposition%important face_of_convex_hull_affine_independent:
```
```  1475   fixes S :: "'a::euclidean_space set"
```
```  1476   assumes "~ affine_dependent S"
```
```  1477     shows "(T face_of (convex hull S) \<longleftrightarrow> (\<exists>c. c \<subseteq> S \<and> T = convex hull c))"
```
```  1478           (is "?lhs = ?rhs")
```
```  1479 proof%unimportant
```
```  1480   assume ?lhs
```
```  1481   then show ?rhs
```
```  1482     by (meson \<open>T face_of convex hull S\<close> aff_independent_finite assms face_of_convex_hull_subset finite_imp_compact)
```
```  1483 next
```
```  1484   assume ?rhs
```
```  1485   then obtain c where "c \<subseteq> S" and T: "T = convex hull c"
```
```  1486     by blast
```
```  1487   have "affine hull c \<inter> affine hull (S - c) = {}"
```
```  1488     apply (rule disjoint_affine_hull [OF assms \<open>c \<subseteq> S\<close>], auto)
```
```  1489     done
```
```  1490   then have "affine hull c \<inter> convex hull (S - c) = {}"
```
```  1491     using convex_hull_subset_affine_hull by fastforce
```
```  1492   then show ?lhs
```
```  1493     by (metis face_of_convex_hulls \<open>c \<subseteq> S\<close> aff_independent_finite assms T)
```
```  1494 qed
```
```  1495
```
```  1496 lemma%unimportant facet_of_convex_hull_affine_independent:
```
```  1497   fixes S :: "'a::euclidean_space set"
```
```  1498   assumes "~ affine_dependent S"
```
```  1499     shows "T facet_of (convex hull S) \<longleftrightarrow>
```
```  1500            T \<noteq> {} \<and> (\<exists>u. u \<in> S \<and> T = convex hull (S - {u}))"
```
```  1501           (is "?lhs = ?rhs")
```
```  1502 proof
```
```  1503   assume ?lhs
```
```  1504   then have "T face_of (convex hull S)" "T \<noteq> {}"
```
```  1505         and afft: "aff_dim T = aff_dim (convex hull S) - 1"
```
```  1506     by (auto simp: facet_of_def)
```
```  1507   then obtain c where "c \<subseteq> S" and c: "T = convex hull c"
```
```  1508     by (auto simp: face_of_convex_hull_affine_independent [OF assms])
```
```  1509   then have affs: "aff_dim S = aff_dim c + 1"
```
```  1510     by (metis aff_dim_convex_hull afft eq_diff_eq)
```
```  1511   have "~ affine_dependent c"
```
```  1512     using \<open>c \<subseteq> S\<close> affine_dependent_subset assms by blast
```
```  1513   with affs have "card (S - c) = 1"
```
```  1514     apply (simp add: aff_dim_affine_independent [symmetric] aff_dim_convex_hull)
```
```  1515     by (metis aff_dim_affine_independent aff_independent_finite One_nat_def \<open>c \<subseteq> S\<close> add.commute
```
```  1516                 add_diff_cancel_right' assms card_Diff_subset card_mono of_nat_1 of_nat_diff of_nat_eq_iff)
```
```  1517   then obtain u where u: "u \<in> S - c"
```
```  1518     by (metis DiffI \<open>c \<subseteq> S\<close> aff_independent_finite assms cancel_comm_monoid_add_class.diff_cancel
```
```  1519                 card_Diff_subset subsetI subset_antisym zero_neq_one)
```
```  1520   then have u: "S = insert u c"
```
```  1521     by (metis Diff_subset \<open>c \<subseteq> S\<close> \<open>card (S - c) = 1\<close> card_1_singletonE double_diff insert_Diff insert_subset singletonD)
```
```  1522   have "T = convex hull (c - {u})"
```
```  1523     by (metis Diff_empty Diff_insert0 \<open>T facet_of convex hull S\<close> c facet_of_irrefl insert_absorb u)
```
```  1524   with \<open>T \<noteq> {}\<close> show ?rhs
```
```  1525     using c u by auto
```
```  1526 next
```
```  1527   assume ?rhs
```
```  1528   then obtain u where "T \<noteq> {}" "u \<in> S" and u: "T = convex hull (S - {u})"
```
```  1529     by (force simp: facet_of_def)
```
```  1530   then have "\<not> S \<subseteq> {u}"
```
```  1531     using \<open>T \<noteq> {}\<close> u by auto
```
```  1532   have [simp]: "aff_dim (convex hull (S - {u})) = aff_dim (convex hull S) - 1"
```
```  1533     using assms \<open>u \<in> S\<close>
```
```  1534     apply (simp add: aff_dim_convex_hull affine_dependent_def)
```
```  1535     apply (drule bspec, assumption)
```
```  1536     by (metis add_diff_cancel_right' aff_dim_insert insert_Diff [of u S])
```
```  1537   show ?lhs
```
```  1538     apply (subst u)
```
```  1539     apply (simp add: \<open>\<not> S \<subseteq> {u}\<close> facet_of_def face_of_convex_hull_affine_independent [OF assms], blast)
```
```  1540     done
```
```  1541 qed
```
```  1542
```
```  1543 lemma%unimportant facet_of_convex_hull_affine_independent_alt:
```
```  1544   fixes S :: "'a::euclidean_space set"
```
```  1545   shows
```
```  1546    "~affine_dependent S
```
```  1547         \<Longrightarrow> (T facet_of (convex hull S) \<longleftrightarrow>
```
```  1548              2 \<le> card S \<and> (\<exists>u. u \<in> S \<and> T = convex hull (S - {u})))"
```
```  1549 apply (simp add: facet_of_convex_hull_affine_independent)
```
```  1550 apply (auto simp: Set.subset_singleton_iff)
```
```  1551 apply (metis Diff_cancel Int_empty_right Int_insert_right_if1  aff_independent_finite card_eq_0_iff card_insert_if card_mono card_subset_eq convex_hull_eq_empty eq_iff equals0D finite_insert finite_subset inf.absorb_iff2 insert_absorb insert_not_empty  not_less_eq_eq numeral_2_eq_2)
```
```  1552 done
```
```  1553
```
```  1554 lemma%unimportant segment_face_of:
```
```  1555   assumes "(closed_segment a b) face_of S"
```
```  1556   shows "a extreme_point_of S" "b extreme_point_of S"
```
```  1557 proof -
```
```  1558   have as: "{a} face_of S"
```
```  1559     by (metis (no_types) assms convex_hull_singleton empty_iff extreme_point_of_convex_hull_insert face_of_face face_of_singleton finite.emptyI finite.insertI insert_absorb insert_iff segment_convex_hull)
```
```  1560   moreover have "{b} face_of S"
```
```  1561   proof -
```
```  1562     have "b \<in> convex hull {a} \<or> b extreme_point_of convex hull {b, a}"
```
```  1563       by (meson extreme_point_of_convex_hull_insert finite.emptyI finite.insertI)
```
```  1564     moreover have "closed_segment a b = convex hull {b, a}"
```
```  1565       using closed_segment_commute segment_convex_hull by blast
```
```  1566     ultimately show ?thesis
```
```  1567       by (metis as assms face_of_face convex_hull_singleton empty_iff face_of_singleton insertE)
```
```  1568     qed
```
```  1569   ultimately show "a extreme_point_of S" "b extreme_point_of S"
```
```  1570     using face_of_singleton by blast+
```
```  1571 qed
```
```  1572
```
```  1573
```
```  1574 lemma%important Krein_Milman_frontier:
```
```  1575   fixes S :: "'a::euclidean_space set"
```
```  1576   assumes "convex S" "compact S"
```
```  1577     shows "S = convex hull (frontier S)"
```
```  1578           (is "?lhs = ?rhs")
```
```  1579 proof%unimportant
```
```  1580   have "?lhs \<subseteq> convex hull {x. x extreme_point_of S}"
```
```  1581     using Krein_Milman_Minkowski assms by blast
```
```  1582   also have "... \<subseteq> ?rhs"
```
```  1583     apply (rule hull_mono)
```
```  1584     apply (auto simp: frontier_def extreme_point_not_in_interior)
```
```  1585     using closure_subset apply (force simp: extreme_point_of_def)
```
```  1586     done
```
```  1587   finally show "?lhs \<subseteq> ?rhs" .
```
```  1588 next
```
```  1589   have "?rhs \<subseteq> convex hull S"
```
```  1590     by (metis Diff_subset \<open>compact S\<close> closure_closed compact_eq_bounded_closed frontier_def hull_mono)
```
```  1591   also have "... \<subseteq> ?lhs"
```
```  1592     by (simp add: \<open>convex S\<close> hull_same)
```
```  1593   finally show "?rhs \<subseteq> ?lhs" .
```
```  1594 qed
```
```  1595
```
```  1596 subsection%important\<open>Polytopes\<close>
```
```  1597
```
```  1598 definition%important polytope where
```
```  1599  "polytope S \<equiv> \<exists>v. finite v \<and> S = convex hull v"
```
```  1600
```
```  1601 lemma%unimportant polytope_translation_eq: "polytope (image (\<lambda>x. a + x) S) \<longleftrightarrow> polytope S"
```
```  1602 apply (simp add: polytope_def, safe)
```
```  1603 apply (metis convex_hull_translation finite_imageI translation_galois)
```
```  1604 by (metis convex_hull_translation finite_imageI)
```
```  1605
```
```  1606 lemma%unimportant polytope_linear_image: "\<lbrakk>linear f; polytope p\<rbrakk> \<Longrightarrow> polytope(image f p)"
```
```  1607   unfolding polytope_def using convex_hull_linear_image by blast
```
```  1608
```
```  1609 lemma%unimportant polytope_empty: "polytope {}"
```
```  1610   using convex_hull_empty polytope_def by blast
```
```  1611
```
```  1612 lemma%unimportant polytope_convex_hull: "finite S \<Longrightarrow> polytope(convex hull S)"
```
```  1613   using polytope_def by auto
```
```  1614
```
```  1615 lemma%unimportant polytope_Times: "\<lbrakk>polytope S; polytope T\<rbrakk> \<Longrightarrow> polytope(S \<times> T)"
```
```  1616   unfolding polytope_def
```
```  1617   by (metis finite_cartesian_product convex_hull_Times)
```
```  1618
```
```  1619 lemma%unimportant face_of_polytope_polytope:
```
```  1620   fixes S :: "'a::euclidean_space set"
```
```  1621   shows "\<lbrakk>polytope S; F face_of S\<rbrakk> \<Longrightarrow> polytope F"
```
```  1622 unfolding polytope_def
```
```  1623 by (meson face_of_convex_hull_subset finite_imp_compact finite_subset)
```
```  1624
```
```  1625 lemma%unimportant finite_polytope_faces:
```
```  1626   fixes S :: "'a::euclidean_space set"
```
```  1627   assumes "polytope S"
```
```  1628   shows "finite {F. F face_of S}"
```
```  1629 proof -
```
```  1630   obtain v where "finite v" "S = convex hull v"
```
```  1631     using assms polytope_def by auto
```
```  1632   have "finite ((hull) convex ` {T. T \<subseteq> v})"
```
```  1633     by (simp add: \<open>finite v\<close>)
```
```  1634   moreover have "{F. F face_of S} \<subseteq> ((hull) convex ` {T. T \<subseteq> v})"
```
```  1635     by (metis (no_types, lifting) \<open>finite v\<close> \<open>S = convex hull v\<close> face_of_convex_hull_subset finite_imp_compact image_eqI mem_Collect_eq subsetI)
```
```  1636   ultimately show ?thesis
```
```  1637     by (blast intro: finite_subset)
```
```  1638 qed
```
```  1639
```
```  1640 lemma%unimportant finite_polytope_facets:
```
```  1641   assumes "polytope S"
```
```  1642   shows "finite {T. T facet_of S}"
```
```  1643 by (simp add: assms facet_of_def finite_polytope_faces)
```
```  1644
```
```  1645 lemma%unimportant polytope_scaling:
```
```  1646   assumes "polytope S"  shows "polytope (image (\<lambda>x. c *\<^sub>R x) S)"
```
```  1647 by (simp add: assms polytope_linear_image)
```
```  1648
```
```  1649 lemma%unimportant polytope_imp_compact:
```
```  1650   fixes S :: "'a::real_normed_vector set"
```
```  1651   shows "polytope S \<Longrightarrow> compact S"
```
```  1652 by (metis finite_imp_compact_convex_hull polytope_def)
```
```  1653
```
```  1654 lemma%unimportant polytope_imp_convex: "polytope S \<Longrightarrow> convex S"
```
```  1655   by (metis convex_convex_hull polytope_def)
```
```  1656
```
```  1657 lemma%unimportant polytope_imp_closed:
```
```  1658   fixes S :: "'a::real_normed_vector set"
```
```  1659   shows "polytope S \<Longrightarrow> closed S"
```
```  1660 by (simp add: compact_imp_closed polytope_imp_compact)
```
```  1661
```
```  1662 lemma%unimportant polytope_imp_bounded:
```
```  1663   fixes S :: "'a::real_normed_vector set"
```
```  1664   shows "polytope S \<Longrightarrow> bounded S"
```
```  1665 by (simp add: compact_imp_bounded polytope_imp_compact)
```
```  1666
```
```  1667 lemma%unimportant polytope_interval: "polytope(cbox a b)"
```
```  1668   unfolding polytope_def by (meson closed_interval_as_convex_hull)
```
```  1669
```
```  1670 lemma%unimportant polytope_sing: "polytope {a}"
```
```  1671   using polytope_def by force
```
```  1672
```
```  1673 lemma%unimportant face_of_polytope_insert:
```
```  1674      "\<lbrakk>polytope S; a \<notin> affine hull S; F face_of S\<rbrakk> \<Longrightarrow> F face_of convex hull (insert a S)"
```
```  1675   by (metis (no_types, lifting) affine_hull_convex_hull face_of_convex_hull_insert hull_insert polytope_def)
```
```  1676
```
```  1677 lemma%important face_of_polytope_insert2:
```
```  1678   fixes a :: "'a :: euclidean_space"
```
```  1679   assumes "polytope S" "a \<notin> affine hull S" "F face_of S"
```
```  1680   shows "convex hull (insert a F) face_of convex hull (insert a S)"
```
```  1681 proof%unimportant -
```
```  1682   obtain V where "finite V" "S = convex hull V"
```
```  1683     using assms by (auto simp: polytope_def)
```
```  1684   then have "convex hull (insert a F) face_of convex hull (insert a V)"
```
```  1685     using affine_hull_convex_hull assms face_of_convex_hull_insert2 by blast
```
```  1686   then show ?thesis
```
```  1687     by (metis \<open>S = convex hull V\<close> hull_insert)
```
```  1688 qed
```
```  1689
```
```  1690
```
```  1691 subsection%important\<open>Polyhedra\<close>
```
```  1692
```
```  1693 definition%important polyhedron where
```
```  1694  "polyhedron S \<equiv>
```
```  1695         \<exists>F. finite F \<and>
```
```  1696             S = \<Inter> F \<and>
```
```  1697             (\<forall>h \<in> F. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b})"
```
```  1698
```
```  1699 lemma%unimportant polyhedron_Int [intro,simp]:
```
```  1700    "\<lbrakk>polyhedron S; polyhedron T\<rbrakk> \<Longrightarrow> polyhedron (S \<inter> T)"
```
```  1701   apply (simp add: polyhedron_def, clarify)
```
```  1702   apply (rename_tac F G)
```
```  1703   apply (rule_tac x="F \<union> G" in exI, auto)
```
```  1704   done
```
```  1705
```
```  1706 lemma%unimportant polyhedron_UNIV [iff]: "polyhedron UNIV"
```
```  1707   unfolding polyhedron_def
```
```  1708   by (rule_tac x="{}" in exI) auto
```
```  1709
```
```  1710 lemma%unimportant polyhedron_Inter [intro,simp]:
```
```  1711    "\<lbrakk>finite F; \<And>S. S \<in> F \<Longrightarrow> polyhedron S\<rbrakk> \<Longrightarrow> polyhedron(\<Inter>F)"
```
```  1712 by (induction F rule: finite_induct) auto
```
```  1713
```
```  1714
```
```  1715 lemma%unimportant polyhedron_empty [iff]: "polyhedron ({} :: 'a :: euclidean_space set)"
```
```  1716 proof -
```
```  1717   have "\<exists>a. a \<noteq> 0 \<and>
```
```  1718              (\<exists>b. {x. (SOME i. i \<in> Basis) \<bullet> x \<le> - 1} = {x. a \<bullet> x \<le> b})"
```
```  1719     by (rule_tac x="(SOME i. i \<in> Basis)" in exI) (force simp: SOME_Basis nonzero_Basis)
```
```  1720   moreover have "\<exists>a b. a \<noteq> 0 \<and>
```
```  1721                        {x. - (SOME i. i \<in> Basis) \<bullet> x \<le> - 1} = {x. a \<bullet> x \<le> b}"
```
```  1722       apply (rule_tac x="-(SOME i. i \<in> Basis)" in exI)
```
```  1723       apply (rule_tac x="-1" in exI)
```
```  1724       apply (simp add: SOME_Basis nonzero_Basis)
```
```  1725       done
```
```  1726   ultimately show ?thesis
```
```  1727     unfolding polyhedron_def
```
```  1728     apply (rule_tac x="{{x. (SOME i. i \<in> Basis) \<bullet> x \<le> -1},
```
```  1729                         {x. -(SOME i. i \<in> Basis) \<bullet> x \<le> -1}}" in exI)
```
```  1730     apply force
```
```  1731     done
```
```  1732 qed
```
```  1733
```
```  1734 lemma%unimportant polyhedron_halfspace_le:
```
```  1735   fixes a :: "'a :: euclidean_space"
```
```  1736   shows "polyhedron {x. a \<bullet> x \<le> b}"
```
```  1737 proof (cases "a = 0")
```
```  1738   case True then show ?thesis by auto
```
```  1739 next
```
```  1740   case False
```
```  1741   then show ?thesis
```
```  1742     unfolding polyhedron_def
```
```  1743     by (rule_tac x="{{x. a \<bullet> x \<le> b}}" in exI) auto
```
```  1744 qed
```
```  1745
```
```  1746 lemma%unimportant polyhedron_halfspace_ge:
```
```  1747   fixes a :: "'a :: euclidean_space"
```
```  1748   shows "polyhedron {x. a \<bullet> x \<ge> b}"
```
```  1749 using polyhedron_halfspace_le [of "-a" "-b"] by simp
```
```  1750
```
```  1751 lemma%important polyhedron_hyperplane:
```
```  1752   fixes a :: "'a :: euclidean_space"
```
```  1753   shows "polyhedron {x. a \<bullet> x = b}"
```
```  1754 proof%unimportant -
```
```  1755   have "{x. a \<bullet> x = b} = {x. a \<bullet> x \<le> b} \<inter> {x. a \<bullet> x \<ge> b}"
```
```  1756     by force
```
```  1757   then show ?thesis
```
```  1758     by (simp add: polyhedron_halfspace_ge polyhedron_halfspace_le)
```
```  1759 qed
```
```  1760
```
```  1761 lemma%unimportant affine_imp_polyhedron:
```
```  1762   fixes S :: "'a :: euclidean_space set"
```
```  1763   shows "affine S \<Longrightarrow> polyhedron S"
```
```  1764 by (metis affine_hull_eq polyhedron_Inter polyhedron_hyperplane affine_hull_finite_intersection_hyperplanes [of S])
```
```  1765
```
```  1766 lemma%unimportant polyhedron_imp_closed:
```
```  1767   fixes S :: "'a :: euclidean_space set"
```
```  1768   shows "polyhedron S \<Longrightarrow> closed S"
```
```  1769 apply (simp add: polyhedron_def)
```
```  1770 using closed_halfspace_le by fastforce
```
```  1771
```
```  1772 lemma%unimportant polyhedron_imp_convex:
```
```  1773   fixes S :: "'a :: euclidean_space set"
```
```  1774   shows "polyhedron S \<Longrightarrow> convex S"
```
```  1775 apply (simp add: polyhedron_def)
```
```  1776 using convex_Inter convex_halfspace_le by fastforce
```
```  1777
```
```  1778 lemma%unimportant polyhedron_affine_hull:
```
```  1779   fixes S :: "'a :: euclidean_space set"
```
```  1780   shows "polyhedron(affine hull S)"
```
```  1781 by (simp add: affine_imp_polyhedron)
```
```  1782
```
```  1783
```
```  1784 subsection%important\<open>Canonical polyhedron representation making facial structure explicit\<close>
```
```  1785
```
```  1786 lemma%important polyhedron_Int_affine:
```
```  1787   fixes S :: "'a :: euclidean_space set"
```
```  1788   shows "polyhedron S \<longleftrightarrow>
```
```  1789            (\<exists>F. finite F \<and> S = (affine hull S) \<inter> \<Inter>F \<and>
```
```  1790                 (\<forall>h \<in> F. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}))"
```
```  1791         (is "?lhs = ?rhs")
```
```  1792 proof%unimportant
```
```  1793   assume ?lhs then show ?rhs
```
```  1794     apply (simp add: polyhedron_def)
```
```  1795     apply (erule ex_forward)
```
```  1796     using hull_subset apply force
```
```  1797     done
```
```  1798 next
```
```  1799   assume ?rhs then show ?lhs
```
```  1800     apply clarify
```
```  1801     apply (erule ssubst)
```
```  1802     apply (force intro: polyhedron_affine_hull polyhedron_halfspace_le)
```
```  1803     done
```
```  1804 qed
```
```  1805
```
```  1806 proposition%important rel_interior_polyhedron_explicit:
```
```  1807   assumes "finite F"
```
```  1808       and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  1809       and faceq: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  1810       and psub: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> affine hull S \<inter> \<Inter>F'"
```
```  1811     shows "rel_interior S = {x \<in> S. \<forall>h \<in> F. a h \<bullet> x < b h}"
```
```  1812 proof%unimportant -
```
```  1813   have rels: "\<And>x. x \<in> rel_interior S \<Longrightarrow> x \<in> S"
```
```  1814     by (meson IntE mem_rel_interior)
```
```  1815   moreover have "a i \<bullet> x < b i" if x: "x \<in> rel_interior S" and "i \<in> F" for x i
```
```  1816   proof -
```
```  1817     have fif: "F - {i} \<subset> F"
```
```  1818       using \<open>i \<in> F\<close> Diff_insert_absorb Diff_subset set_insert psubsetI by blast
```
```  1819     then have "S \<subset> affine hull S \<inter> \<Inter>(F - {i})"
```
```  1820       by (rule psub)
```
```  1821     then obtain z where ssub: "S \<subseteq> \<Inter>(F - {i})" and zint: "z \<in> \<Inter>(F - {i})"
```
```  1822                     and "z \<notin> S" and zaff: "z \<in> affine hull S"
```
```  1823       by auto
```
```  1824     have "z \<noteq> x"
```
```  1825       using \<open>z \<notin> S\<close> rels x by blast
```
```  1826     have "z \<notin> affine hull S \<inter> \<Inter>F"
```
```  1827       using \<open>z \<notin> S\<close> seq by auto
```
```  1828     then have aiz: "a i \<bullet> z > b i"
```
```  1829       using faceq zint zaff by fastforce
```
```  1830     obtain e where "e > 0" "x \<in> S" and e: "ball x e \<inter> affine hull S \<subseteq> S"
```
```  1831       using x by (auto simp: mem_rel_interior_ball)
```
```  1832     then have ins: "\<And>y. \<lbrakk>norm (x - y) < e; y \<in> affine hull S\<rbrakk> \<Longrightarrow> y \<in> S"
```
```  1833       by (metis IntI subsetD dist_norm mem_ball)
```
```  1834     define \<xi> where "\<xi> = min (1/2) (e / 2 / norm(z - x))"
```
```  1835     have "norm (\<xi> *\<^sub>R x - \<xi> *\<^sub>R z) = norm (\<xi> *\<^sub>R (x - z))"
```
```  1836       by (simp add: \<xi>_def algebra_simps norm_mult)
```
```  1837     also have "... = \<xi> * norm (x - z)"
```
```  1838       using \<open>e > 0\<close> by (simp add: \<xi>_def)
```
```  1839     also have "... < e"
```
```  1840       using \<open>z \<noteq> x\<close> \<open>e > 0\<close> by (simp add: \<xi>_def min_def divide_simps norm_minus_commute)
```
```  1841     finally have lte: "norm (\<xi> *\<^sub>R x - \<xi> *\<^sub>R z) < e" .
```
```  1842     have \<xi>_aff: "\<xi> *\<^sub>R z + (1 - \<xi>) *\<^sub>R x \<in> affine hull S"
```
```  1843       by (metis \<open>x \<in> S\<close> add.commute affine_affine_hull diff_add_cancel hull_inc mem_affine zaff)
```
```  1844     have "\<xi> *\<^sub>R z + (1 - \<xi>) *\<^sub>R x \<in> S"
```
```  1845       apply (rule ins [OF _ \<xi>_aff])
```
```  1846       apply (simp add: algebra_simps lte)
```
```  1847       done
```
```  1848     then obtain l where l: "0 < l" "l < 1" and ls: "(l *\<^sub>R z + (1 - l) *\<^sub>R x) \<in> S"
```
```  1849       apply (rule_tac l = \<xi> in that)
```
```  1850       using \<open>e > 0\<close> \<open>z \<noteq> x\<close>  apply (auto simp: \<xi>_def)
```
```  1851       done
```
```  1852     then have i: "l *\<^sub>R z + (1 - l) *\<^sub>R x \<in> i"
```
```  1853       using seq \<open>i \<in> F\<close> by auto
```
```  1854     have "b i * l + (a i \<bullet> x) * (1 - l) < a i \<bullet> (l *\<^sub>R z + (1 - l) *\<^sub>R x)"
```
```  1855       using l by (simp add: algebra_simps aiz)
```
```  1856     also have "\<dots> \<le> b i" using i l
```
```  1857       using faceq mem_Collect_eq \<open>i \<in> F\<close> by blast
```
```  1858     finally have "(a i \<bullet> x) * (1 - l) < b i * (1 - l)"
```
```  1859       by (simp add: algebra_simps)
```
```  1860     with l show ?thesis
```
```  1861       by simp
```
```  1862   qed
```
```  1863   moreover have "x \<in> rel_interior S"
```
```  1864            if "x \<in> S" and less: "\<And>h. h \<in> F \<Longrightarrow> a h \<bullet> x < b h" for x
```
```  1865   proof -
```
```  1866     have 1: "\<And>h. h \<in> F \<Longrightarrow> x \<in> interior h"
```
```  1867       by (metis interior_halfspace_le mem_Collect_eq less faceq)
```
```  1868     have 2: "\<And>y. \<lbrakk>\<forall>h\<in>F. y \<in> interior h; y \<in> affine hull S\<rbrakk> \<Longrightarrow> y \<in> S"
```
```  1869       by (metis IntI Inter_iff contra_subsetD interior_subset seq)
```
```  1870     show ?thesis
```
```  1871       apply (simp add: rel_interior \<open>x \<in> S\<close>)
```
```  1872       apply (rule_tac x="\<Inter>h\<in>F. interior h" in exI)
```
```  1873       apply (auto simp: \<open>finite F\<close> open_INT 1 2)
```
```  1874       done
```
```  1875   qed
```
```  1876   ultimately show ?thesis by blast
```
```  1877 qed
```
```  1878
```
```  1879
```
```  1880 lemma%important polyhedron_Int_affine_parallel:
```
```  1881   fixes S :: "'a :: euclidean_space set"
```
```  1882   shows "polyhedron S \<longleftrightarrow>
```
```  1883          (\<exists>F. finite F \<and>
```
```  1884               S = (affine hull S) \<inter> (\<Inter>F) \<and>
```
```  1885               (\<forall>h \<in> F. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b} \<and>
```
```  1886                              (\<forall>x \<in> affine hull S. (x + a) \<in> affine hull S)))"
```
```  1887     (is "?lhs = ?rhs")
```
```  1888 proof%unimportant
```
```  1889   assume ?lhs
```
```  1890   then obtain F where "finite F" and seq: "S = (affine hull S) \<inter> \<Inter>F"
```
```  1891                   and faces: "\<And>h. h \<in> F \<Longrightarrow> \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}"
```
```  1892     by (fastforce simp add: polyhedron_Int_affine)
```
```  1893   then obtain a b where ab: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  1894     by metis
```
```  1895   show ?rhs
```
```  1896   proof -
```
```  1897     have "\<exists>a' b'. a' \<noteq> 0 \<and>
```
```  1898                   affine hull S \<inter> {x. a' \<bullet> x \<le> b'} = affine hull S \<inter> h \<and>
```
```  1899                   (\<forall>w \<in> affine hull S. (w + a') \<in> affine hull S)"
```
```  1900         if "h \<in> F" "~(affine hull S \<subseteq> h)" for h
```
```  1901     proof -
```
```  1902       have "a h \<noteq> 0" and "h = {x. a h \<bullet> x \<le> b h}" "h \<inter> \<Inter>F = \<Inter>F"
```
```  1903         using \<open>h \<in> F\<close> ab by auto
```
```  1904       then have "(affine hull S) \<inter> {x. a h \<bullet> x \<le> b h} \<noteq> {}"
```
```  1905         by (metis (no_types) affine_hull_eq_empty inf.absorb_iff2 inf_assoc inf_bot_right inf_commute seq that(2))
```
```  1906       moreover have "~ (affine hull S \<subseteq> {x. a h \<bullet> x \<le> b h})"
```
```  1907         using \<open>h = {x. a h \<bullet> x \<le> b h}\<close> that(2) by blast
```
```  1908       ultimately show ?thesis
```
```  1909         using affine_parallel_slice [of "affine hull S"]
```
```  1910         by (metis \<open>h = {x. a h \<bullet> x \<le> b h}\<close> affine_affine_hull)
```
```  1911     qed
```
```  1912     then obtain a b
```
```  1913          where ab: "\<And>h. \<lbrakk>h \<in> F; ~ (affine hull S \<subseteq> h)\<rbrakk>
```
```  1914              \<Longrightarrow> a h \<noteq> 0 \<and>
```
```  1915                   affine hull S \<inter> {x. a h \<bullet> x \<le> b h} = affine hull S \<inter> h \<and>
```
```  1916                   (\<forall>w \<in> affine hull S. (w + a h) \<in> affine hull S)"
```
```  1917       by metis
```
```  1918     have seq2: "S = affine hull S \<inter> (\<Inter>h\<in>{h \<in> F. \<not> affine hull S \<subseteq> h}. {x. a h \<bullet> x \<le> b h})"
```
```  1919       by (subst seq) (auto simp: ab INT_extend_simps)
```
```  1920     show ?thesis
```
```  1921       apply (rule_tac x="(\<lambda>h. {x. a h \<bullet> x \<le> b h}) ` {h. h \<in> F \<and> ~(affine hull S \<subseteq> h)}" in exI)
```
```  1922       apply (intro conjI seq2)
```
```  1923         using \<open>finite F\<close> apply force
```
```  1924        using ab apply blast
```
```  1925        done
```
```  1926   qed
```
```  1927 next
```
```  1928   assume ?rhs then show ?lhs
```
```  1929     apply (simp add: polyhedron_Int_affine)
```
```  1930     by metis
```
```  1931 qed
```
```  1932
```
```  1933
```
```  1934 proposition%important polyhedron_Int_affine_parallel_minimal:
```
```  1935   fixes S :: "'a :: euclidean_space set"
```
```  1936   shows "polyhedron S \<longleftrightarrow>
```
```  1937          (\<exists>F. finite F \<and>
```
```  1938               S = (affine hull S) \<inter> (\<Inter>F) \<and>
```
```  1939               (\<forall>h \<in> F. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b} \<and>
```
```  1940                              (\<forall>x \<in> affine hull S. (x + a) \<in> affine hull S)) \<and>
```
```  1941               (\<forall>F'. F' \<subset> F \<longrightarrow> S \<subset> (affine hull S) \<inter> (\<Inter>F')))"
```
```  1942     (is "?lhs = ?rhs")
```
```  1943 proof%unimportant
```
```  1944   assume ?lhs
```
```  1945   then obtain f0
```
```  1946            where f0: "finite f0"
```
```  1947                  "S = (affine hull S) \<inter> (\<Inter>f0)"
```
```  1948                    (is "?P f0")
```
```  1949                  "\<forall>h \<in> f0. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b} \<and>
```
```  1950                              (\<forall>x \<in> affine hull S. (x + a) \<in> affine hull S)"
```
```  1951                    (is "?Q f0")
```
```  1952     by (force simp: polyhedron_Int_affine_parallel)
```
```  1953   define n where "n = (LEAST n. \<exists>F. card F = n \<and> finite F \<and> ?P F \<and> ?Q F)"
```
```  1954   have nf: "\<exists>F. card F = n \<and> finite F \<and> ?P F \<and> ?Q F"
```
```  1955     apply (simp add: n_def)
```
```  1956     apply (rule LeastI [where k = "card f0"])
```
```  1957     using f0 apply auto
```
```  1958     done
```
```  1959   then obtain F where F: "card F = n" "finite F" and seq: "?P F" and aff: "?Q F"
```
```  1960     by blast
```
```  1961   then have "~ (finite g \<and> ?P g \<and> ?Q g)" if "card g < n" for g
```
```  1962     using that by (auto simp: n_def dest!: not_less_Least)
```
```  1963   then have *: "~ (?P g \<and> ?Q g)" if "g \<subset> F" for g
```
```  1964     using that \<open>finite F\<close> psubset_card_mono \<open>card F = n\<close>
```
```  1965     by (metis finite_Int inf.strict_order_iff)
```
```  1966   have 1: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subseteq> affine hull S \<inter> \<Inter>F'"
```
```  1967     by (subst seq) blast
```
```  1968   have 2: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<noteq> affine hull S \<inter> \<Inter>F'"
```
```  1969     apply (frule *)
```
```  1970     by (metis aff subsetCE subset_iff_psubset_eq)
```
```  1971   show ?rhs
```
```  1972     by (metis \<open>finite F\<close> seq aff psubsetI 1 2)
```
```  1973 next
```
```  1974   assume ?rhs then show ?lhs
```
```  1975     by (auto simp: polyhedron_Int_affine_parallel)
```
```  1976 qed
```
```  1977
```
```  1978
```
```  1979 lemma%unimportant polyhedron_Int_affine_minimal:
```
```  1980   fixes S :: "'a :: euclidean_space set"
```
```  1981   shows "polyhedron S \<longleftrightarrow>
```
```  1982          (\<exists>F. finite F \<and> S = (affine hull S) \<inter> \<Inter>F \<and>
```
```  1983               (\<forall>h \<in> F. \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}) \<and>
```
```  1984               (\<forall>F'. F' \<subset> F \<longrightarrow> S \<subset> (affine hull S) \<inter> \<Inter>F'))"
```
```  1985 apply (rule iffI)
```
```  1986  apply (force simp: polyhedron_Int_affine_parallel_minimal elim!: ex_forward)
```
```  1987 apply (auto simp: polyhedron_Int_affine elim!: ex_forward)
```
```  1988 done
```
```  1989
```
```  1990 proposition%important facet_of_polyhedron_explicit:
```
```  1991   assumes "finite F"
```
```  1992       and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  1993       and faceq: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  1994       and psub: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> affine hull S \<inter> \<Inter>F'"
```
```  1995     shows "c facet_of S \<longleftrightarrow> (\<exists>h. h \<in> F \<and> c = S \<inter> {x. a h \<bullet> x = b h})"
```
```  1996 proof%unimportant (cases "S = {}")
```
```  1997   case True with psub show ?thesis by force
```
```  1998 next
```
```  1999   case False
```
```  2000   have "polyhedron S"
```
```  2001     apply (simp add: polyhedron_Int_affine)
```
```  2002     apply (rule_tac x=F in exI)
```
```  2003     using assms  apply force
```
```  2004     done
```
```  2005   then have "convex S"
```
```  2006     by (rule polyhedron_imp_convex)
```
```  2007   with False rel_interior_eq_empty have "rel_interior S \<noteq> {}" by blast
```
```  2008   then obtain x where "x \<in> rel_interior S" by auto
```
```  2009   then obtain T where "open T" "x \<in> T" "x \<in> S" "T \<inter> affine hull S \<subseteq> S"
```
```  2010     by (force simp: mem_rel_interior)
```
```  2011   then have xaff: "x \<in> affine hull S" and xint: "x \<in> \<Inter>F"
```
```  2012     using seq hull_inc by auto
```
```  2013   have "rel_interior S = {x \<in> S. \<forall>h\<in>F. a h \<bullet> x < b h}"
```
```  2014     by (rule rel_interior_polyhedron_explicit [OF \<open>finite F\<close> seq faceq psub])
```
```  2015   with \<open>x \<in> rel_interior S\<close>
```
```  2016   have [simp]: "\<And>h. h\<in>F \<Longrightarrow> a h \<bullet> x < b h" by blast
```
```  2017   have *: "(S \<inter> {x. a h \<bullet> x = b h}) facet_of S" if "h \<in> F" for h
```
```  2018   proof -
```
```  2019     have "S \<subset> affine hull S \<inter> \<Inter>(F - {h})"
```
```  2020       using psub that by (metis Diff_disjoint Diff_subset insert_disjoint(2) psubsetI)
```
```  2021     then obtain z where zaff: "z \<in> affine hull S" and zint: "z \<in> \<Inter>(F - {h})" and "z \<notin> S"
```
```  2022       by force
```
```  2023     then have "z \<noteq> x" "z \<notin> h" using seq \<open>x \<in> S\<close> by auto
```
```  2024     have "x \<in> h" using that xint by auto
```
```  2025     then have able: "a h \<bullet> x \<le> b h"
```
```  2026       using faceq that by blast
```
```  2027     also have "... < a h \<bullet> z" using \<open>z \<notin> h\<close> faceq [OF that] xint by auto
```
```  2028     finally have xltz: "a h \<bullet> x < a h \<bullet> z" .
```
```  2029     define l where "l = (b h - a h \<bullet> x) / (a h \<bullet> z - a h \<bullet> x)"
```
```  2030     define w where "w = (1 - l) *\<^sub>R x + l *\<^sub>R z"
```
```  2031     have "0 < l" "l < 1"
```
```  2032       using able xltz \<open>b h < a h \<bullet> z\<close> \<open>h \<in> F\<close>
```
```  2033       by (auto simp: l_def divide_simps)
```
```  2034     have awlt: "a i \<bullet> w < b i" if "i \<in> F" "i \<noteq> h" for i
```
```  2035     proof -
```
```  2036       have "(1 - l) * (a i \<bullet> x) < (1 - l) * b i"
```
```  2037         by (simp add: \<open>l < 1\<close> \<open>i \<in> F\<close>)
```
```  2038       moreover have "l * (a i \<bullet> z) \<le> l * b i"
```
```  2039         apply (rule mult_left_mono)
```
```  2040         apply (metis Diff_insert_absorb Inter_iff Set.set_insert \<open>h \<in> F\<close> faceq insertE mem_Collect_eq that zint)
```
```  2041         using \<open>0 < l\<close>
```
```  2042         apply simp
```
```  2043         done
```
```  2044       ultimately show ?thesis by (simp add: w_def algebra_simps)
```
```  2045     qed
```
```  2046     have weq: "a h \<bullet> w = b h"
```
```  2047       using xltz unfolding w_def l_def
```
```  2048       by (simp add: algebra_simps) (simp add: field_simps)
```
```  2049     have "w \<in> affine hull S"
```
```  2050       by (simp add: w_def mem_affine xaff zaff)
```
```  2051     moreover have "w \<in> \<Inter>F"
```
```  2052       using \<open>a h \<bullet> w = b h\<close> awlt faceq less_eq_real_def by blast
```
```  2053     ultimately have "w \<in> S"
```
```  2054       using seq by blast
```
```  2055     with weq have "S \<inter> {x. a h \<bullet> x = b h} \<noteq> {}" by blast
```
```  2056     moreover have "S \<inter> {x. a h \<bullet> x = b h} face_of S"
```
```  2057       apply (rule face_of_Int_supporting_hyperplane_le)
```
```  2058       apply (rule \<open>convex S\<close>)
```
```  2059       apply (subst (asm) seq)
```
```  2060       using faceq that apply fastforce
```
```  2061       done
```
```  2062     moreover have "affine hull (S \<inter> {x. a h \<bullet> x = b h}) =
```
```  2063                    (affine hull S) \<inter> {x. a h \<bullet> x = b h}"
```
```  2064     proof
```
```  2065       show "affine hull (S \<inter> {x. a h \<bullet> x = b h}) \<subseteq> affine hull S \<inter> {x. a h \<bullet> x = b h}"
```
```  2066         apply (intro Int_greatest hull_mono Int_lower1)
```
```  2067         apply (metis affine_hull_eq affine_hyperplane hull_mono inf_le2)
```
```  2068         done
```
```  2069     next
```
```  2070       show "affine hull S \<inter> {x. a h \<bullet> x = b h} \<subseteq> affine hull (S \<inter> {x. a h \<bullet> x = b h})"
```
```  2071       proof
```
```  2072         fix y
```
```  2073         assume yaff: "y \<in> affine hull S \<inter> {y. a h \<bullet> y = b h}"
```
```  2074         obtain T where "0 < T"
```
```  2075                  and T: "\<And>j. \<lbrakk>j \<in> F; j \<noteq> h\<rbrakk> \<Longrightarrow> T * (a j \<bullet> y - a j \<bullet> w) \<le> b j - a j \<bullet> w"
```
```  2076         proof (cases "F - {h} = {}")
```
```  2077           case True then show ?thesis
```
```  2078             by (rule_tac T=1 in that) auto
```
```  2079         next
```
```  2080           case False
```
```  2081           then obtain h' where h': "h' \<in> F - {h}" by auto
```
```  2082           define inff where "inff =
```
```  2083             (INF j\<in>F - {h}.
```
```  2084               if 0 < a j \<bullet> y - a j \<bullet> w
```
```  2085               then (b j - a j \<bullet> w) / (a j \<bullet> y - a j \<bullet> w)
```
```  2086               else 1)"
```
```  2087           have "0 < inff"
```
```  2088             apply (simp add: inff_def)
```
```  2089             apply (rule finite_imp_less_Inf)
```
```  2090               using \<open>finite F\<close> apply blast
```
```  2091              using h' apply blast
```
```  2092             apply simp
```
```  2093             using awlt apply (force simp: divide_simps)
```
```  2094             done
```
```  2095           moreover have "inff * (a j \<bullet> y - a j \<bullet> w) \<le> b j - a j \<bullet> w"
```
```  2096                         if "j \<in> F" "j \<noteq> h" for j
```
```  2097           proof (cases "a j \<bullet> w < a j \<bullet> y")
```
```  2098             case True
```
```  2099             then have "inff \<le> (b j - a j \<bullet> w) / (a j \<bullet> y - a j \<bullet> w)"
```
```  2100               apply (simp add: inff_def)
```
```  2101               apply (rule cInf_le_finite)
```
```  2102               using \<open>finite F\<close> apply blast
```
```  2103               apply (simp add: that split: if_split_asm)
```
```  2104               done
```
```  2105             then show ?thesis
```
```  2106               using \<open>0 < inff\<close> awlt [OF that] mult_strict_left_mono
```
```  2107               by (fastforce simp add: algebra_simps divide_simps split: if_split_asm)
```
```  2108           next
```
```  2109             case False
```
```  2110             with \<open>0 < inff\<close> have "inff * (a j \<bullet> y - a j \<bullet> w) \<le> 0"
```
```  2111               by (simp add: mult_le_0_iff)
```
```  2112             also have "... < b j - a j \<bullet> w"
```
```  2113               by (simp add: awlt that)
```
```  2114             finally show ?thesis by simp
```
```  2115           qed
```
```  2116           ultimately show ?thesis
```
```  2117             by (blast intro: that)
```
```  2118         qed
```
```  2119         define c where "c = (1 - T) *\<^sub>R w + T *\<^sub>R y"
```
```  2120         have "(1 - T) *\<^sub>R w + T *\<^sub>R y \<in> j" if "j \<in> F" for j
```
```  2121         proof (cases "j = h")
```
```  2122           case True
```
```  2123           have "(1 - T) *\<^sub>R w + T *\<^sub>R y \<in> {x. a h \<bullet> x \<le> b h}"
```
```  2124             using weq yaff by (auto simp: algebra_simps)
```
```  2125           with True faceq [OF that] show ?thesis by metis
```
```  2126         next
```
```  2127           case False
```
```  2128           with T that have "(1 - T) *\<^sub>R w + T *\<^sub>R y \<in> {x. a j \<bullet> x \<le> b j}"
```
```  2129             by (simp add: algebra_simps)
```
```  2130           with faceq [OF that] show ?thesis by simp
```
```  2131         qed
```
```  2132         moreover have "(1 - T) *\<^sub>R w + T *\<^sub>R y \<in> affine hull S"
```
```  2133           apply (rule affine_affine_hull [simplified affine_alt, rule_format])
```
```  2134           apply (simp add: \<open>w \<in> affine hull S\<close>)
```
```  2135           using yaff apply blast
```
```  2136           done
```
```  2137         ultimately have "c \<in> S"
```
```  2138           using seq by (force simp: c_def)
```
```  2139         moreover have "a h \<bullet> c = b h"
```
```  2140           using yaff by (force simp: c_def algebra_simps weq)
```
```  2141         ultimately have caff: "c \<in> affine hull (S \<inter> {y. a h \<bullet> y = b h})"
```
```  2142           by (simp add: hull_inc)
```
```  2143         have waff: "w \<in> affine hull (S \<inter> {y. a h \<bullet> y = b h})"
```
```  2144           using \<open>w \<in> S\<close> weq by (blast intro: hull_inc)
```
```  2145         have yeq: "y = (1 - inverse T) *\<^sub>R w + c /\<^sub>R T"
```
```  2146           using \<open>0 < T\<close> by (simp add: c_def algebra_simps)
```
```  2147         show "y \<in> affine hull (S \<inter> {y. a h \<bullet> y = b h})"
```
```  2148           by (metis yeq affine_affine_hull [simplified affine_alt, rule_format, OF waff caff])
```
```  2149       qed
```
```  2150     qed
```
```  2151     ultimately show ?thesis
```
```  2152       apply (simp add: facet_of_def)
```
```  2153       apply (subst aff_dim_affine_hull [symmetric])
```
```  2154       using  \<open>b h < a h \<bullet> z\<close> zaff
```
```  2155       apply (force simp: aff_dim_affine_Int_hyperplane)
```
```  2156       done
```
```  2157   qed
```
```  2158   show ?thesis
```
```  2159   proof
```
```  2160     show "\<exists>h. h \<in> F \<and> c = S \<inter> {x. a h \<bullet> x = b h} \<Longrightarrow> c facet_of S"
```
```  2161       using * by blast
```
```  2162   next
```
```  2163     assume "c facet_of S"
```
```  2164     then have "c face_of S" "convex c" "c \<noteq> {}" and affc: "aff_dim c = aff_dim S - 1"
```
```  2165       by (auto simp: facet_of_def face_of_imp_convex)
```
```  2166     then obtain x where x: "x \<in> rel_interior c"
```
```  2167       by (force simp: rel_interior_eq_empty)
```
```  2168     then have "x \<in> c"
```
```  2169       by (meson subsetD rel_interior_subset)
```
```  2170     then have "x \<in> S"
```
```  2171       using \<open>c facet_of S\<close> facet_of_imp_subset by blast
```
```  2172     have rels: "rel_interior S = {x \<in> S. \<forall>h\<in>F. a h \<bullet> x < b h}"
```
```  2173       by (rule rel_interior_polyhedron_explicit [OF assms])
```
```  2174     have "c \<noteq> S"
```
```  2175       using \<open>c facet_of S\<close> facet_of_irrefl by blast
```
```  2176     then have "x \<notin> rel_interior S"
```
```  2177       by (metis IntI empty_iff \<open>x \<in> c\<close> \<open>c \<noteq> S\<close> \<open>c face_of S\<close> face_of_disjoint_rel_interior)
```
```  2178     with rels \<open>x \<in> S\<close> obtain i where "i \<in> F" and i: "a i \<bullet> x \<ge> b i"
```
```  2179       by force
```
```  2180     have "x \<in> {u. a i \<bullet> u \<le> b i}"
```
```  2181       by (metis IntD2 InterE \<open>i \<in> F\<close> \<open>x \<in> S\<close> faceq seq)
```
```  2182     then have "a i \<bullet> x \<le> b i" by simp
```
```  2183     then have "a i \<bullet> x = b i" using i by auto
```
```  2184     have "c \<subseteq> S \<inter> {x. a i \<bullet> x = b i}"
```
```  2185       apply (rule subset_of_face_of [of _ S])
```
```  2186         apply (simp add: "*" \<open>i \<in> F\<close> facet_of_imp_face_of)
```
```  2187        apply (simp add: \<open>c face_of S\<close> face_of_imp_subset)
```
```  2188       using \<open>a i \<bullet> x = b i\<close> \<open>x \<in> S\<close> x by blast
```
```  2189     then have cface: "c face_of (S \<inter> {x. a i \<bullet> x = b i})"
```
```  2190       by (meson \<open>c face_of S\<close> face_of_subset inf_le1)
```
```  2191     have con: "convex (S \<inter> {x. a i \<bullet> x = b i})"
```
```  2192       by (simp add: \<open>convex S\<close> convex_Int convex_hyperplane)
```
```  2193     show "\<exists>h. h \<in> F \<and> c = S \<inter> {x. a h \<bullet> x = b h}"
```
```  2194       apply (rule_tac x=i in exI)
```
```  2195       apply (simp add: \<open>i \<in> F\<close>)
```
```  2196       by (metis (no_types) * \<open>i \<in> F\<close> affc facet_of_def less_irrefl face_of_aff_dim_lt [OF con cface])
```
```  2197   qed
```
```  2198 qed
```
```  2199
```
```  2200
```
```  2201 lemma%important face_of_polyhedron_subset_explicit:
```
```  2202   fixes S :: "'a :: euclidean_space set"
```
```  2203   assumes "finite F"
```
```  2204       and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2205       and faceq: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2206       and psub: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> affine hull S \<inter> \<Inter>F'"
```
```  2207       and c: "c face_of S" and "c \<noteq> {}" "c \<noteq> S"
```
```  2208    obtains h where "h \<in> F" "c \<subseteq> S \<inter> {x. a h \<bullet> x = b h}"
```
```  2209 proof%unimportant -
```
```  2210   have "c \<subseteq> S" using \<open>c face_of S\<close>
```
```  2211     by (simp add: face_of_imp_subset)
```
```  2212   have "polyhedron S"
```
```  2213     apply (simp add: polyhedron_Int_affine)
```
```  2214     by (metis \<open>finite F\<close> faceq seq)
```
```  2215   then have "convex S"
```
```  2216     by (simp add: polyhedron_imp_convex)
```
```  2217   then have *: "(S \<inter> {x. a h \<bullet> x = b h}) face_of S" if "h \<in> F" for h
```
```  2218     apply (rule face_of_Int_supporting_hyperplane_le)
```
```  2219     using faceq seq that by fastforce
```
```  2220   have "rel_interior c \<noteq> {}"
```
```  2221     using c \<open>c \<noteq> {}\<close> face_of_imp_convex rel_interior_eq_empty by blast
```
```  2222   then obtain x where "x \<in> rel_interior c" by auto
```
```  2223   have rels: "rel_interior S = {x \<in> S. \<forall>h\<in>F. a h \<bullet> x < b h}"
```
```  2224     by (rule rel_interior_polyhedron_explicit [OF \<open>finite F\<close> seq faceq psub])
```
```  2225   then have xnot: "x \<notin> rel_interior S"
```
```  2226     by (metis IntI \<open>x \<in> rel_interior c\<close> c \<open>c \<noteq> S\<close> contra_subsetD empty_iff face_of_disjoint_rel_interior rel_interior_subset)
```
```  2227   then have "x \<in> S"
```
```  2228     using \<open>c \<subseteq> S\<close> \<open>x \<in> rel_interior c\<close> rel_interior_subset by auto
```
```  2229   then have xint: "x \<in> \<Inter>F"
```
```  2230     using seq by blast
```
```  2231   have "F \<noteq> {}" using assms
```
```  2232     by (metis affine_Int affine_Inter affine_affine_hull ex_in_conv face_of_affine_trivial)
```
```  2233   then obtain i where "i \<in> F" "~ (a i \<bullet> x < b i)"
```
```  2234     using \<open>x \<in> S\<close> rels xnot by auto
```
```  2235   with xint have "a i \<bullet> x = b i"
```
```  2236     by (metis eq_iff mem_Collect_eq not_le Inter_iff faceq)
```
```  2237   have face: "S \<inter> {x. a i \<bullet> x = b i} face_of S"
```
```  2238     by (simp add: "*" \<open>i \<in> F\<close>)
```
```  2239   show ?thesis
```
```  2240     apply (rule_tac h = i in that)
```
```  2241      apply (rule \<open>i \<in> F\<close>)
```
```  2242     apply (rule subset_of_face_of [OF face \<open>c \<subseteq> S\<close>])
```
```  2243     using \<open>a i \<bullet> x = b i\<close> \<open>x \<in> rel_interior c\<close> \<open>x \<in> S\<close> apply blast
```
```  2244     done
```
```  2245 qed
```
```  2246
```
```  2247 text\<open>Initial part of proof duplicates that above\<close>
```
```  2248 proposition%important face_of_polyhedron_explicit:
```
```  2249   fixes S :: "'a :: euclidean_space set"
```
```  2250   assumes "finite F"
```
```  2251       and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2252       and faceq: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2253       and psub: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> affine hull S \<inter> \<Inter>F'"
```
```  2254       and c: "c face_of S" and "c \<noteq> {}" "c \<noteq> S"
```
```  2255     shows "c = \<Inter>{S \<inter> {x. a h \<bullet> x = b h} | h. h \<in> F \<and> c \<subseteq> S \<inter> {x. a h \<bullet> x = b h}}"
```
```  2256 proof%unimportant -
```
```  2257   let ?ab = "\<lambda>h. {x. a h \<bullet> x = b h}"
```
```  2258   have "c \<subseteq> S" using \<open>c face_of S\<close>
```
```  2259     by (simp add: face_of_imp_subset)
```
```  2260   have "polyhedron S"
```
```  2261     apply (simp add: polyhedron_Int_affine)
```
```  2262     by (metis \<open>finite F\<close> faceq seq)
```
```  2263   then have "convex S"
```
```  2264     by (simp add: polyhedron_imp_convex)
```
```  2265   then have *: "(S \<inter> ?ab h) face_of S" if "h \<in> F" for h
```
```  2266     apply (rule face_of_Int_supporting_hyperplane_le)
```
```  2267     using faceq seq that by fastforce
```
```  2268   have "rel_interior c \<noteq> {}"
```
```  2269     using c \<open>c \<noteq> {}\<close> face_of_imp_convex rel_interior_eq_empty by blast
```
```  2270   then obtain z where z: "z \<in> rel_interior c" by auto
```
```  2271   have rels: "rel_interior S = {z \<in> S. \<forall>h\<in>F. a h \<bullet> z < b h}"
```
```  2272     by (rule rel_interior_polyhedron_explicit [OF \<open>finite F\<close> seq faceq psub])
```
```  2273   then have xnot: "z \<notin> rel_interior S"
```
```  2274     by (metis IntI \<open>z \<in> rel_interior c\<close> c \<open>c \<noteq> S\<close> contra_subsetD empty_iff face_of_disjoint_rel_interior rel_interior_subset)
```
```  2275   then have "z \<in> S"
```
```  2276     using \<open>c \<subseteq> S\<close> \<open>z \<in> rel_interior c\<close> rel_interior_subset by auto
```
```  2277   with seq have xint: "z \<in> \<Inter>F" by blast
```
```  2278   have "open (\<Inter>h\<in>{h \<in> F. a h \<bullet> z < b h}. {w. a h \<bullet> w < b h})"
```
```  2279     by (auto simp: \<open>finite F\<close> open_halfspace_lt open_INT)
```
```  2280   then obtain e where "0 < e"
```
```  2281                  "ball z e \<subseteq> (\<Inter>h\<in>{h \<in> F. a h \<bullet> z < b h}. {w. a h \<bullet> w < b h})"
```
```  2282     by (auto intro: openE [of _ z])
```
```  2283   then have e: "\<And>h. \<lbrakk>h \<in> F; a h \<bullet> z < b h\<rbrakk> \<Longrightarrow> ball z e \<subseteq> {w. a h \<bullet> w < b h}"
```
```  2284     by blast
```
```  2285   have "c \<subseteq> (S \<inter> ?ab h) \<longleftrightarrow> z \<in> S \<inter> ?ab h" if "h \<in> F" for h
```
```  2286   proof
```
```  2287     show "z \<in> S \<inter> ?ab h \<Longrightarrow> c \<subseteq> S \<inter> ?ab h"
```
```  2288       apply (rule subset_of_face_of [of _ S])
```
```  2289       using that \<open>c \<subseteq> S\<close> \<open>z \<in> rel_interior c\<close>
```
```  2290       using facet_of_polyhedron_explicit [OF \<open>finite F\<close> seq faceq psub]
```
```  2291             unfolding facet_of_def
```
```  2292       apply auto
```
```  2293       done
```
```  2294   next
```
```  2295     show "c \<subseteq> S \<inter> ?ab h \<Longrightarrow> z \<in> S \<inter> ?ab h"
```
```  2296       using \<open>z \<in> rel_interior c\<close> rel_interior_subset by force
```
```  2297   qed
```
```  2298   then have **: "{S \<inter> ?ab h | h. h \<in> F \<and> c \<subseteq> S \<and> c \<subseteq> ?ab h} =
```
```  2299                  {S \<inter> ?ab h |h. h \<in> F \<and> z \<in> S \<inter> ?ab h}"
```
```  2300     by blast
```
```  2301   have bsub: "ball z e \<inter> affine hull \<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}
```
```  2302              \<subseteq> affine hull S \<inter> \<Inter>F \<inter> \<Inter>{?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}"
```
```  2303             if "i \<in> F" and i: "a i \<bullet> z = b i" for i
```
```  2304   proof -
```
```  2305     have sub: "ball z e \<inter> \<Inter>{?ab h |h. h \<in> F \<and> a h \<bullet> z = b h} \<subseteq> j"
```
```  2306              if "j \<in> F" for j
```
```  2307     proof -
```
```  2308       have "a j \<bullet> z \<le> b j" using faceq that xint by auto
```
```  2309       then consider "a j \<bullet> z < b j" | "a j \<bullet> z = b j" by linarith
```
```  2310       then have "\<exists>G. G \<in> {?ab h |h. h \<in> F \<and> a h \<bullet> z = b h} \<and> ball z e \<inter> G \<subseteq> j"
```
```  2311       proof cases
```
```  2312         assume "a j \<bullet> z < b j"
```
```  2313         then have "ball z e \<inter> {x. a i \<bullet> x = b i} \<subseteq> j"
```
```  2314           using e [OF \<open>j \<in> F\<close>] faceq that
```
```  2315           by (fastforce simp: ball_def)
```
```  2316         then show ?thesis
```
```  2317           by (rule_tac x="{x. a i \<bullet> x = b i}" in exI) (force simp: \<open>i \<in> F\<close> i)
```
```  2318       next
```
```  2319         assume eq: "a j \<bullet> z = b j"
```
```  2320         with faceq that show ?thesis
```
```  2321           by (rule_tac x="{x. a j \<bullet> x = b j}" in exI) (fastforce simp add: \<open>j \<in> F\<close>)
```
```  2322       qed
```
```  2323       then show ?thesis  by blast
```
```  2324     qed
```
```  2325     have 1: "affine hull \<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> a h \<bullet> z = b h} \<subseteq> affine hull S"
```
```  2326       apply (rule hull_mono)
```
```  2327       using that \<open>z \<in> S\<close> by auto
```
```  2328     have 2: "affine hull \<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}
```
```  2329           \<subseteq> \<Inter>{?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}"
```
```  2330       by (rule hull_minimal) (auto intro: affine_hyperplane)
```
```  2331     have 3: "ball z e \<inter> \<Inter>{?ab h |h. h \<in> F \<and> a h \<bullet> z = b h} \<subseteq> \<Inter>F"
```
```  2332       by (iprover intro: sub Inter_greatest)
```
```  2333     have *: "\<lbrakk>A \<subseteq> (B :: 'a set); A \<subseteq> C; E \<inter> C \<subseteq> D\<rbrakk> \<Longrightarrow> E \<inter> A \<subseteq> (B \<inter> D) \<inter> C"
```
```  2334              for A B C D E  by blast
```
```  2335     show ?thesis by (intro * 1 2 3)
```
```  2336   qed
```
```  2337   have "\<exists>h. h \<in> F \<and> c \<subseteq> ?ab h"
```
```  2338     apply (rule face_of_polyhedron_subset_explicit [OF \<open>finite F\<close> seq faceq psub])
```
```  2339     using assms by auto
```
```  2340   then have fac: "\<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> c \<subseteq> S \<inter> ?ab h} face_of S"
```
```  2341     using * by (force simp: \<open>c \<subseteq> S\<close> intro: face_of_Inter)
```
```  2342   have red:
```
```  2343      "(\<And>a. P a \<Longrightarrow> T \<subseteq> S \<inter> \<Inter>{F x |x. P x}) \<Longrightarrow> T \<subseteq> \<Inter>{S \<inter> F x |x. P x}"
```
```  2344      for P T F   by blast
```
```  2345   have "ball z e \<inter> affine hull \<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}
```
```  2346         \<subseteq> \<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> a h \<bullet> z = b h}"
```
```  2347     apply (rule red)
```
```  2348     apply (metis seq bsub)
```
```  2349     done
```
```  2350   with \<open>0 < e\<close> have zinrel: "z \<in> rel_interior
```
```  2351                     (\<Inter>{S \<inter> ?ab h |h. h \<in> F \<and> z \<in> S \<and> a h \<bullet> z = b h})"
```
```  2352     by (auto simp: mem_rel_interior_ball \<open>z \<in> S\<close>)
```
```  2353   show ?thesis
```
```  2354     apply (rule face_of_eq [OF c fac])
```
```  2355     using z zinrel apply (force simp: **)
```
```  2356     done
```
```  2357 qed
```
```  2358
```
```  2359
```
```  2360 subsection\<open>More general corollaries from the explicit representation\<close>
```
```  2361
```
```  2362 corollary%important facet_of_polyhedron:
```
```  2363   assumes "polyhedron S" and "c facet_of S"
```
```  2364   obtains a b where "a \<noteq> 0" "S \<subseteq> {x. a \<bullet> x \<le> b}" "c = S \<inter> {x. a \<bullet> x = b}"
```
```  2365 proof%unimportant -
```
```  2366   obtain F where "finite F" and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2367              and faces: "\<And>h. h \<in> F \<Longrightarrow> \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}"
```
```  2368              and min: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> (affine hull S) \<inter> \<Inter>F'"
```
```  2369     using assms by (simp add: polyhedron_Int_affine_minimal) meson
```
```  2370   then obtain a b where ab: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2371     by metis
```
```  2372   obtain i where "i \<in> F" and c: "c = S \<inter> {x. a i \<bullet> x = b i}"
```
```  2373     using facet_of_polyhedron_explicit [OF \<open>finite F\<close> seq ab min] assms
```
```  2374     by force
```
```  2375   moreover have ssub: "S \<subseteq> {x. a i \<bullet> x \<le> b i}"
```
```  2376      apply (subst seq)
```
```  2377      using \<open>i \<in> F\<close> ab by auto
```
```  2378   ultimately show ?thesis
```
```  2379     by (rule_tac a = "a i" and b = "b i" in that) (simp_all add: ab)
```
```  2380 qed
```
```  2381
```
```  2382 corollary%important face_of_polyhedron:
```
```  2383   assumes "polyhedron S" and "c face_of S" and "c \<noteq> {}" and "c \<noteq> S"
```
```  2384     shows "c = \<Inter>{F. F facet_of S \<and> c \<subseteq> F}"
```
```  2385 proof%unimportant -
```
```  2386   obtain F where "finite F" and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2387              and faces: "\<And>h. h \<in> F \<Longrightarrow> \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}"
```
```  2388              and min: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> (affine hull S) \<inter> \<Inter>F'"
```
```  2389     using assms by (simp add: polyhedron_Int_affine_minimal) meson
```
```  2390   then obtain a b where ab: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2391     by metis
```
```  2392   show ?thesis
```
```  2393     apply (subst face_of_polyhedron_explicit [OF \<open>finite F\<close> seq ab min])
```
```  2394     apply (auto simp: assms facet_of_polyhedron_explicit [OF \<open>finite F\<close> seq ab min] cong: Collect_cong)
```
```  2395     done
```
```  2396 qed
```
```  2397
```
```  2398 lemma%unimportant face_of_polyhedron_subset_facet:
```
```  2399   assumes "polyhedron S" and "c face_of S" and "c \<noteq> {}" and "c \<noteq> S"
```
```  2400   obtains F where "F facet_of S" "c \<subseteq> F"
```
```  2401 using face_of_polyhedron assms
```
```  2402 by (metis (no_types, lifting) Inf_greatest antisym_conv face_of_imp_subset mem_Collect_eq)
```
```  2403
```
```  2404
```
```  2405 lemma%unimportant exposed_face_of_polyhedron:
```
```  2406   assumes "polyhedron S"
```
```  2407     shows "F exposed_face_of S \<longleftrightarrow> F face_of S"
```
```  2408 proof
```
```  2409   show "F exposed_face_of S \<Longrightarrow> F face_of S"
```
```  2410     by (simp add: exposed_face_of_def)
```
```  2411 next
```
```  2412   assume "F face_of S"
```
```  2413   show "F exposed_face_of S"
```
```  2414   proof (cases "F = {} \<or> F = S")
```
```  2415     case True then show ?thesis
```
```  2416       using \<open>F face_of S\<close> exposed_face_of by blast
```
```  2417   next
```
```  2418     case False
```
```  2419     then have "{g. g facet_of S \<and> F \<subseteq> g} \<noteq> {}"
```
```  2420       by (metis Collect_empty_eq_bot \<open>F face_of S\<close> assms empty_def face_of_polyhedron_subset_facet)
```
```  2421     moreover have "\<And>T. \<lbrakk>T facet_of S; F \<subseteq> T\<rbrakk> \<Longrightarrow> T exposed_face_of S"
```
```  2422       by (metis assms exposed_face_of facet_of_imp_face_of facet_of_polyhedron)
```
```  2423     ultimately have "\<Inter>{fa.
```
```  2424        fa facet_of S \<and> F \<subseteq> fa} exposed_face_of S"
```
```  2425       by (metis (no_types, lifting) mem_Collect_eq exposed_face_of_Inter)
```
```  2426     then show ?thesis
```
```  2427       using False
```
```  2428       apply (subst face_of_polyhedron [OF assms \<open>F face_of S\<close>], auto)
```
```  2429       done
```
```  2430   qed
```
```  2431 qed
```
```  2432
```
```  2433 lemma%unimportant face_of_polyhedron_polyhedron:
```
```  2434   fixes S :: "'a :: euclidean_space set"
```
```  2435   assumes "polyhedron S" "c face_of S" shows "polyhedron c"
```
```  2436 by (metis assms face_of_imp_eq_affine_Int polyhedron_Int polyhedron_affine_hull polyhedron_imp_convex)
```
```  2437
```
```  2438 lemma%unimportant finite_polyhedron_faces:
```
```  2439   fixes S :: "'a :: euclidean_space set"
```
```  2440   assumes "polyhedron S"
```
```  2441     shows "finite {F. F face_of S}"
```
```  2442 proof -
```
```  2443   obtain F where "finite F" and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2444              and faces: "\<And>h. h \<in> F \<Longrightarrow> \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}"
```
```  2445              and min:   "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> (affine hull S) \<inter> \<Inter>F'"
```
```  2446     using assms by (simp add: polyhedron_Int_affine_minimal) meson
```
```  2447   then obtain a b where ab: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2448     by metis
```
```  2449   have "finite {\<Inter>{S \<inter> {x. a h \<bullet> x = b h} |h. h \<in> F'}| F'. F' \<in> Pow F}"
```
```  2450     by (simp add: \<open>finite F\<close>)
```
```  2451   moreover have "{F. F face_of S} - {{}, S} \<subseteq> {\<Inter>{S \<inter> {x. a h \<bullet> x = b h} |h. h \<in> F'}| F'. F' \<in> Pow F}"
```
```  2452     apply clarify
```
```  2453     apply (rename_tac c)
```
```  2454     apply (drule face_of_polyhedron_explicit [OF \<open>finite F\<close> seq ab min, simplified], simp_all)
```
```  2455     apply (erule ssubst)
```
```  2456     apply (rule_tac x="{h \<in> F. c \<subseteq> S \<inter> {x. a h \<bullet> x = b h}}" in exI, auto)
```
```  2457     done
```
```  2458   ultimately show ?thesis
```
```  2459     by (meson finite.emptyI finite.insertI finite_Diff2 finite_subset)
```
```  2460 qed
```
```  2461
```
```  2462 lemma%unimportant finite_polyhedron_exposed_faces:
```
```  2463    "polyhedron S \<Longrightarrow> finite {F. F exposed_face_of S}"
```
```  2464 using exposed_face_of_polyhedron finite_polyhedron_faces by fastforce
```
```  2465
```
```  2466 lemma%unimportant finite_polyhedron_extreme_points:
```
```  2467   fixes S :: "'a :: euclidean_space set"
```
```  2468   shows "polyhedron S \<Longrightarrow> finite {v. v extreme_point_of S}"
```
```  2469 apply (simp add: face_of_singleton [symmetric])
```
```  2470 apply (rule finite_subset [OF _ finite_vimageI [OF finite_polyhedron_faces]], auto)
```
```  2471 done
```
```  2472
```
```  2473 lemma%unimportant finite_polyhedron_facets:
```
```  2474   fixes S :: "'a :: euclidean_space set"
```
```  2475   shows "polyhedron S \<Longrightarrow> finite {F. F facet_of S}"
```
```  2476 unfolding facet_of_def
```
```  2477 by (blast intro: finite_subset [OF _ finite_polyhedron_faces])
```
```  2478
```
```  2479
```
```  2480 proposition%important rel_interior_of_polyhedron:
```
```  2481   fixes S :: "'a :: euclidean_space set"
```
```  2482   assumes "polyhedron S"
```
```  2483     shows "rel_interior S = S - \<Union>{F. F facet_of S}"
```
```  2484 proof%unimportant -
```
```  2485   obtain F where "finite F" and seq: "S = affine hull S \<inter> \<Inter>F"
```
```  2486              and faces: "\<And>h. h \<in> F \<Longrightarrow> \<exists>a b. a \<noteq> 0 \<and> h = {x. a \<bullet> x \<le> b}"
```
```  2487              and min: "\<And>F'. F' \<subset> F \<Longrightarrow> S \<subset> (affine hull S) \<inter> \<Inter>F'"
```
```  2488     using assms by (simp add: polyhedron_Int_affine_minimal) meson
```
```  2489   then obtain a b where ab: "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> h = {x. a h \<bullet> x \<le> b h}"
```
```  2490     by metis
```
```  2491   have facet: "(c facet_of S) \<longleftrightarrow> (\<exists>h. h \<in> F \<and> c = S \<inter> {x. a h \<bullet> x = b h})" for c
```
```  2492     by (rule facet_of_polyhedron_explicit [OF \<open>finite F\<close> seq ab min])
```
```  2493   have rel: "rel_interior S = {x \<in> S. \<forall>h\<in>F. a h \<bullet> x < b h}"
```
```  2494     by (rule rel_interior_polyhedron_explicit [OF \<open>finite F\<close> seq ab min])
```
```  2495   have "a h \<bullet> x < b h" if "x \<in> S" "h \<in> F" and xnot: "x \<notin> \<Union>{F. F facet_of S}" for x h
```
```  2496   proof -
```
```  2497     have "x \<in> \<Inter>F" using seq that by force
```
```  2498     with \<open>h \<in> F\<close> ab have "a h \<bullet> x \<le> b h" by auto
```
```  2499     then consider "a h \<bullet> x < b h" | "a h \<bullet> x = b h" by linarith
```
```  2500     then show ?thesis
```
```  2501     proof cases
```
```  2502       case 1 then show ?thesis .
```
```  2503     next
```
```  2504       case 2
```
```  2505       have "Collect ((\<in>) x) \<notin> Collect ((\<in>) (\<Union>{A. A facet_of S}))"
```
```  2506         using xnot by fastforce
```
```  2507       then have "F \<notin> Collect ((\<in>) h)"
```
```  2508         using 2 \<open>x \<in> S\<close> facet by blast
```
```  2509       with \<open>h \<in> F\<close> have "\<Inter>F \<subseteq> S \<inter> {x. a h \<bullet> x = b h}" by blast
```
```  2510       with 2 that \<open>x \<in> \<Inter>F\<close> show ?thesis
```
```  2511         apply simp
```
```  2512         apply (drule_tac x="\<Inter>F" in spec)
```
```  2513         apply (simp add: facet)
```
```  2514         apply (drule_tac x=h in spec)
```
```  2515         using seq by auto
```
```  2516       qed
```
```  2517   qed
```
```  2518   moreover have "\<exists>h\<in>F. a h \<bullet> x \<ge> b h" if "x \<in> \<Union>{F. F facet_of S}" for x
```
```  2519     using that by (force simp: facet)
```
```  2520   ultimately show ?thesis
```
```  2521     by (force simp: rel)
```
```  2522 qed
```
```  2523
```
```  2524 lemma%unimportant rel_boundary_of_polyhedron:
```
```  2525   fixes S :: "'a :: euclidean_space set"
```
```  2526   assumes "polyhedron S"
```
```  2527     shows "S - rel_interior S = \<Union> {F. F facet_of S}"
```
```  2528 using facet_of_imp_subset by (fastforce simp add: rel_interior_of_polyhedron assms)
```
```  2529
```
```  2530 lemma%unimportant rel_frontier_of_polyhedron:
```
```  2531   fixes S :: "'a :: euclidean_space set"
```
```  2532   assumes "polyhedron S"
```
```  2533     shows "rel_frontier S = \<Union> {F. F facet_of S}"
```
```  2534 by (simp add: assms rel_frontier_def polyhedron_imp_closed rel_boundary_of_polyhedron)
```
```  2535
```
```  2536 lemma%unimportant rel_frontier_of_polyhedron_alt:
```
```  2537   fixes S :: "'a :: euclidean_space set"
```
```  2538   assumes "polyhedron S"
```
```  2539     shows "rel_frontier S = \<Union> {F. F face_of S \<and> (F \<noteq> S)}"
```
```  2540 apply (rule subset_antisym)
```
```  2541   apply (force simp: rel_frontier_of_polyhedron facet_of_def assms)
```
```  2542 using face_of_subset_rel_frontier by fastforce
```
```  2543
```
```  2544
```
```  2545 text\<open>A characterization of polyhedra as having finitely many faces\<close>
```
```  2546
```
```  2547 proposition%important polyhedron_eq_finite_exposed_faces:
```
```  2548   fixes S :: "'a :: euclidean_space set"
```
```  2549   shows "polyhedron S \<longleftrightarrow> closed S \<and> convex S \<and> finite {F. F exposed_face_of S}"
```
```  2550          (is "?lhs = ?rhs")
```
```  2551 proof%unimportant
```
```  2552   assume ?lhs
```
```  2553   then show ?rhs
```
```  2554     by (auto simp: polyhedron_imp_closed polyhedron_imp_convex finite_polyhedron_exposed_faces)
```
```  2555 next
```
```  2556   assume ?rhs
```
```  2557   then have "closed S" "convex S" and fin: "finite {F. F exposed_face_of S}" by auto
```
```  2558   show ?lhs
```
```  2559   proof (cases "S = {}")
```
```  2560     case True then show ?thesis by auto
```
```  2561   next
```
```  2562     case False
```
```  2563     define F where "F = {h. h exposed_face_of S \<and> h \<noteq> {} \<and> h \<noteq> S}"
```
```  2564     have "finite F" by (simp add: fin F_def)
```
```  2565     have hface: "h face_of S"
```
```  2566       and "\<exists>a b. a \<noteq> 0 \<and> S \<subseteq> {x. a \<bullet> x \<le> b} \<and> h = S \<inter> {x. a \<bullet> x = b}"
```
```  2567       if "h \<in> F" for h
```
```  2568       using exposed_face_of F_def that by simp_all auto
```
```  2569     then obtain a b where ab:
```
```  2570       "\<And>h. h \<in> F \<Longrightarrow> a h \<noteq> 0 \<and> S \<subseteq> {x. a h \<bullet> x \<le> b h} \<and> h = S \<inter> {x. a h \<bullet> x = b h}"
```
```  2571       by metis
```
```  2572     have *: "False"
```
```  2573       if paff: "p \<in> affine hull S" and "p \<notin> S"
```
```  2574       and pint: "p \<in> \<Inter>{{x. a h \<bullet> x \<le> b h} |h. h \<in> F}" for p
```
```  2575     proof -
```
```  2576       have "rel_interior S \<noteq> {}"
```
```  2577         by (simp add: \<open>S \<noteq> {}\<close> \<open>convex S\<close> rel_interior_eq_empty)
```
```  2578       then obtain c where c: "c \<in> rel_interior S" by auto
```
```  2579       with rel_interior_subset have "c \<in> S"  by blast
```
```  2580       have ccp: "closed_segment c p \<subseteq> affine hull S"
```
```  2581         by (meson affine_affine_hull affine_imp_convex c closed_segment_subset hull_subset paff rel_interior_subset subsetCE)
```
```  2582       obtain x where xcl: "x \<in> closed_segment c p" and "x \<in> S" and xnot: "x \<notin> rel_interior S"
```
```  2583         using connected_openin [of "closed_segment c p"]
```
```  2584         apply simp
```
```  2585         apply (drule_tac x="closed_segment c p \<inter> rel_interior S" in spec)
```
```  2586         apply (erule impE)
```
```  2587          apply (force simp: openin_rel_interior openin_Int intro: openin_subtopology_Int_subset [OF _ ccp])
```
```  2588         apply (drule_tac x="closed_segment c p \<inter> (- S)" in spec)
```
```  2589         using rel_interior_subset \<open>closed S\<close> c \<open>p \<notin> S\<close> apply blast
```
```  2590         done
```
```  2591       then obtain \<mu> where "0 \<le> \<mu>" "\<mu> \<le> 1" and xeq: "x = (1 - \<mu>) *\<^sub>R c + \<mu> *\<^sub>R p"
```
```  2592         by (auto simp: in_segment)
```
```  2593       show False
```
```  2594       proof (cases "\<mu>=0 \<or> \<mu>=1")
```
```  2595         case True with xeq c xnot \<open>x \<in> S\<close> \<open>p \<notin> S\<close>
```
```  2596         show False by auto
```
```  2597       next
```
```  2598         case False
```
```  2599         then have xos: "x \<in> open_segment c p"
```
```  2600           using \<open>x \<in> S\<close> c open_segment_def that(2) xcl xnot by auto
```
```  2601         have xclo: "x \<in> closure S"
```
```  2602           using \<open>x \<in> S\<close> closure_subset by blast
```
```  2603         obtain d where "d \<noteq> 0"
```
```  2604               and dle: "\<And>y. y \<in> closure S \<Longrightarrow> d \<bullet> x \<le> d \<bullet> y"
```
```  2605               and dless: "\<And>y. y \<in> rel_interior S \<Longrightarrow> d \<bullet> x < d \<bullet> y"
```
```  2606           by (metis supporting_hyperplane_relative_frontier [OF \<open>convex S\<close> xclo xnot])
```
```  2607         have sex: "S \<inter> {y. d \<bullet> y = d \<bullet> x} exposed_face_of S"
```
```  2608           by (simp add: \<open>closed S\<close> dle exposed_face_of_Int_supporting_hyperplane_ge [OF \<open>convex S\<close>])
```
```  2609         have sne: "S \<inter> {y. d \<bullet> y = d \<bullet> x} \<noteq> {}"
```
```  2610           using \<open>x \<in> S\<close> by blast
```
```  2611         have sns: "S \<inter> {y. d \<bullet> y = d \<bullet> x} \<noteq> S"
```
```  2612           by (metis (mono_tags) Int_Collect c subsetD dless not_le order_refl rel_interior_subset)
```
```  2613         obtain h where "h \<in> F" "x \<in> h"
```
```  2614           apply (rule_tac h="S \<inter> {y. d \<bullet> y = d \<bullet> x}" in that)
```
```  2615           apply (simp_all add: F_def sex sne sns \<open>x \<in> S\<close>)
```
```  2616           done
```
```  2617         have abface: "{y. a h \<bullet> y = b h} face_of {y. a h \<bullet> y \<le> b h}"
```
```  2618           using hyperplane_face_of_halfspace_le by blast
```
```  2619         then have "c \<in> h"
```
```  2620           using face_ofD [OF abface xos] \<open>c \<in> S\<close> \<open>h \<in> F\<close> ab pint \<open>x \<in> h\<close> by blast
```
```  2621         with c have "h \<inter> rel_interior S \<noteq> {}" by blast
```
```  2622         then show False
```
```  2623           using \<open>h \<in> F\<close> F_def face_of_disjoint_rel_interior hface by auto
```
```  2624       qed
```
```  2625     qed
```
```  2626     have "S \<subseteq> affine hull S \<inter> \<Inter>{{x. a h \<bullet> x \<le> b h} |h. h \<in> F}"
```
```  2627       using ab by (auto simp: hull_subset)
```
```  2628     moreover have "affine hull S \<inter> \<Inter>{{x. a h \<bullet> x \<le> b h} |h. h \<in> F} \<subseteq> S"
```
```  2629       using * by blast
```
```  2630     ultimately have "S = affine hull S \<inter> \<Inter> {{x. a h \<bullet> x \<le> b h} |h. h \<in> F}" ..
```
```  2631     then show ?thesis
```
```  2632       apply (rule ssubst)
```
```  2633       apply (force intro: polyhedron_affine_hull polyhedron_halfspace_le simp: \<open>finite F\<close>)
```
```  2634       done
```
```  2635   qed
```
```  2636 qed
```
```  2637
```
```  2638 corollary%important polyhedron_eq_finite_faces:
```
```  2639   fixes S :: "'a :: euclidean_space set"
```
```  2640   shows "polyhedron S \<longleftrightarrow> closed S \<and> convex S \<and> finite {F. F face_of S}"
```
```  2641          (is "?lhs = ?rhs")
```
```  2642 proof%unimportant
```
```  2643   assume ?lhs
```
```  2644   then show ?rhs
```
```  2645     by (simp add: finite_polyhedron_faces polyhedron_imp_closed polyhedron_imp_convex)
```
```  2646 next
```
```  2647   assume ?rhs
```
```  2648   then show ?lhs
```
```  2649     by (force simp: polyhedron_eq_finite_exposed_faces exposed_face_of intro: finite_subset)
```
```  2650 qed
```
```  2651
```
```  2652 lemma%unimportant polyhedron_linear_image_eq:
```
```  2653   fixes h :: "'a :: euclidean_space \<Rightarrow> 'b :: euclidean_space"
```
```  2654   assumes "linear h" "bij h"
```
```  2655     shows "polyhedron (h ` S) \<longleftrightarrow> polyhedron S"
```
```  2656 proof -
```
```  2657   have *: "{f. P f} = (image h) ` {f. P (h ` f)}" for P
```
```  2658     apply safe
```
```  2659     apply (rule_tac x="inv h ` x" in image_eqI)
```
```  2660     apply (auto simp: \<open>bij h\<close> bij_is_surj image_f_inv_f)
```
```  2661     done
```
```  2662   have "inj h" using bij_is_inj assms by blast
```
```  2663   then have injim: "inj_on ((`) h) A" for A
```
```  2664     by (simp add: inj_on_def inj_image_eq_iff)
```
```  2665   show ?thesis
```
```  2666     using \<open>linear h\<close> \<open>inj h\<close>
```
```  2667     apply (simp add: polyhedron_eq_finite_faces closed_injective_linear_image_eq)
```
```  2668     apply (simp add: * face_of_linear_image [of h _ S, symmetric] finite_image_iff injim)
```
```  2669     done
```
```  2670 qed
```
```  2671
```
```  2672 lemma%unimportant polyhedron_negations:
```
```  2673   fixes S :: "'a :: euclidean_space set"
```
```  2674   shows   "polyhedron S \<Longrightarrow> polyhedron(image uminus S)"
```
```  2675   by (subst polyhedron_linear_image_eq)
```
```  2676     (auto simp: bij_uminus intro!: linear_uminus)
```
```  2677
```
```  2678 subsection%important\<open>Relation between polytopes and polyhedra\<close>
```
```  2679
```
```  2680 lemma%important polytope_eq_bounded_polyhedron:
```
```  2681   fixes S :: "'a :: euclidean_space set"
```
```  2682   shows "polytope S \<longleftrightarrow> polyhedron S \<and> bounded S"
```
```  2683          (is "?lhs = ?rhs")
```
```  2684 proof%unimportant
```
```  2685   assume ?lhs
```
```  2686   then show ?rhs
```
```  2687     by (simp add: finite_polytope_faces polyhedron_eq_finite_faces
```
```  2688                   polytope_imp_closed polytope_imp_convex polytope_imp_bounded)
```
```  2689 next
```
```  2690   assume ?rhs then show ?lhs
```
```  2691     unfolding polytope_def
```
```  2692     apply (rule_tac x="{v. v extreme_point_of S}" in exI)
```
```  2693     apply (simp add: finite_polyhedron_extreme_points Krein_Milman_Minkowski compact_eq_bounded_closed polyhedron_imp_closed polyhedron_imp_convex)
```
```  2694     done
```
```  2695 qed
```
```  2696
```
```  2697 lemma%unimportant polytope_Int:
```
```  2698   fixes S :: "'a :: euclidean_space set"
```
```  2699   shows "\<lbrakk>polytope S; polytope T\<rbrakk> \<Longrightarrow> polytope(S \<inter> T)"
```
```  2700 by (simp add: polytope_eq_bounded_polyhedron bounded_Int)
```
```  2701
```
```  2702
```
```  2703 lemma%important polytope_Int_polyhedron:
```
```  2704   fixes S :: "'a :: euclidean_space set"
```
```  2705   shows "\<lbrakk>polytope S; polyhedron T\<rbrakk> \<Longrightarrow> polytope(S \<inter> T)"
```
```  2706 by%unimportant (simp add: bounded_Int polytope_eq_bounded_polyhedron)
```
```  2707
```
```  2708 lemma%important polyhedron_Int_polytope:
```
```  2709   fixes S :: "'a :: euclidean_space set"
```
```  2710   shows "\<lbrakk>polyhedron S; polytope T\<rbrakk> \<Longrightarrow> polytope(S \<inter> T)"
```
```  2711 by%unimportant (simp add: bounded_Int polytope_eq_bounded_polyhedron)
```
```  2712
```
```  2713 lemma%important polytope_imp_polyhedron:
```
```  2714   fixes S :: "'a :: euclidean_space set"
```
```  2715   shows "polytope S \<Longrightarrow> polyhedron S"
```
```  2716 by%unimportant (simp add: polytope_eq_bounded_polyhedron)
```
```  2717
```
```  2718 lemma%unimportant polytope_facet_exists:
```
```  2719   fixes p :: "'a :: euclidean_space set"
```
```  2720   assumes "polytope p" "0 < aff_dim p"
```
```  2721   obtains F where "F facet_of p"
```
```  2722 proof (cases "p = {}")
```
```  2723   case True with assms show ?thesis by auto
```
```  2724 next
```
```  2725   case False
```
```  2726   then obtain v where "v extreme_point_of p"
```
```  2727     using extreme_point_exists_convex
```
```  2728     by (blast intro: \<open>polytope p\<close> polytope_imp_compact polytope_imp_convex)
```
```  2729   then
```
```  2730   show ?thesis
```
```  2731     by (metis face_of_polyhedron_subset_facet polytope_imp_polyhedron aff_dim_sing
```
```  2732        all_not_in_conv assms face_of_singleton less_irrefl singletonI that)
```
```  2733 qed
```
```  2734
```
```  2735 lemma%unimportant polyhedron_interval [iff]: "polyhedron(cbox a b)"
```
```  2736 by (metis polytope_imp_polyhedron polytope_interval)
```
```  2737
```
```  2738 lemma%unimportant polyhedron_convex_hull:
```
```  2739   fixes S :: "'a :: euclidean_space set"
```
```  2740   shows "finite S \<Longrightarrow> polyhedron(convex hull S)"
```
```  2741 by (simp add: polytope_convex_hull polytope_imp_polyhedron)
```
```  2742
```
```  2743
```
```  2744 subsection%important\<open>Relative and absolute frontier of a polytope\<close>
```
```  2745
```
```  2746 lemma rel_boundary_of_convex_hull:
```
```  2747     fixes S :: "'a::euclidean_space set"
```
```  2748     assumes "~ affine_dependent S"
```
```  2749       shows "(convex hull S) - rel_interior(convex hull S) = (\<Union>a\<in>S. convex hull (S - {a}))"
```
```  2750 proof -
```
```  2751   have "finite S" by (metis assms aff_independent_finite)
```
```  2752   then consider "card S = 0" | "card S = 1" | "2 \<le> card S" by arith
```
```  2753   then show ?thesis
```
```  2754   proof cases
```
```  2755     case 1 then have "S = {}" by (simp add: \<open>finite S\<close>)
```
```  2756     then show ?thesis by simp
```
```  2757   next
```
```  2758     case 2 show ?thesis
```
```  2759       by (auto intro: card_1_singletonE [OF \<open>card S = 1\<close>])
```
```  2760   next
```
```  2761     case 3
```
```  2762     with assms show ?thesis
```
```  2763       by (auto simp: polyhedron_convex_hull rel_boundary_of_polyhedron facet_of_convex_hull_affine_independent_alt \<open>finite S\<close>)
```
```  2764   qed
```
```  2765 qed
```
```  2766
```
```  2767 proposition frontier_of_convex_hull:
```
```  2768     fixes S :: "'a::euclidean_space set"
```
```  2769     assumes "card S = Suc (DIM('a))"
```
```  2770       shows "frontier(convex hull S) = \<Union> {convex hull (S - {a}) | a. a \<in> S}"
```
```  2771 proof (cases "affine_dependent S")
```
```  2772   case True
```
```  2773     have [iff]: "finite S"
```
```  2774       using assms using card_infinite by force
```
```  2775     then have ccs: "closed (convex hull S)"
```
```  2776       by (simp add: compact_imp_closed finite_imp_compact_convex_hull)
```
```  2777     { fix x T
```
```  2778       assume "finite T" "T \<subseteq> S" "int (card T) \<le> aff_dim S + 1" "x \<in> convex hull T"
```
```  2779       then have "S \<noteq> T"
```
```  2780         using True \<open>finite S\<close> aff_dim_le_card affine_independent_iff_card by fastforce
```
```  2781       then obtain a where "a \<in> S" "a \<notin> T"
```
```  2782         using \<open>T \<subseteq> S\<close> by blast
```
```  2783       then have "x \<in> (\<Union>a\<in>S. convex hull (S - {a}))"
```
```  2784         using True affine_independent_iff_card [of S]
```
```  2785         apply simp
```
```  2786         apply (metis (no_types, hide_lams) Diff_eq_empty_iff Diff_insert0 \<open>a \<notin> T\<close> \<open>T \<subseteq> S\<close> \<open>x \<in> convex hull T\<close>  hull_mono insert_Diff_single   subsetCE)
```
```  2787         done
```
```  2788     } note * = this
```
```  2789     have 1: "convex hull S \<subseteq> (\<Union> a\<in>S. convex hull (S - {a}))"
```
```  2790       apply (subst caratheodory_aff_dim)
```
```  2791       apply (blast intro: *)
```
```  2792       done
```
```  2793     have 2: "\<Union>((\<lambda>a. convex hull (S - {a})) ` S) \<subseteq> convex hull S"
```
```  2794       by (rule Union_least) (metis (no_types, lifting)  Diff_subset hull_mono imageE)
```
```  2795     show ?thesis using True
```
```  2796       apply (simp add: segment_convex_hull frontier_def)
```
```  2797       using interior_convex_hull_eq_empty [OF assms]
```
```  2798       apply (simp add: closure_closed [OF ccs])
```
```  2799       apply (rule subset_antisym)
```
```  2800       using 1 apply blast
```
```  2801       using 2 apply blast
```
```  2802       done
```
```  2803 next
```
```  2804   case False
```
```  2805   then have "frontier (convex hull S) = (convex hull S) - rel_interior(convex hull S)"
```
```  2806     apply (simp add: rel_boundary_of_convex_hull [symmetric] frontier_def)
```
```  2807     by (metis aff_independent_finite assms closure_convex_hull finite_imp_compact_convex_hull hull_hull interior_convex_hull_eq_empty rel_interior_nonempty_interior)
```
```  2808   also have "... = \<Union>{convex hull (S - {a}) |a. a \<in> S}"
```
```  2809   proof -
```
```  2810     have "convex hull S - rel_interior (convex hull S) = rel_frontier (convex hull S)"
```
```  2811       by (simp add: False aff_independent_finite polyhedron_convex_hull rel_boundary_of_polyhedron rel_frontier_of_polyhedron)
```
```  2812     then show ?thesis
```
```  2813       by (simp add: False rel_frontier_convex_hull_cases)
```
```  2814   qed
```
```  2815   finally show ?thesis .
```
```  2816 qed
```
```  2817
```
```  2818 subsection\<open>Special case of a triangle\<close>
```
```  2819
```
```  2820 proposition frontier_of_triangle:
```
```  2821     fixes a :: "'a::euclidean_space"
```
```  2822     assumes "DIM('a) = 2"
```
```  2823     shows "frontier(convex hull {a,b,c}) = closed_segment a b \<union> closed_segment b c \<union> closed_segment c a"
```
```  2824           (is "?lhs = ?rhs")
```
```  2825 proof (cases "b = a \<or> c = a \<or> c = b")
```
```  2826   case True then show ?thesis
```
```  2827     by (auto simp: assms segment_convex_hull frontier_def empty_interior_convex_hull insert_commute card_insert_le_m1 hull_inc insert_absorb)
```
```  2828 next
```
```  2829   case False then have [simp]: "card {a, b, c} = Suc (DIM('a))"
```
```  2830     by (simp add: card_insert Set.insert_Diff_if assms)
```
```  2831   show ?thesis
```
```  2832   proof
```
```  2833     show "?lhs \<subseteq> ?rhs"
```
```  2834       using False
```
```  2835       by (force simp: segment_convex_hull frontier_of_convex_hull insert_Diff_if insert_commute split: if_split_asm)
```
```  2836     show "?rhs \<subseteq> ?lhs"
```
```  2837       using False
```
```  2838       apply (simp add: frontier_of_convex_hull segment_convex_hull)
```
```  2839       apply (intro conjI subsetI)
```
```  2840         apply (rule_tac X="convex hull {a,b}" in UnionI; force simp: Set.insert_Diff_if)
```
```  2841        apply (rule_tac X="convex hull {b,c}" in UnionI; force)
```
```  2842       apply (rule_tac X="convex hull {a,c}" in UnionI; force simp: insert_commute Set.insert_Diff_if)
```
```  2843       done
```
```  2844   qed
```
```  2845 qed
```
```  2846
```
```  2847 corollary inside_of_triangle:
```
```  2848     fixes a :: "'a::euclidean_space"
```
```  2849     assumes "DIM('a) = 2"
```
```  2850     shows "inside (closed_segment a b \<union> closed_segment b c \<union> closed_segment c a) = interior(convex hull {a,b,c})"
```
```  2851 by (metis assms frontier_of_triangle bounded_empty bounded_insert convex_convex_hull inside_frontier_eq_interior bounded_convex_hull)
```
```  2852
```
```  2853 corollary interior_of_triangle:
```
```  2854     fixes a :: "'a::euclidean_space"
```
```  2855     assumes "DIM('a) = 2"
```
```  2856     shows "interior(convex hull {a,b,c}) =
```
```  2857            convex hull {a,b,c} - (closed_segment a b \<union> closed_segment b c \<union> closed_segment c a)"
```
```  2858   using interior_subset
```
```  2859   by (force simp: frontier_of_triangle [OF assms, symmetric] frontier_def Diff_Diff_Int)
```
```  2860
```
```  2861 subsection\<open>Subdividing a cell complex\<close>
```
```  2862
```
```  2863 lemma subdivide_interval:
```
```  2864   fixes x::real
```
```  2865   assumes "a < \<bar>x - y\<bar>" "0 < a"
```
```  2866   obtains n where "n \<in> \<int>" "x < n * a \<and> n * a < y \<or> y <  n * a \<and> n * a < x"
```
```  2867 proof -
```
```  2868   consider "a + x < y" | "a + y < x"
```
```  2869     using assms by linarith
```
```  2870   then show ?thesis
```
```  2871   proof cases
```
```  2872     case 1
```
```  2873     let ?n = "of_int (floor (x/a)) + 1"
```
```  2874     have x: "x < ?n * a"
```
```  2875       by (meson \<open>0 < a\<close> divide_less_eq floor_eq_iff)
```
```  2876     have "?n * a \<le> a + x"
```
```  2877       apply (simp add: algebra_simps)
```
```  2878       by (metis \<open>0 < a\<close> floor_correct less_irrefl nonzero_mult_div_cancel_left real_mult_le_cancel_iff2 times_divide_eq_right)
```
```  2879     also have "... < y"
```
```  2880       by (rule 1)
```
```  2881     finally have "?n * a < y" .
```
```  2882     with x show ?thesis
```
```  2883       using Ints_1 Ints_add Ints_of_int that by blast
```
```  2884   next
```
```  2885     case 2
```
```  2886     let ?n = "of_int (floor (y/a)) + 1"
```
```  2887     have y: "y < ?n * a"
```
```  2888       by (meson \<open>0 < a\<close> divide_less_eq floor_eq_iff)
```
```  2889     have "?n * a \<le> a + y"
```
```  2890       apply (simp add: algebra_simps)
```
```  2891       by (metis \<open>0 < a\<close> floor_correct less_irrefl nonzero_mult_div_cancel_left real_mult_le_cancel_iff2 times_divide_eq_right)
```
```  2892     also have "... < x"
```
```  2893       by (rule 2)
```
```  2894     finally have "?n * a < x" .
```
```  2895     then show ?thesis
```
```  2896       using Ints_1 Ints_add Ints_of_int that y by blast
```
```  2897   qed
```
```  2898 qed
```
```  2899
```
```  2900 lemma cell_subdivision_lemma:
```
```  2901   assumes "finite \<F>"
```
```  2902       and "\<And>X. X \<in> \<F> \<Longrightarrow> polytope X"
```
```  2903       and "\<And>X. X \<in> \<F> \<Longrightarrow> aff_dim X \<le> d"
```
```  2904       and "\<And>X Y. \<lbrakk>X \<in> \<F>; Y \<in> \<F>\<rbrakk> \<Longrightarrow> (X \<inter> Y) face_of X \<and> (X \<inter> Y) face_of Y"
```
```  2905       and "finite I"
```
```  2906     shows "\<exists>\<G>. \<Union>\<G> = \<Union>\<F> \<and>
```
```  2907                  finite \<G> \<and>
```
```  2908                  (\<forall>C \<in> \<G>. \<exists>D. D \<in> \<F> \<and> C \<subseteq> D) \<and>
```
```  2909                  (\<forall>C \<in> \<F>. \<forall>x \<in> C. \<exists>D. D \<in> \<G> \<and> x \<in> D \<and> D \<subseteq> C) \<and>
```
```  2910                  (\<forall>X \<in> \<G>. polytope X) \<and>
```
```  2911                  (\<forall>X \<in> \<G>. aff_dim X \<le> d) \<and>
```
```  2912                  (\<forall>X \<in> \<G>. \<forall>Y \<in> \<G>. X \<inter> Y face_of X \<and> X \<inter> Y face_of Y) \<and>
```
```  2913                  (\<forall>X \<in> \<G>. \<forall>x \<in> X. \<forall>y \<in> X. \<forall>a b.
```
```  2914                           (a,b) \<in> I \<longrightarrow> a \<bullet> x \<le> b \<and> a \<bullet> y \<le> b \<or>
```
```  2915                                         a \<bullet> x \<ge> b \<and> a \<bullet> y \<ge> b)"
```
```  2916   using \<open>finite I\<close>
```
```  2917 proof induction
```
```  2918   case empty
```
```  2919   then show ?case
```
```  2920     by (rule_tac x="\<F>" in exI) (auto simp: assms)
```
```  2921 next
```
```  2922   case (insert ab I)
```
```  2923   then obtain \<G> where eq: "\<Union>\<G> = \<Union>\<F>" and "finite \<G>"
```
```  2924                    and sub1: "\<And>C. C \<in> \<G> \<Longrightarrow> \<exists>D. D \<in> \<F> \<and> C \<subseteq> D"
```
```  2925                    and sub2: "\<And>C x. C \<in> \<F> \<and> x \<in> C \<Longrightarrow> \<exists>D. D \<in> \<G> \<and> x \<in> D \<and> D \<subseteq> C"
```
```  2926                    and poly: "\<And>X. X \<in> \<G> \<Longrightarrow> polytope X"
```
```  2927                    and aff: "\<And>X. X \<in> \<G> \<Longrightarrow> aff_dim X \<le> d"
```
```  2928                    and face: "\<And>X Y. \<lbrakk>X \<in> \<G>; Y \<in> \<G>\<rbrakk> \<Longrightarrow> X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  2929                    and I: "\<And>X x y a b.  \<lbrakk>X \<in> \<G>; x \<in> X; y \<in> X; (a,b) \<in> I\<rbrakk> \<Longrightarrow>
```
```  2930                                     a \<bullet> x \<le> b \<and> a \<bullet> y \<le> b \<or> a \<bullet> x \<ge> b \<and> a \<bullet> y \<ge> b"
```
```  2931     by (auto simp: that)
```
```  2932   obtain a b where "ab = (a,b)"
```
```  2933     by fastforce
```
```  2934   let ?\<G> = "(\<lambda>X. X \<inter> {x. a \<bullet> x \<le> b}) ` \<G> \<union> (\<lambda>X. X \<inter> {x. a \<bullet> x \<ge> b}) ` \<G>"
```
```  2935   have eqInt: "(S \<inter> Collect P) \<inter> (T \<inter> Collect Q) = (S \<inter> T) \<inter> (Collect P \<inter> Collect Q)" for S T::"'a set" and P Q
```
```  2936     by blast
```
```  2937   show ?case
```
```  2938   proof (intro conjI exI)
```
```  2939     show "\<Union>?\<G> = \<Union>\<F>"
```
```  2940       by (force simp: eq [symmetric])
```
```  2941     show "finite ?\<G>"
```
```  2942       using \<open>finite \<G>\<close> by force
```
```  2943     show "\<forall>X \<in> ?\<G>. polytope X"
```
```  2944       by (force simp: poly polytope_Int_polyhedron polyhedron_halfspace_le polyhedron_halfspace_ge)
```
```  2945     show "\<forall>X \<in> ?\<G>. aff_dim X \<le> d"
```
```  2946       by (auto; metis order_trans aff aff_dim_subset inf_le1)
```
```  2947     show "\<forall>X \<in> ?\<G>. \<forall>x \<in> X. \<forall>y \<in> X. \<forall>a b.
```
```  2948                           (a,b) \<in> insert ab I \<longrightarrow> a \<bullet> x \<le> b \<and> a \<bullet> y \<le> b \<or>
```
```  2949                                                   a \<bullet> x \<ge> b \<and> a \<bullet> y \<ge> b"
```
```  2950       using \<open>ab = (a, b)\<close> I by fastforce
```
```  2951     show "\<forall>X \<in> ?\<G>. \<forall>Y \<in> ?\<G>. X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  2952       by (auto simp: eqInt halfspace_Int_eq face_of_Int_Int face face_of_halfspace_le face_of_halfspace_ge)
```
```  2953     show "\<forall>C \<in> ?\<G>. \<exists>D. D \<in> \<F> \<and> C \<subseteq> D"
```
```  2954       using sub1 by force
```
```  2955     show "\<forall>C\<in>\<F>. \<forall>x\<in>C. \<exists>D. D \<in> ?\<G> \<and> x \<in> D \<and> D \<subseteq> C"
```
```  2956     proof (intro ballI)
```
```  2957       fix C z
```
```  2958       assume "C \<in> \<F>" "z \<in> C"
```
```  2959       with sub2 obtain D where D: "D \<in> \<G>" "z \<in> D" "D \<subseteq> C" by blast
```
```  2960       have "D \<in> \<G> \<and> z \<in> D \<inter> {x. a \<bullet> x \<le> b} \<and> D \<inter> {x. a \<bullet> x \<le> b} \<subseteq> C \<or>
```
```  2961             D \<in> \<G> \<and> z \<in> D \<inter> {x. a \<bullet> x \<ge> b} \<and> D \<inter> {x. a \<bullet> x \<ge> b} \<subseteq> C"
```
```  2962         using linorder_class.linear [of "a \<bullet> z" b] D by blast
```
```  2963       then show "\<exists>D. D \<in> ?\<G> \<and> z \<in> D \<and> D \<subseteq> C"
```
```  2964         by blast
```
```  2965     qed
```
```  2966   qed
```
```  2967 qed
```
```  2968
```
```  2969
```
```  2970 proposition cell_complex_subdivision_exists:
```
```  2971   fixes \<F> :: "'a::euclidean_space set set"
```
```  2972   assumes "0 < e" "finite \<F>"
```
```  2973       and poly: "\<And>X. X \<in> \<F> \<Longrightarrow> polytope X"
```
```  2974       and aff: "\<And>X. X \<in> \<F> \<Longrightarrow> aff_dim X \<le> d"
```
```  2975       and face: "\<And>X Y. \<lbrakk>X \<in> \<F>; Y \<in> \<F>\<rbrakk> \<Longrightarrow> X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  2976   obtains "\<F>'" where "finite \<F>'" "\<Union>\<F>' = \<Union>\<F>" "\<And>X. X \<in> \<F>' \<Longrightarrow> diameter X < e"
```
```  2977                 "\<And>X. X \<in> \<F>' \<Longrightarrow> polytope X" "\<And>X. X \<in> \<F>' \<Longrightarrow> aff_dim X \<le> d"
```
```  2978                 "\<And>X Y. \<lbrakk>X \<in> \<F>'; Y \<in> \<F>'\<rbrakk> \<Longrightarrow> X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  2979                 "\<And>C. C \<in> \<F>' \<Longrightarrow> \<exists>D. D \<in> \<F> \<and> C \<subseteq> D"
```
```  2980                 "\<And>C x. C \<in> \<F> \<and> x \<in> C \<Longrightarrow> \<exists>D. D \<in> \<F>' \<and> x \<in> D \<and> D \<subseteq> C"
```
```  2981 proof -
```
```  2982   have "bounded(\<Union>\<F>)"
```
```  2983     by (simp add: \<open>finite \<F>\<close> poly bounded_Union polytope_imp_bounded)
```
```  2984   then obtain B where "B > 0" and B: "\<And>x. x \<in> \<Union>\<F> \<Longrightarrow> norm x < B"
```
```  2985     by (meson bounded_pos_less)
```
```  2986   define C where "C \<equiv> {z \<in> \<int>. \<bar>z * e / 2 / real DIM('a)\<bar> \<le> B}"
```
```  2987   define I where "I \<equiv> \<Union>i \<in> Basis. \<Union>j \<in> C. { (i::'a, j * e / 2 / DIM('a)) }"
```
```  2988   have "finite C"
```
```  2989     using finite_int_segment [of "-B / (e / 2 / DIM('a))" "B / (e / 2 / DIM('a))"]
```
```  2990     apply (simp add: C_def)
```
```  2991     apply (erule rev_finite_subset)
```
```  2992     using \<open>0 < e\<close>
```
```  2993     apply (auto simp: divide_simps)
```
```  2994     done
```
```  2995   then have "finite I"
```
```  2996     by (simp add: I_def)
```
```  2997   obtain \<F>' where eq: "\<Union>\<F>' = \<Union>\<F>" and "finite \<F>'"
```
```  2998               and poly: "\<And>X. X \<in> \<F>' \<Longrightarrow> polytope X"
```
```  2999               and aff: "\<And>X. X \<in> \<F>' \<Longrightarrow> aff_dim X \<le> d"
```
```  3000               and face: "\<And>X Y. \<lbrakk>X \<in> \<F>'; Y \<in> \<F>'\<rbrakk> \<Longrightarrow> X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  3001               and I: "\<And>X x y a b.  \<lbrakk>X \<in> \<F>'; x \<in> X; y \<in> X; (a,b) \<in> I\<rbrakk> \<Longrightarrow>
```
```  3002                                      a \<bullet> x \<le> b \<and> a \<bullet> y \<le> b \<or> a \<bullet> x \<ge> b \<and> a \<bullet> y \<ge> b"
```
```  3003               and sub1: "\<And>C. C \<in> \<F>' \<Longrightarrow> \<exists>D. D \<in> \<F> \<and> C \<subseteq> D"
```
```  3004               and sub2: "\<And>C x. C \<in> \<F> \<and> x \<in> C \<Longrightarrow> \<exists>D. D \<in> \<F>' \<and> x \<in> D \<and> D \<subseteq> C"
```
```  3005     apply (rule exE [OF cell_subdivision_lemma])
```
```  3006     using assms \<open>finite I\<close> apply auto
```
```  3007     done
```
```  3008   show ?thesis
```
```  3009   proof (rule_tac \<F>'="\<F>'" in that)
```
```  3010     show "diameter X < e" if "X \<in> \<F>'" for X
```
```  3011     proof -
```
```  3012       have "diameter X \<le> e/2"
```
```  3013       proof (rule diameter_le)
```
```  3014         show "norm (x - y) \<le> e / 2" if "x \<in> X" "y \<in> X" for x y
```
```  3015         proof -
```
```  3016           have "norm x < B" "norm y < B"
```
```  3017             using B \<open>X \<in> \<F>'\<close> eq that by fastforce+
```
```  3018           have "norm (x - y) \<le> (\<Sum>b\<in>Basis. \<bar>(x-y) \<bullet> b\<bar>)"
```
```  3019             by (rule norm_le_l1)
```
```  3020           also have "... \<le> of_nat (DIM('a)) * (e / 2 / DIM('a))"
```
```  3021           proof (rule sum_bounded_above)
```
```  3022             fix i::'a
```
```  3023             assume "i \<in> Basis"
```
```  3024             then have I': "\<And>z b. \<lbrakk>z \<in> C; b = z * e / (2 * real DIM('a))\<rbrakk> \<Longrightarrow> i \<bullet> x \<le> b \<and> i \<bullet> y \<le> b \<or> i \<bullet> x \<ge> b \<and> i \<bullet> y \<ge> b"
```
```  3025               using I \<open>X \<in> \<F>'\<close> that
```
```  3026               by (fastforce simp: I_def)
```
```  3027             show "\<bar>(x - y) \<bullet> i\<bar> \<le> e / 2 / real DIM('a)"
```
```  3028             proof (rule ccontr)
```
```  3029               assume "\<not> \<bar>(x - y) \<bullet> i\<bar> \<le> e / 2 / real DIM('a)"
```
```  3030               then have xyi: "\<bar>i \<bullet> x - i \<bullet> y\<bar> > e / 2 / real DIM('a)"
```
```  3031                 by (simp add: inner_commute inner_diff_right)
```
```  3032               obtain n where "n \<in> \<int>" and n: "i \<bullet> x < n * (e / 2 / real DIM('a)) \<and> n * (e / 2 / real DIM('a)) < i \<bullet> y \<or> i \<bullet> y < n * (e / 2 / real DIM('a)) \<and> n * (e / 2 / real DIM('a)) < i \<bullet> x"
```
```  3033                 using subdivide_interval [OF xyi] DIM_positive \<open>0 < e\<close>
```
```  3034                 by (auto simp: zero_less_divide_iff)
```
```  3035               have "\<bar>i \<bullet> x\<bar> < B"
```
```  3036                 by (metis \<open>i \<in> Basis\<close> \<open>norm x < B\<close> inner_commute norm_bound_Basis_lt)
```
```  3037               have "\<bar>i \<bullet> y\<bar> < B"
```
```  3038                 by (metis \<open>i \<in> Basis\<close> \<open>norm y < B\<close> inner_commute norm_bound_Basis_lt)
```
```  3039               have *: "\<bar>n * e\<bar> \<le> B * (2 * real DIM('a))"
```
```  3040                       if "\<bar>ix\<bar> < B" "\<bar>iy\<bar> < B"
```
```  3041                          and ix: "ix * (2 * real DIM('a)) < n * e"
```
```  3042                          and iy: "n * e < iy * (2 * real DIM('a))" for ix iy
```
```  3043               proof (rule abs_leI)
```
```  3044                 have "iy * (2 * real DIM('a)) \<le> B * (2 * real DIM('a))"
```
```  3045                   by (rule mult_right_mono) (use \<open>\<bar>iy\<bar> < B\<close> in linarith)+
```
```  3046                 then show "n * e \<le> B * (2 * real DIM('a))"
```
```  3047                   using iy by linarith
```
```  3048               next
```
```  3049                 have "- ix * (2 * real DIM('a)) \<le> B * (2 * real DIM('a))"
```
```  3050                   by (rule mult_right_mono) (use \<open>\<bar>ix\<bar> < B\<close> in linarith)+
```
```  3051                 then show "- (n * e) \<le> B * (2 * real DIM('a))"
```
```  3052                   using ix by linarith
```
```  3053               qed
```
```  3054               have "n \<in> C"
```
```  3055                 using \<open>n \<in> \<int>\<close> n  by (auto simp: C_def divide_simps intro: * \<open>\<bar>i \<bullet> x\<bar> < B\<close> \<open>\<bar>i \<bullet> y\<bar> < B\<close>)
```
```  3056               show False
```
```  3057                 using  I' [OF \<open>n \<in> C\<close> refl] n  by auto
```
```  3058             qed
```
```  3059           qed
```
```  3060           also have "... = e / 2"
```
```  3061             by simp
```
```  3062           finally show ?thesis .
```
```  3063         qed
```
```  3064       qed (use \<open>0 < e\<close> in force)
```
```  3065       also have "... < e"
```
```  3066         by (simp add: \<open>0 < e\<close>)
```
```  3067       finally show ?thesis .
```
```  3068     qed
```
```  3069   qed (auto simp: eq poly aff face sub1 sub2 \<open>finite \<F>'\<close>)
```
```  3070 qed
```
```  3071
```
```  3072
```
```  3073 subsection\<open>Simplexes\<close>
```
```  3074
```
```  3075 text\<open>The notion of n-simplex for integer @{term"n \<ge> -1"}\<close>
```
```  3076 definition simplex :: "int \<Rightarrow> 'a::euclidean_space set \<Rightarrow> bool" (infix "simplex" 50)
```
```  3077   where "n simplex S \<equiv> \<exists>C. ~(affine_dependent C) \<and> int(card C) = n + 1 \<and> S = convex hull C"
```
```  3078
```
```  3079 lemma simplex:
```
```  3080     "n simplex S \<longleftrightarrow> (\<exists>C. finite C \<and>
```
```  3081                        ~(affine_dependent C) \<and>
```
```  3082                        int(card C) = n + 1 \<and>
```
```  3083                        S = convex hull C)"
```
```  3084   by (auto simp add: simplex_def intro: aff_independent_finite)
```
```  3085
```
```  3086 lemma simplex_convex_hull:
```
```  3087    "~affine_dependent C \<and> int(card C) = n + 1 \<Longrightarrow> n simplex (convex hull C)"
```
```  3088   by (auto simp add: simplex_def)
```
```  3089
```
```  3090 lemma convex_simplex: "n simplex S \<Longrightarrow> convex S"
```
```  3091   by (metis convex_convex_hull simplex_def)
```
```  3092
```
```  3093 lemma compact_simplex: "n simplex S \<Longrightarrow> compact S"
```
```  3094   unfolding simplex
```
```  3095   using finite_imp_compact_convex_hull by blast
```
```  3096
```
```  3097 lemma closed_simplex: "n simplex S \<Longrightarrow> closed S"
```
```  3098   by (simp add: compact_imp_closed compact_simplex)
```
```  3099
```
```  3100 lemma simplex_imp_polytope:
```
```  3101    "n simplex S \<Longrightarrow> polytope S"
```
```  3102   unfolding simplex_def polytope_def
```
```  3103   using aff_independent_finite by blast
```
```  3104
```
```  3105 lemma simplex_imp_polyhedron:
```
```  3106    "n simplex S \<Longrightarrow> polyhedron S"
```
```  3107   by (simp add: polytope_imp_polyhedron simplex_imp_polytope)
```
```  3108
```
```  3109 lemma simplex_dim_ge: "n simplex S \<Longrightarrow> -1 \<le> n"
```
```  3110   by (metis (no_types, hide_lams) aff_dim_geq affine_independent_iff_card diff_add_cancel diff_diff_eq2 simplex_def)
```
```  3111
```
```  3112 lemma simplex_empty [simp]: "n simplex {} \<longleftrightarrow> n = -1"
```
```  3113 proof
```
```  3114   assume "n simplex {}"
```
```  3115   then show "n = -1"
```
```  3116     unfolding simplex by (metis card_empty convex_hull_eq_empty diff_0 diff_eq_eq of_nat_0)
```
```  3117 next
```
```  3118   assume "n = -1" then show "n simplex {}"
```
```  3119     by (fastforce simp: simplex)
```
```  3120 qed
```
```  3121
```
```  3122 lemma simplex_minus_1 [simp]: "-1 simplex S \<longleftrightarrow> S = {}"
```
```  3123   by (metis simplex cancel_comm_monoid_add_class.diff_cancel card_0_eq diff_minus_eq_add of_nat_eq_0_iff simplex_empty)
```
```  3124
```
```  3125
```
```  3126 lemma aff_dim_simplex:
```
```  3127    "n simplex S \<Longrightarrow> aff_dim S = n"
```
```  3128   by (metis simplex add.commute add_diff_cancel_left' aff_dim_convex_hull affine_independent_iff_card)
```
```  3129
```
```  3130 lemma zero_simplex_sing: "0 simplex {a}"
```
```  3131   apply (simp add: simplex_def)
```
```  3132   by (metis affine_independent_1 card_empty card_insert_disjoint convex_hull_singleton empty_iff finite.emptyI)
```
```  3133
```
```  3134 lemma simplex_sing [simp]: "n simplex {a} \<longleftrightarrow> n = 0"
```
```  3135   using aff_dim_simplex aff_dim_sing zero_simplex_sing by blast
```
```  3136
```
```  3137 lemma simplex_zero: "0 simplex S \<longleftrightarrow> (\<exists>a. S = {a})"
```
```  3138 apply (auto simp: )
```
```  3139   using aff_dim_eq_0 aff_dim_simplex by blast
```
```  3140
```
```  3141 lemma one_simplex_segment: "a \<noteq> b \<Longrightarrow> 1 simplex closed_segment a b"
```
```  3142   apply (simp add: simplex_def)
```
```  3143   apply (rule_tac x="{a,b}" in exI)
```
```  3144   apply (auto simp: segment_convex_hull)
```
```  3145   done
```
```  3146
```
```  3147 lemma simplex_segment_cases:
```
```  3148    "(if a = b then 0 else 1) simplex closed_segment a b"
```
```  3149   by (auto simp: one_simplex_segment)
```
```  3150
```
```  3151 lemma simplex_segment:
```
```  3152    "\<exists>n. n simplex closed_segment a b"
```
```  3153   using simplex_segment_cases by metis
```
```  3154
```
```  3155 lemma polytope_lowdim_imp_simplex:
```
```  3156   assumes "polytope P" "aff_dim P \<le> 1"
```
```  3157   obtains n where "n simplex P"
```
```  3158 proof (cases "P = {}")
```
```  3159   case True
```
```  3160   then show ?thesis
```
```  3161     by (simp add: that)
```
```  3162 next
```
```  3163   case False
```
```  3164   then show ?thesis
```
```  3165     by (metis assms compact_convex_collinear_segment collinear_aff_dim polytope_imp_compact polytope_imp_convex simplex_segment_cases that)
```
```  3166 qed
```
```  3167
```
```  3168 lemma simplex_insert_dimplus1:
```
```  3169   fixes n::int
```
```  3170   assumes "n simplex S" and a: "a \<notin> affine hull S"
```
```  3171   shows "(n+1) simplex (convex hull (insert a S))"
```
```  3172 proof -
```
```  3173   obtain C where C: "finite C" "~(affine_dependent C)" "int(card C) = n+1" and S: "S = convex hull C"
```
```  3174     using assms unfolding simplex by force
```
```  3175   show ?thesis
```
```  3176     unfolding simplex
```
```  3177   proof (intro exI conjI)
```
```  3178       have "aff_dim S = n"
```
```  3179         using aff_dim_simplex assms(1) by blast
```
```  3180       moreover have "a \<notin> affine hull C"
```
```  3181         using S a affine_hull_convex_hull by blast
```
```  3182       moreover have "a \<notin> C"
```
```  3183           using S a hull_inc by fastforce
```
```  3184       ultimately show "\<not> affine_dependent (insert a C)"
```
```  3185         by (simp add: C S aff_dim_convex_hull aff_dim_insert affine_independent_iff_card)
```
```  3186   next
```
```  3187     have "a \<notin> C"
```
```  3188       using S a hull_inc by fastforce
```
```  3189     then show "int (card (insert a C)) = n + 1 + 1"
```
```  3190       by (simp add: C)
```
```  3191   next
```
```  3192     show "convex hull insert a S = convex hull (insert a C)"
```
```  3193       by (simp add: S convex_hull_insert_segments)
```
```  3194   qed (use C in auto)
```
```  3195 qed
```
```  3196
```
```  3197 subsection%important \<open>Simplicial complexes and triangulations\<close>
```
```  3198
```
```  3199 definition%important simplicial_complex where
```
```  3200  "simplicial_complex \<C> \<equiv>
```
```  3201         finite \<C> \<and>
```
```  3202         (\<forall>S \<in> \<C>. \<exists>n. n simplex S) \<and>
```
```  3203         (\<forall>F S. S \<in> \<C> \<and> F face_of S \<longrightarrow> F \<in> \<C>) \<and>
```
```  3204         (\<forall>S S'. S \<in> \<C> \<and> S' \<in> \<C>
```
```  3205                 \<longrightarrow> (S \<inter> S') face_of S \<and> (S \<inter> S') face_of S')"
```
```  3206
```
```  3207 definition%important triangulation where
```
```  3208  "triangulation \<T> \<equiv>
```
```  3209         finite \<T> \<and>
```
```  3210         (\<forall>T \<in> \<T>. \<exists>n. n simplex T) \<and>
```
```  3211         (\<forall>T T'. T \<in> \<T> \<and> T' \<in> \<T>
```
```  3212                 \<longrightarrow> (T \<inter> T') face_of T \<and> (T \<inter> T') face_of T')"
```
```  3213
```
```  3214
```
```  3215 subsection%important\<open>Refining a cell complex to a simplicial complex\<close>
```
```  3216
```
```  3217 lemma%important convex_hull_insert_Int_eq:
```
```  3218   fixes z :: "'a :: euclidean_space"
```
```  3219   assumes z: "z \<in> rel_interior S"
```
```  3220       and T: "T \<subseteq> rel_frontier S"
```
```  3221       and U: "U \<subseteq> rel_frontier S"
```
```  3222       and "convex S" "convex T" "convex U"
```
```  3223   shows "convex hull (insert z T) \<inter> convex hull (insert z U) = convex hull (insert z (T \<inter> U))"
```
```  3224     (is "?lhs = ?rhs")
```
```  3225 proof%unimportant
```
```  3226   show "?lhs \<subseteq> ?rhs"
```
```  3227   proof (cases "T={} \<or> U={}")
```
```  3228     case True then show ?thesis by auto
```
```  3229   next
```
```  3230     case False
```
```  3231     then have "T \<noteq> {}" "U \<noteq> {}" by auto
```
```  3232     have TU: "convex (T \<inter> U)"
```
```  3233       by (simp add: \<open>convex T\<close> \<open>convex U\<close> convex_Int)
```
```  3234     have "(\<Union>x\<in>T. closed_segment z x) \<inter> (\<Union>x\<in>U. closed_segment z x)
```
```  3235           \<subseteq> (if T \<inter> U = {} then {z} else \<Union>((closed_segment z) ` (T \<inter> U)))" (is "_ \<subseteq> ?IF")
```
```  3236     proof clarify
```
```  3237       fix x t u
```
```  3238       assume xt: "x \<in> closed_segment z t"
```
```  3239         and xu: "x \<in> closed_segment z u"
```
```  3240         and "t \<in> T" "u \<in> U"
```
```  3241       then have ne: "t \<noteq> z" "u \<noteq> z"
```
```  3242         using T U z unfolding rel_frontier_def by blast+
```
```  3243       show "x \<in> ?IF"
```
```  3244       proof (cases "x = z")
```
```  3245         case True then show ?thesis by auto
```
```  3246       next
```
```  3247         case False
```
```  3248         have t: "t \<in> closure S"
```
```  3249           using T \<open>t \<in> T\<close> rel_frontier_def by auto
```
```  3250         have u: "u \<in> closure S"
```
```  3251           using U \<open>u \<in> U\<close> rel_frontier_def by auto
```
```  3252         show ?thesis
```
```  3253         proof (cases "t = u")
```
```  3254           case True
```
```  3255           then show ?thesis
```
```  3256             using \<open>t \<in> T\<close> \<open>u \<in> U\<close> xt by auto
```
```  3257         next
```
```  3258           case False
```
```  3259           have tnot: "t \<notin> closed_segment u z"
```
```  3260           proof -
```
```  3261             have "t \<in> closure S - rel_interior S"
```
```  3262               using T \<open>t \<in> T\<close> rel_frontier_def by blast
```
```  3263             then have "t \<notin> open_segment z u"
```
```  3264               by (meson DiffD2 rel_interior_closure_convex_segment [OF \<open>convex S\<close> z u] subsetD)
```
```  3265             then show ?thesis
```
```  3266               by (simp add: \<open>t \<noteq> u\<close> \<open>t \<noteq> z\<close> open_segment_commute open_segment_def)
```
```  3267           qed
```
```  3268           moreover have "u \<notin> closed_segment z t"
```
```  3269             using rel_interior_closure_convex_segment [OF \<open>convex S\<close> z t] \<open>u \<in> U\<close> \<open>u \<noteq> z\<close>
```
```  3270               U [unfolded rel_frontier_def] tnot
```
```  3271             by (auto simp: closed_segment_eq_open)
```
```  3272           ultimately
```
```  3273           have "~(between (t,u) z | between (u,z) t | between (z,t) u)" if "x \<noteq> z"
```
```  3274             using that xt xu
```
```  3275             apply (simp add: between_mem_segment [symmetric])
```
```  3276             by (metis between_commute between_trans_2 between_antisym)
```
```  3277           then have "~ collinear {t, z, u}" if "x \<noteq> z"
```
```  3278             by (auto simp: that collinear_between_cases between_commute)
```
```  3279           moreover have "collinear {t, z, x}"
```
```  3280             by (metis closed_segment_commute collinear_2 collinear_closed_segment collinear_triples ends_in_segment(1) insert_absorb insert_absorb2 xt)
```
```  3281           moreover have "collinear {z, x, u}"
```
```  3282             by (metis closed_segment_commute collinear_2 collinear_closed_segment collinear_triples ends_in_segment(1) insert_absorb insert_absorb2 xu)
```
```  3283           ultimately have False
```
```  3284             using collinear_3_trans [of t z x u] \<open>x \<noteq> z\<close> by blast
```
```  3285           then show ?thesis by metis
```
```  3286         qed
```
```  3287       qed
```
```  3288     qed
```
```  3289     then show ?thesis
```
```  3290       using False \<open>convex T\<close> \<open>convex U\<close> TU
```
```  3291       by (simp add: convex_hull_insert_segments hull_same split: if_split_asm)
```
```  3292   qed
```
```  3293   show "?rhs \<subseteq> ?lhs"
```
```  3294     by (metis inf_greatest hull_mono inf.cobounded1 inf.cobounded2 insert_mono)
```
```  3295 qed
```
```  3296
```
```  3297 lemma%important simplicial_subdivision_aux:
```
```  3298   assumes "finite \<M>"
```
```  3299       and "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3300       and "\<And>C. C \<in> \<M> \<Longrightarrow> aff_dim C \<le> of_nat n"
```
```  3301       and "\<And>C F. \<lbrakk>C \<in> \<M>; F face_of C\<rbrakk> \<Longrightarrow> F \<in> \<M>"
```
```  3302       and "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3303     shows "\<exists>\<T>. simplicial_complex \<T> \<and>
```
```  3304                 (\<forall>K \<in> \<T>. aff_dim K \<le> of_nat n) \<and>
```
```  3305                 \<Union>\<T> = \<Union>\<M> \<and>
```
```  3306                 (\<forall>C \<in> \<M>. \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F) \<and>
```
```  3307                 (\<forall>K \<in> \<T>. \<exists>C. C \<in> \<M> \<and> K \<subseteq> C)"
```
```  3308   using assms
```
```  3309 proof%unimportant (induction n arbitrary: \<M> rule: less_induct)
```
```  3310   case (less n)
```
```  3311   then have poly\<M>: "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3312       and aff\<M>:    "\<And>C. C \<in> \<M> \<Longrightarrow> aff_dim C \<le> of_nat n"
```
```  3313       and face\<M>:   "\<And>C F. \<lbrakk>C \<in> \<M>; F face_of C\<rbrakk> \<Longrightarrow> F \<in> \<M>"
```
```  3314       and intface\<M>: "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3315     by metis+
```
```  3316   show ?case
```
```  3317   proof (cases "n \<le> 1")
```
```  3318     case True
```
```  3319     have "\<And>s. \<lbrakk>n \<le> 1; s \<in> \<M>\<rbrakk> \<Longrightarrow> \<exists>m. m simplex s"
```
```  3320       using poly\<M> aff\<M> by (force intro: polytope_lowdim_imp_simplex)
```
```  3321     then show ?thesis
```
```  3322       unfolding simplicial_complex_def
```
```  3323       apply (rule_tac x="\<M>" in exI)
```
```  3324       using True by (auto simp: less.prems)
```
```  3325   next
```
```  3326     case False
```
```  3327     define \<S> where "\<S> \<equiv> {C \<in> \<M>. aff_dim C < n}"
```
```  3328     have "finite \<S>" "\<And>C. C \<in> \<S> \<Longrightarrow> polytope C" "\<And>C. C \<in> \<S> \<Longrightarrow> aff_dim C \<le> int (n - 1)"
```
```  3329          "\<And>C F. \<lbrakk>C \<in> \<S>; F face_of C\<rbrakk> \<Longrightarrow> F \<in> \<S>"
```
```  3330          "\<And>C1 C2. \<lbrakk>C1 \<in> \<S>; C2 \<in> \<S>\<rbrakk>  \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3331       using less.prems
```
```  3332       apply (auto simp: \<S>_def)
```
```  3333       by (metis aff_dim_subset face_of_imp_subset less_le not_le)
```
```  3334     with less.IH [of "n-1" \<S>] False
```
```  3335     obtain \<U> where "simplicial_complex \<U>"
```
```  3336            and aff_dim\<U>: "\<And>K. K \<in> \<U> \<Longrightarrow> aff_dim K \<le> int (n - 1)"
```
```  3337            and        "\<Union>\<U> = \<Union>\<S>"
```
```  3338            and fin\<U>:  "\<And>C. C \<in> \<S> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<U> \<and> C = \<Union>F"
```
```  3339            and C\<U>:    "\<And>K. K \<in> \<U> \<Longrightarrow> \<exists>C. C \<in> \<S> \<and> K \<subseteq> C"
```
```  3340       by auto
```
```  3341     then have "finite \<U>"
```
```  3342          and simpl\<U>: "\<And>S. S \<in> \<U> \<Longrightarrow> \<exists>n. n simplex S"
```
```  3343          and face\<U>:  "\<And>F S. \<lbrakk>S \<in> \<U>; F face_of S\<rbrakk> \<Longrightarrow> F \<in> \<U>"
```
```  3344          and faceI\<U>: "\<And>S S'. \<lbrakk>S \<in> \<U>; S' \<in> \<U>\<rbrakk> \<Longrightarrow> (S \<inter> S') face_of S \<and> (S \<inter> S') face_of S'"
```
```  3345       by (auto simp: simplicial_complex_def)
```
```  3346     define \<N> where "\<N> \<equiv> {C \<in> \<M>. aff_dim C = n}"
```
```  3347     have "finite \<N>"
```
```  3348       by (simp add: \<N>_def less.prems(1))
```
```  3349     have poly\<N>: "\<And>C. C \<in> \<N> \<Longrightarrow> polytope C"
```
```  3350       and convex\<N>: "\<And>C. C \<in> \<N> \<Longrightarrow> convex C"
```
```  3351       and closed\<N>: "\<And>C. C \<in> \<N> \<Longrightarrow> closed C"
```
```  3352       by (auto simp: \<N>_def poly\<M> polytope_imp_convex polytope_imp_closed)
```
```  3353     have in_rel_interior: "(SOME z. z \<in> rel_interior C) \<in> rel_interior C" if "C \<in> \<N>" for C
```
```  3354         using that poly\<M> polytope_imp_convex rel_interior_aff_dim some_in_eq by (fastforce simp: \<N>_def)
```
```  3355     have *: "\<exists>T. ~affine_dependent T \<and> card T \<le> n \<and> aff_dim K < n \<and> K = convex hull T"
```
```  3356       if "K \<in> \<U>" for K
```
```  3357     proof -
```
```  3358       obtain r where r: "r simplex K"
```
```  3359         using \<open>K \<in> \<U>\<close> simpl\<U> by blast
```
```  3360       have "r = aff_dim K"
```
```  3361         using \<open>r simplex K\<close> aff_dim_simplex by blast
```
```  3362       with r
```
```  3363       show ?thesis
```
```  3364         unfolding simplex_def
```
```  3365         using False \<open>\<And>K. K \<in> \<U> \<Longrightarrow> aff_dim K \<le> int (n - 1)\<close> that by fastforce
```
```  3366     qed
```
```  3367     have ahK_C_disjoint: "affine hull K \<inter> rel_interior C = {}"
```
```  3368       if "C \<in> \<N>" "K \<in> \<U>" "K \<subseteq> rel_frontier C" for C K
```
```  3369     proof -
```
```  3370       have "convex C" "closed C"
```
```  3371         by (auto simp: convex\<N> closed\<N> \<open>C \<in> \<N>\<close>)
```
```  3372       obtain F where F: "F face_of C" and "F \<noteq> C" "K \<subseteq> F"
```
```  3373       proof -
```
```  3374         obtain L where "L \<in> \<S>" "K \<subseteq> L"
```
```  3375           using \<open>K \<in> \<U>\<close> C\<U> by blast
```
```  3376         have "K \<le> rel_frontier C"
```
```  3377           by (simp add: \<open>K \<subseteq> rel_frontier C\<close>)
```
```  3378         also have "... \<le> C"
```
```  3379           by (simp add: \<open>closed C\<close> rel_frontier_def subset_iff)
```
```  3380         finally have "K \<subseteq> C" .
```
```  3381         have "L \<inter> C face_of C"
```
```  3382           using \<N>_def \<S>_def \<open>C \<in> \<N>\<close> \<open>L \<in> \<S>\<close> intface\<M> by auto
```
```  3383         moreover have "L \<inter> C \<noteq> C"
```
```  3384           using \<open>C \<in> \<N>\<close> \<open>L \<in> \<S>\<close>
```
```  3385           apply (clarsimp simp: \<N>_def \<S>_def)
```
```  3386           by (metis aff_dim_subset inf_le1 not_le)
```
```  3387         moreover have "K \<subseteq> L \<inter> C"
```
```  3388           using \<open>C \<in> \<N>\<close> \<open>L \<in> \<S>\<close> \<open>K \<subseteq> C\<close> \<open>K \<subseteq> L\<close>
```
```  3389           by (auto simp: \<N>_def \<S>_def)
```
```  3390         ultimately show ?thesis using that by metis
```
```  3391       qed
```
```  3392       have "affine hull F \<inter> rel_interior C = {}"
```
```  3393         by (rule affine_hull_face_of_disjoint_rel_interior [OF \<open>convex C\<close> F \<open>F \<noteq> C\<close>])
```
```  3394       with hull_mono [OF \<open>K \<subseteq> F\<close>]
```
```  3395       show "affine hull K \<inter> rel_interior C = {}"
```
```  3396         by fastforce
```
```  3397     qed
```
```  3398     let ?\<T> = "(\<Union>C \<in> \<N>. \<Union>K \<in> \<U> \<inter> Pow (rel_frontier C).
```
```  3399                      {convex hull (insert (SOME z. z \<in> rel_interior C) K)})"
```
```  3400     have "\<exists>\<T>. simplicial_complex \<T> \<and>
```
```  3401               (\<forall>K \<in> \<T>. aff_dim K \<le> of_nat n) \<and>
```
```  3402               (\<forall>C \<in> \<M>. \<exists>F. F \<subseteq> \<T> \<and> C = \<Union>F) \<and>
```
```  3403               (\<forall>K \<in> \<T>. \<exists>C. C \<in> \<M> \<and> K \<subseteq> C)"
```
```  3404     proof (rule exI, intro conjI ballI)
```
```  3405       show "simplicial_complex (\<U> \<union> ?\<T>)"
```
```  3406         unfolding simplicial_complex_def
```
```  3407       proof (intro conjI impI ballI allI)
```
```  3408         show "finite (\<U> \<union> ?\<T>)"
```
```  3409           using \<open>finite \<U>\<close> \<open>finite \<N>\<close> by simp
```
```  3410         show "\<exists>n. n simplex S" if "S \<in> \<U> \<union> ?\<T>" for S
```
```  3411           using that ahK_C_disjoint in_rel_interior simpl\<U> simplex_insert_dimplus1 by fastforce
```
```  3412         show "F \<in> \<U> \<union> ?\<T>" if S: "S \<in> \<U> \<union> ?\<T> \<and> F face_of S" for F S
```
```  3413         proof -
```
```  3414           have "F \<in> \<U>" if "S \<in> \<U>"
```
```  3415             using S face\<U> that by blast
```
```  3416           moreover have "F \<in> \<U> \<union> ?\<T>"
```
```  3417             if "F face_of S" "C \<in> \<N>" "K \<in> \<U>" and "K \<subseteq> rel_frontier C"
```
```  3418               and S: "S = convex hull insert (SOME z. z \<in> rel_interior C) K" for C K
```
```  3419           proof -
```
```  3420             let ?z = "SOME z. z \<in> rel_interior C"
```
```  3421             have "?z \<in> rel_interior C"
```
```  3422               by (simp add: in_rel_interior \<open>C \<in> \<N>\<close>)
```
```  3423             moreover
```
```  3424             obtain I where "\<not> affine_dependent I" "card I \<le> n" "aff_dim K < int n" "K = convex hull I"
```
```  3425               using * [OF \<open>K \<in> \<U>\<close>] by auto
```
```  3426             ultimately have "?z \<notin> affine hull I"
```
```  3427               using ahK_C_disjoint affine_hull_convex_hull that by blast
```
```  3428             have "compact I" "finite I"
```
```  3429               by (auto simp: \<open>\<not> affine_dependent I\<close> aff_independent_finite finite_imp_compact)
```
```  3430             moreover have "F face_of convex hull insert ?z I"
```
```  3431               by (metis S \<open>F face_of S\<close> \<open>K = convex hull I\<close> convex_hull_eq_empty convex_hull_insert_segments hull_hull)
```
```  3432             ultimately obtain J where "J \<subseteq> insert ?z I" "F = convex hull J"
```
```  3433               using face_of_convex_hull_subset [of "insert ?z I" F] by auto
```
```  3434             show ?thesis
```
```  3435             proof (cases "?z \<in> J")
```
```  3436               case True
```
```  3437               have "F \<in> (\<Union>K\<in>\<U> \<inter> Pow (rel_frontier C). {convex hull insert ?z K})"
```
```  3438               proof
```
```  3439                 have "convex hull (J - {?z}) face_of K"
```
```  3440                   by (metis True \<open>J \<subseteq> insert ?z I\<close> \<open>K = convex hull I\<close> \<open>\<not> affine_dependent I\<close> face_of_convex_hull_affine_independent subset_insert_iff)
```
```  3441                 then have "convex hull (J - {?z}) \<in> \<U>"
```
```  3442                   by (rule face\<U> [OF \<open>K \<in> \<U>\<close>])
```
```  3443                 moreover
```
```  3444                 have "\<And>x. x \<in> convex hull (J - {?z}) \<Longrightarrow> x \<in> rel_frontier C"
```
```  3445                   by (metis True \<open>J \<subseteq> insert ?z I\<close> \<open>K = convex hull I\<close> subsetD hull_mono subset_insert_iff that(4))
```
```  3446                 ultimately show "convex hull (J - {?z}) \<in> \<U> \<inter> Pow (rel_frontier C)" by auto
```
```  3447                 let ?F = "convex hull insert ?z (convex hull (J - {?z}))"
```
```  3448                 have "F \<subseteq> ?F"
```
```  3449                   apply (clarsimp simp: \<open>F = convex hull J\<close>)
```
```  3450                   by (metis True subsetD hull_mono hull_subset subset_insert_iff)
```
```  3451                 moreover have "?F \<subseteq> F"
```
```  3452                   apply (clarsimp simp: \<open>F = convex hull J\<close>)
```
```  3453                   by (metis (no_types, lifting) True convex_hull_eq_empty convex_hull_insert_segments hull_hull insert_Diff)
```
```  3454                 ultimately
```
```  3455                 show "F \<in> {?F}" by auto
```
```  3456               qed
```
```  3457               with \<open>C\<in>\<N>\<close> show ?thesis by auto
```
```  3458             next
```
```  3459               case False
```
```  3460               then have "F \<in> \<U>"
```
```  3461                 using face_of_convex_hull_affine_independent [OF \<open>\<not> affine_dependent I\<close>]
```
```  3462                 by (metis Int_absorb2 Int_insert_right_if0 \<open>F = convex hull J\<close> \<open>J \<subseteq> insert ?z I\<close> \<open>K = convex hull I\<close> face\<U> inf_le2 \<open>K \<in> \<U>\<close>)
```
```  3463               then show "F \<in> \<U> \<union> ?\<T>"
```
```  3464                 by blast
```
```  3465             qed
```
```  3466           qed
```
```  3467           ultimately show ?thesis
```
```  3468             using that by auto
```
```  3469         qed
```
```  3470         have "(S \<inter> S' face_of S) \<and> (S \<inter> S' face_of S')"
```
```  3471           if "S \<in> \<U> \<union> ?\<T>" "S' \<in> \<U> \<union> ?\<T>" for S S'
```
```  3472         proof -
```
```  3473           have symmy: "\<lbrakk>\<And>X Y. R X Y \<Longrightarrow> R Y X;
```
```  3474                         \<And>X Y. \<lbrakk>X \<in> \<U>; Y \<in> \<U>\<rbrakk> \<Longrightarrow> R X Y;
```
```  3475                         \<And>X Y. \<lbrakk>X \<in> \<U>; Y \<in> ?\<T>\<rbrakk> \<Longrightarrow> R X Y;
```
```  3476                         \<And>X Y. \<lbrakk>X \<in> ?\<T>; Y \<in> ?\<T>\<rbrakk> \<Longrightarrow> R X Y\<rbrakk> \<Longrightarrow> R S S'" for R
```
```  3477             using that by (metis (no_types, lifting) Un_iff)
```
```  3478           show ?thesis
```
```  3479           proof (rule symmy)
```
```  3480             show "Y \<inter> X face_of Y \<and> Y \<inter> X face_of X"
```
```  3481               if "X \<inter> Y face_of X \<and> X \<inter> Y face_of Y" for X Y :: "'a set"
```
```  3482               by (simp add: inf_commute that)
```
```  3483           next
```
```  3484             show "X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  3485               if "X \<in> \<U>" and "Y \<in> \<U>" for X Y
```
```  3486               by (simp add: faceI\<U> that)
```
```  3487           next
```
```  3488             show "X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  3489               if XY: "X \<in> \<U>" "Y \<in> ?\<T>" for X Y
```
```  3490             proof -
```
```  3491               obtain C K
```
```  3492                 where "C \<in> \<N>" "K \<in> \<U>" "K \<subseteq> rel_frontier C"
```
```  3493                 and Y: "Y = convex hull insert (SOME z. z \<in> rel_interior C) K"
```
```  3494                 using XY by blast
```
```  3495               have "convex C"
```
```  3496                 by (simp add: \<open>C \<in> \<N>\<close> convex\<N>)
```
```  3497               have "K \<subseteq> C"
```
```  3498                 by (metis DiffE \<open>C \<in> \<N>\<close> \<open>K \<subseteq> rel_frontier C\<close> closed\<N> closure_closed rel_frontier_def subset_iff)
```
```  3499               let ?z = "(SOME z. z \<in> rel_interior C)"
```
```  3500               have z: "?z \<in> rel_interior C"
```
```  3501                 using \<open>C \<in> \<N>\<close> in_rel_interior by blast
```
```  3502               obtain D where "D \<in> \<S>" "X \<subseteq> D"
```
```  3503                 using C\<U> \<open>X \<in> \<U>\<close> by blast
```
```  3504               have "D \<inter> rel_interior C = (C \<inter> D) \<inter> rel_interior C"
```
```  3505                 using rel_interior_subset by blast
```
```  3506               also have "(C \<inter> D) \<inter> rel_interior C = {}"
```
```  3507               proof (rule face_of_disjoint_rel_interior)
```
```  3508                 show "C \<inter> D face_of C"
```
```  3509                   using \<N>_def \<S>_def \<open>C \<in> \<N>\<close> \<open>D \<in> \<S>\<close> intface\<M> by blast
```
```  3510                 show "C \<inter> D \<noteq> C"
```
```  3511                   by (metis (mono_tags, lifting) Int_lower2 \<N>_def \<S>_def \<open>C \<in> \<N>\<close> \<open>D \<in> \<S>\<close> aff_dim_subset mem_Collect_eq not_le)
```
```  3512               qed
```
```  3513               finally have DC: "D \<inter> rel_interior C = {}" .
```
```  3514               have eq: "X \<inter> convex hull (insert ?z K) = X \<inter> convex hull K"
```
```  3515                 apply (rule Int_convex_hull_insert_rel_exterior [OF \<open>convex C\<close> \<open>K \<subseteq> C\<close> z])
```
```  3516                 using DC by (meson \<open>X \<subseteq> D\<close> disjnt_def disjnt_subset1)
```
```  3517               obtain I where I: "\<not> affine_dependent I"
```
```  3518                          and Keq: "K = convex hull I" and [simp]: "convex hull K = K"
```
```  3519                 using "*" \<open>K \<in> \<U>\<close> by force
```
```  3520               then have "?z \<notin> affine hull I"
```
```  3521                 using ahK_C_disjoint \<open>C \<in> \<N>\<close> \<open>K \<in> \<U>\<close> \<open>K \<subseteq> rel_frontier C\<close> affine_hull_convex_hull z by blast
```
```  3522               have "X \<inter> K face_of K"
```
```  3523                 by (simp add: \<open>K \<in> \<U>\<close> faceI\<U> \<open>X \<in> \<U>\<close>)
```
```  3524               also have "... face_of convex hull insert ?z K"
```
```  3525                 by (metis I Keq \<open>?z \<notin> affine hull I\<close> aff_independent_finite convex_convex_hull face_of_convex_hull_insert face_of_refl hull_insert)
```
```  3526               finally have "X \<inter> K face_of convex hull insert ?z K" .
```
```  3527               then show ?thesis
```
```  3528                 using "*" \<open>K \<in> \<U>\<close> faceI\<U> that(1) by (fastforce simp add: Y eq)
```
```  3529             qed
```
```  3530           next
```
```  3531             show "X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  3532               if XY: "X \<in> ?\<T>" "Y \<in> ?\<T>" for X Y
```
```  3533             proof -
```
```  3534               obtain C K D L
```
```  3535                 where "C \<in> \<N>" "K \<in> \<U>" "K \<subseteq> rel_frontier C"
```
```  3536                 and X: "X = convex hull insert (SOME z. z \<in> rel_interior C) K"
```
```  3537                 and "D \<in> \<N>" "L \<in> \<U>" "L \<subseteq> rel_frontier D"
```
```  3538                 and Y: "Y = convex hull insert (SOME z. z \<in> rel_interior D) L"
```
```  3539                 using XY by blast
```
```  3540               let ?z = "(SOME z. z \<in> rel_interior C)"
```
```  3541               have z: "?z \<in> rel_interior C"
```
```  3542                 using \<open>C \<in> \<N>\<close> in_rel_interior by blast
```
```  3543               have "convex C"
```
```  3544                 by (simp add: \<open>C \<in> \<N>\<close> convex\<N>)
```
```  3545               have "convex K"
```
```  3546                 using "*" \<open>K \<in> \<U>\<close> by blast
```
```  3547               have "convex L"
```
```  3548                 by (meson \<open>L \<in> \<U>\<close> convex_simplex simpl\<U>)
```
```  3549               show ?thesis
```
```  3550               proof (cases "D=C")
```
```  3551                 case True
```
```  3552                 then have "L \<subseteq> rel_frontier C"
```
```  3553                   using \<open>L \<subseteq> rel_frontier D\<close> by auto
```
```  3554                 show ?thesis
```
```  3555                   apply (simp add: X Y True)
```
```  3556                   apply (simp add: convex_hull_insert_Int_eq [OF z] \<open>K \<subseteq> rel_frontier C\<close> \<open>L \<subseteq> rel_frontier C\<close> \<open>convex C\<close> \<open>convex K\<close> \<open>convex L\<close>)
```
```  3557                   using face_of_polytope_insert2
```
```  3558                   by (metis "*" IntI \<open>C \<in> \<N>\<close> \<open>K \<in> \<U>\<close> \<open>L \<in> \<U>\<close>\<open>K \<subseteq> rel_frontier C\<close> \<open>L \<subseteq> rel_frontier C\<close> aff_independent_finite ahK_C_disjoint empty_iff faceI\<U> polytope_convex_hull z)
```
```  3559               next
```
```  3560                 case False
```
```  3561                 have "convex D"
```
```  3562                   by (simp add: \<open>D \<in> \<N>\<close> convex\<N>)
```
```  3563                 have "K \<subseteq> C"
```
```  3564                   by (metis DiffE \<open>C \<in> \<N>\<close> \<open>K \<subseteq> rel_frontier C\<close> closed\<N> closure_closed rel_frontier_def subset_eq)
```
```  3565                 have "L \<subseteq> D"
```
```  3566                   by (metis DiffE \<open>D \<in> \<N>\<close> \<open>L \<subseteq> rel_frontier D\<close> closed\<N> closure_closed rel_frontier_def subset_eq)
```
```  3567                 let ?w = "(SOME w. w \<in> rel_interior D)"
```
```  3568                 have w: "?w \<in> rel_interior D"
```
```  3569                   using \<open>D \<in> \<N>\<close> in_rel_interior by blast
```
```  3570                 have "C \<inter> rel_interior D = (D \<inter> C) \<inter> rel_interior D"
```
```  3571                   using rel_interior_subset by blast
```
```  3572                 also have "(D \<inter> C) \<inter> rel_interior D = {}"
```
```  3573                 proof (rule face_of_disjoint_rel_interior)
```
```  3574                   show "D \<inter> C face_of D"
```
```  3575                     using \<N>_def \<open>C \<in> \<N>\<close> \<open>D \<in> \<N>\<close> intface\<M> by blast
```
```  3576                   have "D \<in> \<M> \<and> aff_dim D = int n"
```
```  3577                     using \<N>_def \<open>D \<in> \<N>\<close> by blast
```
```  3578                   moreover have "C \<in> \<M> \<and> aff_dim C = int n"
```
```  3579                     using \<N>_def \<open>C \<in> \<N>\<close> by blast
```
```  3580                   ultimately show "D \<inter> C \<noteq> D"
```
```  3581                     by (metis False face_of_aff_dim_lt inf.idem inf_le1 intface\<M> not_le poly\<M> polytope_imp_convex)
```
```  3582                 qed
```
```  3583                 finally have CD: "C \<inter> (rel_interior D) = {}" .
```
```  3584                 have zKC: "(convex hull insert ?z K) \<subseteq> C"
```
```  3585                   by (metis DiffE \<open>C \<in> \<N>\<close> \<open>K \<subseteq> rel_frontier C\<close> closed\<N> closure_closed convex\<N> hull_minimal insert_subset rel_frontier_def rel_interior_subset subset_iff z)
```
```  3586                 have eq: "convex hull (insert ?z K) \<inter> convex hull (insert ?w L) =
```
```  3587                           convex hull (insert ?z K) \<inter> convex hull L"
```
```  3588                   apply (rule Int_convex_hull_insert_rel_exterior [OF \<open>convex D\<close> \<open>L \<subseteq> D\<close> w])
```
```  3589                   using zKC CD apply (force simp: disjnt_def)
```
```  3590                   done
```
```  3591                 have ch_id: "convex hull K = K" "convex hull L = L"
```
```  3592                   using "*" \<open>K \<in> \<U>\<close> \<open>L \<in> \<U>\<close> hull_same by auto
```
```  3593                 have "convex C"
```
```  3594                   by (simp add: \<open>C \<in> \<N>\<close> convex\<N>)
```
```  3595                 have "convex hull (insert ?z K) \<inter> L = L \<inter> convex hull (insert ?z K)"
```
```  3596                   by blast
```
```  3597                 also have "... = convex hull K \<inter> L"
```
```  3598                 proof (subst Int_convex_hull_insert_rel_exterior [OF \<open>convex C\<close> \<open>K \<subseteq> C\<close> z])
```
```  3599                   have "(C \<inter> D) \<inter> rel_interior C = {}"
```
```  3600                   proof (rule face_of_disjoint_rel_interior)
```
```  3601                     show "C \<inter> D face_of C"
```
```  3602                       using \<N>_def \<open>C \<in> \<N>\<close> \<open>D \<in> \<N>\<close> intface\<M> by blast
```
```  3603                     have "D \<in> \<M>" "aff_dim D = int n"
```
```  3604                       using \<N>_def \<open>D \<in> \<N>\<close> by fastforce+
```
```  3605                     moreover have "C \<in> \<M>" "aff_dim C = int n"
```
```  3606                       using \<N>_def \<open>C \<in> \<N>\<close> by fastforce+
```
```  3607                     ultimately have "aff_dim D + - 1 * aff_dim C \<le> 0"
```
```  3608                       by fastforce
```
```  3609                     then have "\<not> C face_of D"
```
```  3610                       using False \<open>convex D\<close> face_of_aff_dim_lt by fastforce
```
```  3611                     show "C \<inter> D \<noteq> C"
```
```  3612                       using \<open>C \<in> \<M>\<close> \<open>D \<in> \<M>\<close> \<open>\<not> C face_of D\<close> intface\<M> by fastforce
```
```  3613                   qed
```
```  3614                   then have "D \<inter> rel_interior C = {}"
```
```  3615                     by (metis inf.absorb_iff2 inf_assoc inf_sup_aci(1) rel_interior_subset)
```
```  3616                   then show "disjnt L (rel_interior C)"
```
```  3617                     by (meson \<open>L \<subseteq> D\<close> disjnt_def disjnt_subset1)
```
```  3618                 next
```
```  3619                   show "L \<inter> convex hull K = convex hull K \<inter> L"
```
```  3620                     by force
```
```  3621                 qed
```
```  3622                 finally have chKL: "convex hull (insert ?z K) \<inter> L = convex hull K \<inter> L" .
```
```  3623                 have "convex hull insert ?z K \<inter> convex hull L face_of K"
```
```  3624                   by (simp add: \<open>K \<in> \<U>\<close> \<open>L \<in> \<U>\<close> ch_id chKL faceI\<U>)
```
```  3625                 also have "... face_of convex hull insert ?z K"
```
```  3626                 proof -
```
```  3627                   obtain I where I: "\<not> affine_dependent I" "K = convex hull I"
```
```  3628                     using * [OF \<open>K \<in> \<U>\<close>] by auto
```
```  3629                   then have "\<And>a. a \<notin> rel_interior C \<or> a \<notin> affine hull I"
```
```  3630                     using ahK_C_disjoint \<open>C \<in> \<N>\<close> \<open>K \<in> \<U>\<close> \<open>K \<subseteq> rel_frontier C\<close> affine_hull_convex_hull by blast
```
```  3631                   then show ?thesis
```
```  3632                     by (metis I affine_independent_insert face_of_convex_hull_affine_independent hull_insert subset_insertI z)
```
```  3633                 qed
```
```  3634                 finally have 1: "convex hull insert ?z K \<inter> convex hull L face_of convex hull insert ?z K" .
```
```  3635                 have "convex hull insert ?z K \<inter> convex hull L face_of L"
```
```  3636                   by (simp add: \<open>K \<in> \<U>\<close> \<open>L \<in> \<U>\<close> ch_id chKL faceI\<U>)
```
```  3637                 also have "... face_of convex hull insert ?w L"
```
```  3638                 proof -
```
```  3639                   obtain I where I: "\<not> affine_dependent I" "L = convex hull I"
```
```  3640                     using * [OF \<open>L \<in> \<U>\<close>] by auto
```
```  3641                   then have "\<And>a. a \<notin> rel_interior D \<or> a \<notin> affine hull I"
```
```  3642                     using \<open>D \<in> \<N>\<close> \<open>L \<in> \<U>\<close> \<open>L \<subseteq> rel_frontier D\<close> affine_hull_convex_hull ahK_C_disjoint by blast
```
```  3643                   then show ?thesis
```
```  3644                     by (metis I aff_independent_finite convex_convex_hull face_of_convex_hull_insert face_of_refl hull_insert w)
```
```  3645                 qed
```
```  3646                 finally have 2: "convex hull insert ?z K \<inter> convex hull L face_of convex hull insert ?w L" .
```
```  3647                 show ?thesis
```
```  3648                   by (simp add: X Y eq 1 2)
```
```  3649               qed
```
```  3650             qed
```
```  3651           qed
```
```  3652         qed
```
```  3653         then
```
```  3654         show "S \<inter> S' face_of S" "S \<inter> S' face_of S'" if "S \<in> \<U> \<union> ?\<T> \<and> S' \<in> \<U> \<union> ?\<T>" for S S'
```
```  3655           using that by auto
```
```  3656       qed
```
```  3657       show "\<exists>F \<subseteq> \<U> \<union> ?\<T>. C = \<Union>F" if "C \<in> \<M>" for C
```
```  3658       proof (cases "C \<in> \<S>")
```
```  3659         case True
```
```  3660         then show ?thesis
```
```  3661           by (meson UnCI fin\<U> subsetD subsetI)
```
```  3662       next
```
```  3663         case False
```
```  3664         then have "C \<in> \<N>"
```
```  3665           by (simp add: \<N>_def \<S>_def aff\<M> less_le that)
```
```  3666         let ?z = "SOME z. z \<in> rel_interior C"
```
```  3667         have z: "?z \<in> rel_interior C"
```
```  3668           using \<open>C \<in> \<N>\<close> in_rel_interior by blast
```
```  3669         let ?F = "\<Union>K \<in> \<U> \<inter> Pow (rel_frontier C). {convex hull (insert ?z K)}"
```
```  3670         have "?F \<subseteq> ?\<T>"
```
```  3671           using \<open>C \<in> \<N>\<close> by blast
```
```  3672         moreover have "C \<subseteq> \<Union>?F"
```
```  3673         proof
```
```  3674           fix x
```
```  3675           assume "x \<in> C"
```
```  3676           have "convex C"
```
```  3677             using \<open>C \<in> \<N>\<close> convex\<N> by blast
```
```  3678           have "bounded C"
```
```  3679             using \<open>C \<in> \<N>\<close> by (simp add: poly\<M> polytope_imp_bounded that)
```
```  3680           have "polytope C"
```
```  3681             using \<open>C \<in> \<N>\<close> poly\<N> by auto
```
```  3682           have "\<not> (?z = x \<and> C = {?z})"
```
```  3683             using \<open>C \<in> \<N>\<close> aff_dim_sing [of ?z] \<open>\<not> n \<le> 1\<close> by (force simp: \<N>_def)
```
```  3684           then obtain y where y: "y \<in> rel_frontier C" and xzy: "x \<in> closed_segment ?z y"
```
```  3685             and sub: "open_segment ?z y \<subseteq> rel_interior C"
```
```  3686             by (blast intro: segment_to_rel_frontier [OF \<open>convex C\<close> \<open>bounded C\<close> z \<open>x \<in> C\<close>])
```
```  3687           then obtain F where "y \<in> F" "F face_of C" "F \<noteq> C"
```
```  3688             by (auto simp: rel_frontier_of_polyhedron_alt [OF polytope_imp_polyhedron [OF \<open>polytope C\<close>]])
```
```  3689           then obtain \<G> where "finite \<G>" "\<G> \<subseteq> \<U>" "F = \<Union>\<G>"
```
```  3690             by (metis (mono_tags, lifting) \<S>_def \<open>C \<in> \<M>\<close> \<open>convex C\<close> aff\<M> face\<M> face_of_aff_dim_lt fin\<U> le_less_trans mem_Collect_eq not_less)
```
```  3691           then obtain K where "y \<in> K" "K \<in> \<G>"
```
```  3692             using \<open>y \<in> F\<close> by blast
```
```  3693           moreover have x: "x \<in> convex hull {?z,y}"
```
```  3694             using segment_convex_hull xzy by auto
```
```  3695           moreover have "convex hull {?z,y} \<subseteq> convex hull insert ?z K"
```
```  3696             by (metis (full_types) \<open>y \<in> K\<close> hull_mono empty_subsetI insertCI insert_subset)
```
```  3697           moreover have "K \<in> \<U>"
```
```  3698             using \<open>K \<in> \<G>\<close> \<open>\<G> \<subseteq> \<U>\<close> by blast
```
```  3699           moreover have "K \<subseteq> rel_frontier C"
```
```  3700             using \<open>F = \<Union>\<G>\<close> \<open>F \<noteq> C\<close> \<open>F face_of C\<close> \<open>K \<in> \<G>\<close> face_of_subset_rel_frontier by fastforce
```
```  3701           ultimately show "x \<in> \<Union>?F"
```
```  3702             by force
```
```  3703         qed
```
```  3704         moreover
```
```  3705         have "convex hull insert (SOME z. z \<in> rel_interior C) K \<subseteq> C"
```
```  3706           if "K \<in> \<U>" "K \<subseteq> rel_frontier C" for K
```
```  3707         proof (rule hull_minimal)
```
```  3708           show "insert (SOME z. z \<in> rel_interior C) K \<subseteq> C"
```
```  3709             using that \<open>C \<in> \<N>\<close> in_rel_interior rel_interior_subset
```
```  3710             by (force simp: closure_eq rel_frontier_def closed\<N>)
```
```  3711           show "convex C"
```
```  3712             by (simp add: \<open>C \<in> \<N>\<close> convex\<N>)
```
```  3713         qed
```
```  3714         then have "\<Union>?F \<subseteq> C"
```
```  3715           by auto
```
```  3716         ultimately show ?thesis
```
```  3717           by blast
```
```  3718       qed
```
```  3719
```
```  3720       have "(\<exists>C. C \<in> \<M> \<and> L \<subseteq> C) \<and> aff_dim L \<le> int n"  if "L \<in> \<U> \<union> ?\<T>" for L
```
```  3721         using that
```
```  3722       proof
```
```  3723         assume "L \<in> \<U>"
```
```  3724         then show ?thesis
```
```  3725           using C\<U> \<S>_def "*" by fastforce
```
```  3726       next
```
```  3727         assume "L \<in> ?\<T>"
```
```  3728         then obtain C K where "C \<in> \<N>"
```
```  3729           and L: "L = convex hull insert (SOME z. z \<in> rel_interior C) K"
```
```  3730           and K: "K \<in> \<U>" "K \<subseteq> rel_frontier C"
```
```  3731           by auto
```
```  3732         then have "convex hull C = C"
```
```  3733           by (meson convex\<N> convex_hull_eq)
```
```  3734         then have "convex C"
```
```  3735           by (metis (no_types) convex_convex_hull)
```
```  3736         have "rel_frontier C \<subseteq> C"
```
```  3737           by (metis DiffE closed\<N> \<open>C \<in> \<N>\<close> closure_closed rel_frontier_def subsetI)
```
```  3738         have "K \<subseteq> C"
```
```  3739           using K \<open>rel_frontier C \<subseteq> C\<close> by blast
```
```  3740         have "C \<in> \<M>"
```
```  3741           using \<N>_def \<open>C \<in> \<N>\<close> by auto
```
```  3742         moreover have "L \<subseteq> C"
```
```  3743           using K L \<open>C \<in> \<N>\<close>
```
```  3744           by (metis \<open>K \<subseteq> C\<close> \<open>convex hull C = C\<close> contra_subsetD hull_mono in_rel_interior insert_subset rel_interior_subset)
```
```  3745         ultimately show ?thesis
```
```  3746           using \<open>rel_frontier C \<subseteq> C\<close> \<open>L \<subseteq> C\<close> aff\<M> aff_dim_subset \<open>C \<in> \<M>\<close> dual_order.trans by blast
```
```  3747       qed
```
```  3748       then show "\<exists>C. C \<in> \<M> \<and> L \<subseteq> C" "aff_dim L \<le> int n" if "L \<in> \<U> \<union> ?\<T>" for L
```
```  3749         using that by auto
```
```  3750     qed
```
```  3751     then show ?thesis
```
```  3752       apply (rule ex_forward, safe)
```
```  3753         apply (meson Union_iff subsetCE, fastforce)
```
```  3754       by (meson infinite_super simplicial_complex_def)
```
```  3755   qed
```
```  3756 qed
```
```  3757
```
```  3758
```
```  3759 lemma%important simplicial_subdivision_of_cell_complex_lowdim:
```
```  3760   assumes "finite \<M>"
```
```  3761       and poly: "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3762       and face: "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3763       and aff: "\<And>C. C \<in> \<M> \<Longrightarrow> aff_dim C \<le> d"
```
```  3764   obtains \<T> where "simplicial_complex \<T>" "\<And>K. K \<in> \<T> \<Longrightarrow> aff_dim K \<le> d"
```
```  3765                   "\<Union>\<T> = \<Union>\<M>"
```
```  3766                   "\<And>C. C \<in> \<M> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F"
```
```  3767                   "\<And>K. K \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<M> \<and> K \<subseteq> C"
```
```  3768 proof%unimportant (cases "d \<ge> 0")
```
```  3769   case True
```
```  3770   then obtain n where n: "d = of_nat n"
```
```  3771     using zero_le_imp_eq_int by blast
```
```  3772   have "\<exists>\<T>. simplicial_complex \<T> \<and>
```
```  3773             (\<forall>K\<in>\<T>. aff_dim K \<le> int n) \<and>
```
```  3774             \<Union>\<T> = \<Union>(\<Union>C\<in>\<M>. {F. F face_of C}) \<and>
```
```  3775             (\<forall>C\<in>\<Union>C\<in>\<M>. {F. F face_of C}.
```
```  3776                 \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F) \<and>
```
```  3777             (\<forall>K\<in>\<T>. \<exists>C. C \<in> (\<Union>C\<in>\<M>. {F. F face_of C}) \<and> K \<subseteq> C)"
```
```  3778   proof (rule simplicial_subdivision_aux)
```
```  3779     show "finite (\<Union>C\<in>\<M>. {F. F face_of C})"
```
```  3780       using \<open>finite \<M>\<close> poly polyhedron_eq_finite_faces polytope_imp_polyhedron by fastforce
```
```  3781     show "polytope F" if "F \<in> (\<Union>C\<in>\<M>. {F. F face_of C})" for F
```
```  3782       using poly that face_of_polytope_polytope by blast
```
```  3783     show "aff_dim F \<le> int n" if "F \<in> (\<Union>C\<in>\<M>. {F. F face_of C})" for F
```
```  3784       using that
```
```  3785       by clarify (metis n aff_dim_subset aff face_of_imp_subset order_trans)
```
```  3786     show "F \<in> (\<Union>C\<in>\<M>. {F. F face_of C})"
```
```  3787       if "G \<in> (\<Union>C\<in>\<M>. {F. F face_of C})" and "F face_of G" for F G
```
```  3788       using that face_of_trans by blast
```
```  3789   next
```
```  3790     show "F1 \<inter> F2 face_of F1 \<and> F1 \<inter> F2 face_of F2"
```
```  3791       if "F1 \<in> (\<Union>C\<in>\<M>. {F. F face_of C})" and "F2 \<in> (\<Union>C\<in>\<M>. {F. F face_of C})" for F1 F2
```
```  3792       using that
```
```  3793       by safe (meson face face_of_Int_subface)+
```
```  3794   qed
```
```  3795   moreover
```
```  3796   have "\<Union>(\<Union>C\<in>\<M>. {F. F face_of C}) = \<Union>\<M>"
```
```  3797     using face_of_imp_subset face by blast
```
```  3798   ultimately show ?thesis
```
```  3799     apply clarify
```
```  3800     apply (rule that, assumption+)
```
```  3801        using n apply blast
```
```  3802       apply (simp_all add: poly face_of_refl polytope_imp_convex)
```
```  3803     using face_of_imp_subset by fastforce
```
```  3804 next
```
```  3805   case False
```
```  3806   then have m1: "\<And>C. C \<in> \<M> \<Longrightarrow> aff_dim C = -1"
```
```  3807     by (metis aff aff_dim_empty_eq aff_dim_negative_iff dual_order.trans not_less)
```
```  3808   then have face\<M>: "\<And>F S. \<lbrakk>S \<in> \<M>; F face_of S\<rbrakk> \<Longrightarrow> F \<in> \<M>"
```
```  3809     by (metis aff_dim_empty face_of_empty)
```
```  3810   show ?thesis
```
```  3811   proof
```
```  3812     have "\<And>S. S \<in> \<M> \<Longrightarrow> \<exists>n. n simplex S"
```
```  3813       by (metis (no_types) m1 aff_dim_empty simplex_minus_1)
```
```  3814     then show "simplicial_complex \<M>"
```
```  3815       by (auto simp: simplicial_complex_def \<open>finite \<M>\<close> face intro: face\<M>)
```
```  3816     show "aff_dim K \<le> d" if "K \<in> \<M>" for K
```
```  3817       by (simp add: that aff)
```
```  3818     show "\<exists>F. finite F \<and> F \<subseteq> \<M> \<and> C = \<Union>F" if "C \<in> \<M>" for C
```
```  3819       using \<open>C \<in> \<M>\<close> equals0I by auto
```
```  3820     show "\<exists>C. C \<in> \<M> \<and> K \<subseteq> C" if "K \<in> \<M>" for K
```
```  3821       using \<open>K \<in> \<M>\<close> by blast
```
```  3822   qed auto
```
```  3823 qed
```
```  3824
```
```  3825 proposition%important simplicial_subdivision_of_cell_complex:
```
```  3826   assumes "finite \<M>"
```
```  3827       and poly: "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3828       and face: "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3829   obtains \<T> where "simplicial_complex \<T>"
```
```  3830                   "\<Union>\<T> = \<Union>\<M>"
```
```  3831                   "\<And>C. C \<in> \<M> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F"
```
```  3832                   "\<And>K. K \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<M> \<and> K \<subseteq> C"
```
```  3833   by%unimportant (blast intro: simplicial_subdivision_of_cell_complex_lowdim [OF assms aff_dim_le_DIM])
```
```  3834
```
```  3835 corollary%important fine_simplicial_subdivision_of_cell_complex:
```
```  3836   assumes "0 < e" "finite \<M>"
```
```  3837       and poly: "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3838       and face: "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3839   obtains \<T> where "simplicial_complex \<T>"
```
```  3840                   "\<And>K. K \<in> \<T> \<Longrightarrow> diameter K < e"
```
```  3841                   "\<Union>\<T> = \<Union>\<M>"
```
```  3842                   "\<And>C. C \<in> \<M> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F"
```
```  3843                   "\<And>K. K \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<M> \<and> K \<subseteq> C"
```
```  3844 proof%unimportant -
```
```  3845   obtain \<N> where \<N>: "finite \<N>" "\<Union>\<N> = \<Union>\<M>"
```
```  3846               and diapoly: "\<And>X. X \<in> \<N> \<Longrightarrow> diameter X < e" "\<And>X. X \<in> \<N> \<Longrightarrow> polytope X"
```
```  3847                and      "\<And>X Y. \<lbrakk>X \<in> \<N>; Y \<in> \<N>\<rbrakk> \<Longrightarrow> X \<inter> Y face_of X \<and> X \<inter> Y face_of Y"
```
```  3848                and \<N>covers: "\<And>C x. C \<in> \<M> \<and> x \<in> C \<Longrightarrow> \<exists>D. D \<in> \<N> \<and> x \<in> D \<and> D \<subseteq> C"
```
```  3849                and \<N>covered: "\<And>C. C \<in> \<N> \<Longrightarrow> \<exists>D. D \<in> \<M> \<and> C \<subseteq> D"
```
```  3850     by (blast intro: cell_complex_subdivision_exists [OF \<open>0 < e\<close> \<open>finite \<M>\<close> poly aff_dim_le_DIM face])
```
```  3851   then obtain \<T> where \<T>: "simplicial_complex \<T>" "\<Union>\<T> = \<Union>\<N>"
```
```  3852                    and \<T>covers: "\<And>C. C \<in> \<N> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F"
```
```  3853                    and \<T>covered: "\<And>K. K \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<N> \<and> K \<subseteq> C"
```
```  3854     using simplicial_subdivision_of_cell_complex [OF \<open>finite \<N>\<close>] by metis
```
```  3855   show ?thesis
```
```  3856   proof
```
```  3857     show "simplicial_complex \<T>"
```
```  3858       by (rule \<T>)
```
```  3859     show "diameter K < e" if "K \<in> \<T>" for K
```
```  3860       by (metis le_less_trans diapoly \<T>covered diameter_subset polytope_imp_bounded that)
```
```  3861     show "\<Union>\<T> = \<Union>\<M>"
```
```  3862       by (simp add: \<N>(2) \<open>\<Union>\<T> = \<Union>\<N>\<close>)
```
```  3863     show "\<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F" if "C \<in> \<M>" for C
```
```  3864     proof -
```
```  3865       { fix x
```
```  3866         assume "x \<in> C"
```
```  3867         then obtain D where "D \<in> \<T>" "x \<in> D" "D \<subseteq> C"
```
```  3868           using \<N>covers \<open>C \<in> \<M>\<close> \<T>covers by force
```
```  3869         then have "\<exists>X\<in>\<T> \<inter> Pow C. x \<in> X"
```
```  3870           using \<open>D \<in> \<T>\<close> \<open>D \<subseteq> C\<close> \<open>x \<in> D\<close> by blast
```
```  3871       }
```
```  3872       moreover
```
```  3873       have "finite (\<T> \<inter> Pow C)"
```
```  3874         using \<open>simplicial_complex \<T>\<close> simplicial_complex_def by auto
```
```  3875       ultimately show ?thesis
```
```  3876         by (rule_tac x="(\<T> \<inter> Pow C)" in exI) auto
```
```  3877     qed
```
```  3878     show "\<exists>C. C \<in> \<M> \<and> K \<subseteq> C" if "K \<in> \<T>" for K
```
```  3879       by (meson \<N>covered \<T>covered order_trans that)
```
```  3880   qed
```
```  3881 qed
```
```  3882
```
```  3883 subsection%important\<open>Some results on cell division with full-dimensional cells only\<close>
```
```  3884
```
```  3885 lemma%important convex_Union_fulldim_cells:
```
```  3886   assumes "finite \<S>" and clo: "\<And>C. C \<in> \<S> \<Longrightarrow> closed C" and con: "\<And>C. C \<in> \<S> \<Longrightarrow> convex C"
```
```  3887       and eq: "\<Union>\<S> = U"and  "convex U"
```
```  3888  shows "\<Union>{C \<in> \<S>. aff_dim C = aff_dim U} = U"  (is "?lhs = U")
```
```  3889 proof%unimportant -
```
```  3890   have "closed U"
```
```  3891     using \<open>finite \<S>\<close> clo eq by blast
```
```  3892   have "?lhs \<subseteq> U"
```
```  3893     using eq by blast
```
```  3894   moreover have "U \<subseteq> ?lhs"
```
```  3895   proof (cases "\<forall>C \<in> \<S>. aff_dim C = aff_dim U")
```
```  3896     case True
```
```  3897     then show ?thesis
```
```  3898       using eq by blast
```
```  3899   next
```
```  3900     case False
```
```  3901     have "closed ?lhs"
```
```  3902       by (simp add: \<open>finite \<S>\<close> clo closed_Union)
```
```  3903     moreover have "U \<subseteq> closure ?lhs"
```
```  3904     proof -
```
```  3905       have "U \<subseteq> closure(\<Inter>{U - C |C. C \<in> \<S> \<and> aff_dim C < aff_dim U})"
```
```  3906       proof (rule Baire [OF \<open>closed U\<close>])
```
```  3907         show "countable {U - C |C. C \<in> \<S> \<and> aff_dim C < aff_dim U}"
```
```  3908           using \<open>finite \<S>\<close> uncountable_infinite by fastforce
```
```  3909         have "\<And>C. C \<in> \<S> \<Longrightarrow> openin (subtopology euclidean U) (U-C)"
```
```  3910           by (metis Sup_upper clo closed_limpt closedin_limpt eq openin_diff openin_subtopology_self)
```
```  3911         then show "openin (subtopology euclidean U) T \<and> U \<subseteq> closure T"
```
```  3912           if "T \<in> {U - C |C. C \<in> \<S> \<and> aff_dim C < aff_dim U}" for T
```
```  3913           using that dense_complement_convex_closed \<open>closed U\<close> \<open>convex U\<close> by auto
```
```  3914       qed
```
```  3915       also have "... \<subseteq> closure ?lhs"
```
```  3916       proof -
```
```  3917         obtain C where "C \<in> \<S>" "aff_dim C < aff_dim U"
```
```  3918           by (metis False Sup_upper aff_dim_subset eq eq_iff not_le)
```
```  3919         have "\<exists>X. X \<in> \<S> \<and> aff_dim X = aff_dim U \<and> x \<in> X"
```
```  3920           if "\<And>V. (\<exists>C. V = U - C \<and> C \<in> \<S> \<and> aff_dim C < aff_dim U) \<Longrightarrow> x \<in> V" for x
```
```  3921         proof -
```
```  3922           have "x \<in> U \<and> x \<in> \<Union>\<S>"
```
```  3923             using \<open>C \<in> \<S>\<close> \<open>aff_dim C < aff_dim U\<close> eq that by blast
```
```  3924           then show ?thesis
```
```  3925             by (metis Diff_iff Sup_upper Union_iff aff_dim_subset dual_order.order_iff_strict eq that)
```
```  3926         qed
```
```  3927         then show ?thesis
```
```  3928           by (auto intro!: closure_mono)
```
```  3929       qed
```
```  3930       finally show ?thesis .
```
```  3931     qed
```
```  3932     ultimately show ?thesis
```
```  3933       using closure_subset_eq by blast
```
```  3934   qed
```
```  3935   ultimately show ?thesis by blast
```
```  3936 qed
```
```  3937
```
```  3938 proposition%important fine_triangular_subdivision_of_cell_complex:
```
```  3939   assumes "0 < e" "finite \<M>"
```
```  3940       and poly: "\<And>C. C \<in> \<M> \<Longrightarrow> polytope C"
```
```  3941       and aff: "\<And>C. C \<in> \<M> \<Longrightarrow> aff_dim C = d"
```
```  3942       and face: "\<And>C1 C2. \<lbrakk>C1 \<in> \<M>; C2 \<in> \<M>\<rbrakk> \<Longrightarrow> C1 \<inter> C2 face_of C1 \<and> C1 \<inter> C2 face_of C2"
```
```  3943   obtains \<T> where "triangulation \<T>" "\<And>k. k \<in> \<T> \<Longrightarrow> diameter k < e"
```
```  3944                  "\<And>k. k \<in> \<T> \<Longrightarrow> aff_dim k = d" "\<Union>\<T> = \<Union>\<M>"
```
```  3945                  "\<And>C. C \<in> \<M> \<Longrightarrow> \<exists>f. finite f \<and> f \<subseteq> \<T> \<and> C = \<Union>f"
```
```  3946                  "\<And>k. k \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<M> \<and> k \<subseteq> C"
```
```  3947 proof%unimportant -
```
```  3948   obtain \<T> where "simplicial_complex \<T>"
```
```  3949              and dia\<T>: "\<And>K. K \<in> \<T> \<Longrightarrow> diameter K < e"
```
```  3950              and "\<Union>\<T> = \<Union>\<M>"
```
```  3951              and in\<M>: "\<And>C. C \<in> \<M> \<Longrightarrow> \<exists>F. finite F \<and> F \<subseteq> \<T> \<and> C = \<Union>F"
```
```  3952              and in\<T>: "\<And>K. K \<in> \<T> \<Longrightarrow> \<exists>C. C \<in> \<M> \<and> K \<subseteq> C"
```
```  3953     by (blast intro: fine_simplicial_subdivision_of_cell_complex [OF \<open>e > 0\<close> \<open>finite \<M>\<close> poly face])
```
```  3954   let ?\<T> = "{K \<in> \<T>. aff_dim K = d}"
```
```  3955   show thesis
```
```  3956   proof
```
```  3957     show "triangulation ?\<T>"
```
```  3958       using \<open>simplicial_complex \<T>\<close> by (auto simp: triangulation_def simplicial_complex_def)
```
```  3959     show "diameter L < e" if "L \<in> {K \<in> \<T>. aff_dim K = d}" for L
```
```  3960       using that by (auto simp: dia\<T>)
```
```  3961     show "aff_dim L = d" if "L \<in> {K \<in> \<T>. aff_dim K = d}" for L
```
```  3962       using that by auto
```
```  3963     show "\<exists>F. finite F \<and> F \<subseteq> {K \<in> \<T>. aff_dim K = d} \<and> C = \<Union>F" if "C \<in> \<M>" for C
```
```  3964     proof -
```
```  3965       obtain F where "finite F" "F \<subseteq> \<T>" "C = \<Union>F"
```
```  3966         using in\<M> [OF \<open>C \<in> \<M>\<close>] by auto
```
```  3967       show ?thesis
```
```  3968       proof (intro exI conjI)
```
```  3969         show "finite {K \<in> F. aff_dim K = d}"
```
```  3970           by (simp add: \<open>finite F\<close>)
```
```  3971         show "{K \<in> F. aff_dim K = d} \<subseteq> {K \<in> \<T>. aff_dim K = d}"
```
```  3972           using \<open>F \<subseteq> \<T>\<close> by blast
```
```  3973         have "d = aff_dim C"
```
```  3974           by (simp add: aff that)
```
```  3975         moreover have "\<And>K. K \<in> F \<Longrightarrow> closed K \<and> convex K"
```
```  3976           using \<open>simplicial_complex \<T>\<close> \<open>F \<subseteq> \<T>\<close>
```
```  3977           unfolding simplicial_complex_def by (metis subsetCE \<open>F \<subseteq> \<T>\<close> closed_simplex convex_simplex)
```
```  3978         moreover have "convex (\<Union>F)"
```
```  3979           using \<open>C = \<Union>F\<close> poly polytope_imp_convex that by blast
```
```  3980         ultimately show "C = \<Union>{K \<in> F. aff_dim K = d}"
```
```  3981           by (simp add: convex_Union_fulldim_cells \<open>C = \<Union>F\<close> \<open>finite F\<close>)
```
```  3982       qed
```
```  3983     qed
```
```  3984     then show "\<Union>{K \<in> \<T>. aff_dim K = d} = \<Union>\<M>"
```
```  3985       by auto (meson in\<T> subsetCE)
```
```  3986     show "\<exists>C. C \<in> \<M> \<and> L \<subseteq> C"
```
```  3987       if "L \<in> {K \<in> \<T>. aff_dim K = d}" for L
```
```  3988       using that by (auto simp: in\<T>)
```
```  3989   qed
```
```  3990 qed
```
```  3991
```
```  3992 end
```