src/HOL/Hilbert_Choice.thy
 author hoelzl Mon Jun 30 15:45:25 2014 +0200 (2014-06-30) changeset 57448 159e45728ceb parent 57275 0ddb5b755cdc child 58074 87a8cc594bf6 permissions -rw-r--r--
more equalities of topological filters; strengthen dependent_nat_choice; tuned a couple of proofs
```     1 (*  Title:      HOL/Hilbert_Choice.thy
```
```     2     Author:     Lawrence C Paulson, Tobias Nipkow
```
```     3     Copyright   2001  University of Cambridge
```
```     4 *)
```
```     5
```
```     6 header {* Hilbert's Epsilon-Operator and the Axiom of Choice *}
```
```     7
```
```     8 theory Hilbert_Choice
```
```     9 imports Nat Wellfounded
```
```    10 keywords "specification" :: thy_goal
```
```    11 begin
```
```    12
```
```    13 subsection {* Hilbert's epsilon *}
```
```    14
```
```    15 axiomatization Eps :: "('a => bool) => 'a" where
```
```    16   someI: "P x ==> P (Eps P)"
```
```    17
```
```    18 syntax (epsilon)
```
```    19   "_Eps"        :: "[pttrn, bool] => 'a"    ("(3\<some>_./ _)" [0, 10] 10)
```
```    20 syntax (HOL)
```
```    21   "_Eps"        :: "[pttrn, bool] => 'a"    ("(3@ _./ _)" [0, 10] 10)
```
```    22 syntax
```
```    23   "_Eps"        :: "[pttrn, bool] => 'a"    ("(3SOME _./ _)" [0, 10] 10)
```
```    24 translations
```
```    25   "SOME x. P" == "CONST Eps (%x. P)"
```
```    26
```
```    27 print_translation {*
```
```    28   [(@{const_syntax Eps}, fn _ => fn [Abs abs] =>
```
```    29       let val (x, t) = Syntax_Trans.atomic_abs_tr' abs
```
```    30       in Syntax.const @{syntax_const "_Eps"} \$ x \$ t end)]
```
```    31 *} -- {* to avoid eta-contraction of body *}
```
```    32
```
```    33 definition inv_into :: "'a set => ('a => 'b) => ('b => 'a)" where
```
```    34 "inv_into A f == %x. SOME y. y : A & f y = x"
```
```    35
```
```    36 abbreviation inv :: "('a => 'b) => ('b => 'a)" where
```
```    37 "inv == inv_into UNIV"
```
```    38
```
```    39
```
```    40 subsection {*Hilbert's Epsilon-operator*}
```
```    41
```
```    42 text{*Easier to apply than @{text someI} if the witness comes from an
```
```    43 existential formula*}
```
```    44 lemma someI_ex [elim?]: "\<exists>x. P x ==> P (SOME x. P x)"
```
```    45 apply (erule exE)
```
```    46 apply (erule someI)
```
```    47 done
```
```    48
```
```    49 text{*Easier to apply than @{text someI} because the conclusion has only one
```
```    50 occurrence of @{term P}.*}
```
```    51 lemma someI2: "[| P a;  !!x. P x ==> Q x |] ==> Q (SOME x. P x)"
```
```    52 by (blast intro: someI)
```
```    53
```
```    54 text{*Easier to apply than @{text someI2} if the witness comes from an
```
```    55 existential formula*}
```
```    56 lemma someI2_ex: "[| \<exists>a. P a; !!x. P x ==> Q x |] ==> Q (SOME x. P x)"
```
```    57 by (blast intro: someI2)
```
```    58
```
```    59 lemma some_equality [intro]:
```
```    60      "[| P a;  !!x. P x ==> x=a |] ==> (SOME x. P x) = a"
```
```    61 by (blast intro: someI2)
```
```    62
```
```    63 lemma some1_equality: "[| EX!x. P x; P a |] ==> (SOME x. P x) = a"
```
```    64 by blast
```
```    65
```
```    66 lemma some_eq_ex: "P (SOME x. P x) =  (\<exists>x. P x)"
```
```    67 by (blast intro: someI)
```
```    68
```
```    69 lemma some_eq_trivial [simp]: "(SOME y. y=x) = x"
```
```    70 apply (rule some_equality)
```
```    71 apply (rule refl, assumption)
```
```    72 done
```
```    73
```
```    74 lemma some_sym_eq_trivial [simp]: "(SOME y. x=y) = x"
```
```    75 apply (rule some_equality)
```
```    76 apply (rule refl)
```
```    77 apply (erule sym)
```
```    78 done
```
```    79
```
```    80
```
```    81 subsection{*Axiom of Choice, Proved Using the Description Operator*}
```
```    82
```
```    83 lemma choice: "\<forall>x. \<exists>y. Q x y ==> \<exists>f. \<forall>x. Q x (f x)"
```
```    84 by (fast elim: someI)
```
```    85
```
```    86 lemma bchoice: "\<forall>x\<in>S. \<exists>y. Q x y ==> \<exists>f. \<forall>x\<in>S. Q x (f x)"
```
```    87 by (fast elim: someI)
```
```    88
```
```    89 lemma choice_iff: "(\<forall>x. \<exists>y. Q x y) \<longleftrightarrow> (\<exists>f. \<forall>x. Q x (f x))"
```
```    90 by (fast elim: someI)
```
```    91
```
```    92 lemma choice_iff': "(\<forall>x. P x \<longrightarrow> (\<exists>y. Q x y)) \<longleftrightarrow> (\<exists>f. \<forall>x. P x \<longrightarrow> Q x (f x))"
```
```    93 by (fast elim: someI)
```
```    94
```
```    95 lemma bchoice_iff: "(\<forall>x\<in>S. \<exists>y. Q x y) \<longleftrightarrow> (\<exists>f. \<forall>x\<in>S. Q x (f x))"
```
```    96 by (fast elim: someI)
```
```    97
```
```    98 lemma bchoice_iff': "(\<forall>x\<in>S. P x \<longrightarrow> (\<exists>y. Q x y)) \<longleftrightarrow> (\<exists>f. \<forall>x\<in>S. P x \<longrightarrow> Q x (f x))"
```
```    99 by (fast elim: someI)
```
```   100
```
```   101 lemma dependent_nat_choice:
```
```   102   assumes  1: "\<exists>x. P 0 x" and
```
```   103            2: "\<And>x n. P n x \<Longrightarrow> \<exists>y. P (Suc n) y \<and> Q n x y"
```
```   104   shows "\<exists>f. \<forall>n. P n (f n) \<and> Q n (f n) (f (Suc n))"
```
```   105 proof (intro exI allI conjI)
```
```   106   fix n def f \<equiv> "rec_nat (SOME x. P 0 x) (\<lambda>n x. SOME y. P (Suc n) y \<and> Q n x y)"
```
```   107   have "P 0 (f 0)" "\<And>n. P n (f n) \<Longrightarrow> P (Suc n) (f (Suc n)) \<and> Q n (f n) (f (Suc n))"
```
```   108     using someI_ex[OF 1] someI_ex[OF 2] by (simp_all add: f_def)
```
```   109   then show "P n (f n)" "Q n (f n) (f (Suc n))"
```
```   110     by (induct n) auto
```
```   111 qed
```
```   112
```
```   113 subsection {*Function Inverse*}
```
```   114
```
```   115 lemma inv_def: "inv f = (%y. SOME x. f x = y)"
```
```   116 by(simp add: inv_into_def)
```
```   117
```
```   118 lemma inv_into_into: "x : f ` A ==> inv_into A f x : A"
```
```   119 apply (simp add: inv_into_def)
```
```   120 apply (fast intro: someI2)
```
```   121 done
```
```   122
```
```   123 lemma inv_id [simp]: "inv id = id"
```
```   124 by (simp add: inv_into_def id_def)
```
```   125
```
```   126 lemma inv_into_f_f [simp]:
```
```   127   "[| inj_on f A;  x : A |] ==> inv_into A f (f x) = x"
```
```   128 apply (simp add: inv_into_def inj_on_def)
```
```   129 apply (blast intro: someI2)
```
```   130 done
```
```   131
```
```   132 lemma inv_f_f: "inj f ==> inv f (f x) = x"
```
```   133 by simp
```
```   134
```
```   135 lemma f_inv_into_f: "y : f`A  ==> f (inv_into A f y) = y"
```
```   136 apply (simp add: inv_into_def)
```
```   137 apply (fast intro: someI2)
```
```   138 done
```
```   139
```
```   140 lemma inv_into_f_eq: "[| inj_on f A; x : A; f x = y |] ==> inv_into A f y = x"
```
```   141 apply (erule subst)
```
```   142 apply (fast intro: inv_into_f_f)
```
```   143 done
```
```   144
```
```   145 lemma inv_f_eq: "[| inj f; f x = y |] ==> inv f y = x"
```
```   146 by (simp add:inv_into_f_eq)
```
```   147
```
```   148 lemma inj_imp_inv_eq: "[| inj f; ALL x. f(g x) = x |] ==> inv f = g"
```
```   149   by (blast intro: inv_into_f_eq)
```
```   150
```
```   151 text{*But is it useful?*}
```
```   152 lemma inj_transfer:
```
```   153   assumes injf: "inj f" and minor: "!!y. y \<in> range(f) ==> P(inv f y)"
```
```   154   shows "P x"
```
```   155 proof -
```
```   156   have "f x \<in> range f" by auto
```
```   157   hence "P(inv f (f x))" by (rule minor)
```
```   158   thus "P x" by (simp add: inv_into_f_f [OF injf])
```
```   159 qed
```
```   160
```
```   161 lemma inj_iff: "(inj f) = (inv f o f = id)"
```
```   162 apply (simp add: o_def fun_eq_iff)
```
```   163 apply (blast intro: inj_on_inverseI inv_into_f_f)
```
```   164 done
```
```   165
```
```   166 lemma inv_o_cancel[simp]: "inj f ==> inv f o f = id"
```
```   167 by (simp add: inj_iff)
```
```   168
```
```   169 lemma o_inv_o_cancel[simp]: "inj f ==> g o inv f o f = g"
```
```   170 by (simp add: comp_assoc)
```
```   171
```
```   172 lemma inv_into_image_cancel[simp]:
```
```   173   "inj_on f A ==> S <= A ==> inv_into A f ` f ` S = S"
```
```   174 by(fastforce simp: image_def)
```
```   175
```
```   176 lemma inj_imp_surj_inv: "inj f ==> surj (inv f)"
```
```   177 by (blast intro!: surjI inv_into_f_f)
```
```   178
```
```   179 lemma surj_f_inv_f: "surj f ==> f(inv f y) = y"
```
```   180 by (simp add: f_inv_into_f)
```
```   181
```
```   182 lemma inv_into_injective:
```
```   183   assumes eq: "inv_into A f x = inv_into A f y"
```
```   184       and x: "x: f`A"
```
```   185       and y: "y: f`A"
```
```   186   shows "x=y"
```
```   187 proof -
```
```   188   have "f (inv_into A f x) = f (inv_into A f y)" using eq by simp
```
```   189   thus ?thesis by (simp add: f_inv_into_f x y)
```
```   190 qed
```
```   191
```
```   192 lemma inj_on_inv_into: "B <= f`A ==> inj_on (inv_into A f) B"
```
```   193 by (blast intro: inj_onI dest: inv_into_injective injD)
```
```   194
```
```   195 lemma bij_betw_inv_into: "bij_betw f A B ==> bij_betw (inv_into A f) B A"
```
```   196 by (auto simp add: bij_betw_def inj_on_inv_into)
```
```   197
```
```   198 lemma surj_imp_inj_inv: "surj f ==> inj (inv f)"
```
```   199 by (simp add: inj_on_inv_into)
```
```   200
```
```   201 lemma surj_iff: "(surj f) = (f o inv f = id)"
```
```   202 by (auto intro!: surjI simp: surj_f_inv_f fun_eq_iff[where 'b='a])
```
```   203
```
```   204 lemma surj_iff_all: "surj f \<longleftrightarrow> (\<forall>x. f (inv f x) = x)"
```
```   205   unfolding surj_iff by (simp add: o_def fun_eq_iff)
```
```   206
```
```   207 lemma surj_imp_inv_eq: "[| surj f; \<forall>x. g(f x) = x |] ==> inv f = g"
```
```   208 apply (rule ext)
```
```   209 apply (drule_tac x = "inv f x" in spec)
```
```   210 apply (simp add: surj_f_inv_f)
```
```   211 done
```
```   212
```
```   213 lemma bij_imp_bij_inv: "bij f ==> bij (inv f)"
```
```   214 by (simp add: bij_def inj_imp_surj_inv surj_imp_inj_inv)
```
```   215
```
```   216 lemma inv_equality: "[| !!x. g (f x) = x;  !!y. f (g y) = y |] ==> inv f = g"
```
```   217 apply (rule ext)
```
```   218 apply (auto simp add: inv_into_def)
```
```   219 done
```
```   220
```
```   221 lemma inv_inv_eq: "bij f ==> inv (inv f) = f"
```
```   222 apply (rule inv_equality)
```
```   223 apply (auto simp add: bij_def surj_f_inv_f)
```
```   224 done
```
```   225
```
```   226 (** bij(inv f) implies little about f.  Consider f::bool=>bool such that
```
```   227     f(True)=f(False)=True.  Then it's consistent with axiom someI that
```
```   228     inv f could be any function at all, including the identity function.
```
```   229     If inv f=id then inv f is a bijection, but inj f, surj(f) and
```
```   230     inv(inv f)=f all fail.
```
```   231 **)
```
```   232
```
```   233 lemma inv_into_comp:
```
```   234   "[| inj_on f (g ` A); inj_on g A; x : f ` g ` A |] ==>
```
```   235   inv_into A (f o g) x = (inv_into A g o inv_into (g ` A) f) x"
```
```   236 apply (rule inv_into_f_eq)
```
```   237   apply (fast intro: comp_inj_on)
```
```   238  apply (simp add: inv_into_into)
```
```   239 apply (simp add: f_inv_into_f inv_into_into)
```
```   240 done
```
```   241
```
```   242 lemma o_inv_distrib: "[| bij f; bij g |] ==> inv (f o g) = inv g o inv f"
```
```   243 apply (rule inv_equality)
```
```   244 apply (auto simp add: bij_def surj_f_inv_f)
```
```   245 done
```
```   246
```
```   247 lemma image_surj_f_inv_f: "surj f ==> f ` (inv f ` A) = A"
```
```   248 by (simp add: image_eq_UN surj_f_inv_f)
```
```   249
```
```   250 lemma image_inv_f_f: "inj f ==> inv f ` (f ` A) = A"
```
```   251   by (simp add: image_eq_UN)
```
```   252
```
```   253 lemma inv_image_comp: "inj f ==> inv f ` (f ` X) = X"
```
```   254   by (fact image_inv_f_f)
```
```   255
```
```   256 lemma bij_image_Collect_eq: "bij f ==> f ` Collect P = {y. P (inv f y)}"
```
```   257 apply auto
```
```   258 apply (force simp add: bij_is_inj)
```
```   259 apply (blast intro: bij_is_surj [THEN surj_f_inv_f, symmetric])
```
```   260 done
```
```   261
```
```   262 lemma bij_vimage_eq_inv_image: "bij f ==> f -` A = inv f ` A"
```
```   263 apply (auto simp add: bij_is_surj [THEN surj_f_inv_f])
```
```   264 apply (blast intro: bij_is_inj [THEN inv_into_f_f, symmetric])
```
```   265 done
```
```   266
```
```   267 lemma finite_fun_UNIVD1:
```
```   268   assumes fin: "finite (UNIV :: ('a \<Rightarrow> 'b) set)"
```
```   269   and card: "card (UNIV :: 'b set) \<noteq> Suc 0"
```
```   270   shows "finite (UNIV :: 'a set)"
```
```   271 proof -
```
```   272   from fin have finb: "finite (UNIV :: 'b set)" by (rule finite_fun_UNIVD2)
```
```   273   with card have "card (UNIV :: 'b set) \<ge> Suc (Suc 0)"
```
```   274     by (cases "card (UNIV :: 'b set)") (auto simp add: card_eq_0_iff)
```
```   275   then obtain n where "card (UNIV :: 'b set) = Suc (Suc n)" "n = card (UNIV :: 'b set) - Suc (Suc 0)" by auto
```
```   276   then obtain b1 b2 where b1b2: "(b1 :: 'b) \<noteq> (b2 :: 'b)" by (auto simp add: card_Suc_eq)
```
```   277   from fin have "finite (range (\<lambda>f :: 'a \<Rightarrow> 'b. inv f b1))" by (rule finite_imageI)
```
```   278   moreover have "UNIV = range (\<lambda>f :: 'a \<Rightarrow> 'b. inv f b1)"
```
```   279   proof (rule UNIV_eq_I)
```
```   280     fix x :: 'a
```
```   281     from b1b2 have "x = inv (\<lambda>y. if y = x then b1 else b2) b1" by (simp add: inv_into_def)
```
```   282     thus "x \<in> range (\<lambda>f\<Colon>'a \<Rightarrow> 'b. inv f b1)" by blast
```
```   283   qed
```
```   284   ultimately show "finite (UNIV :: 'a set)" by simp
```
```   285 qed
```
```   286
```
```   287 text {*
```
```   288   Every infinite set contains a countable subset. More precisely we
```
```   289   show that a set @{text S} is infinite if and only if there exists an
```
```   290   injective function from the naturals into @{text S}.
```
```   291
```
```   292   The ``only if'' direction is harder because it requires the
```
```   293   construction of a sequence of pairwise different elements of an
```
```   294   infinite set @{text S}. The idea is to construct a sequence of
```
```   295   non-empty and infinite subsets of @{text S} obtained by successively
```
```   296   removing elements of @{text S}.
```
```   297 *}
```
```   298
```
```   299 lemma infinite_countable_subset:
```
```   300   assumes inf: "\<not> finite (S::'a set)"
```
```   301   shows "\<exists>f. inj (f::nat \<Rightarrow> 'a) \<and> range f \<subseteq> S"
```
```   302   -- {* Courtesy of Stephan Merz *}
```
```   303 proof -
```
```   304   def Sseq \<equiv> "rec_nat S (\<lambda>n T. T - {SOME e. e \<in> T})"
```
```   305   def pick \<equiv> "\<lambda>n. (SOME e. e \<in> Sseq n)"
```
```   306   { fix n have "Sseq n \<subseteq> S" "\<not> finite (Sseq n)" by (induct n) (auto simp add: Sseq_def inf) }
```
```   307   moreover then have *: "\<And>n. pick n \<in> Sseq n"
```
```   308     unfolding pick_def by (subst (asm) finite.simps) (auto simp add: ex_in_conv intro: someI_ex)
```
```   309   ultimately have "range pick \<subseteq> S" by auto
```
```   310   moreover
```
```   311   { fix n m
```
```   312     have "pick n \<notin> Sseq (n + Suc m)" by (induct m) (auto simp add: Sseq_def pick_def)
```
```   313     with * have "pick n \<noteq> pick (n + Suc m)" by auto
```
```   314   }
```
```   315   then have "inj pick" by (intro linorder_injI) (auto simp add: less_iff_Suc_add)
```
```   316   ultimately show ?thesis by blast
```
```   317 qed
```
```   318
```
```   319 lemma infinite_iff_countable_subset: "\<not> finite S \<longleftrightarrow> (\<exists>f. inj (f::nat \<Rightarrow> 'a) \<and> range f \<subseteq> S)"
```
```   320   -- {* Courtesy of Stephan Merz *}
```
```   321   using finite_imageD finite_subset infinite_UNIV_char_0 infinite_countable_subset by auto
```
```   322
```
```   323 lemma image_inv_into_cancel:
```
```   324   assumes SURJ: "f`A=A'" and SUB: "B' \<le> A'"
```
```   325   shows "f `((inv_into A f)`B') = B'"
```
```   326   using assms
```
```   327 proof (auto simp add: f_inv_into_f)
```
```   328   let ?f' = "(inv_into A f)"
```
```   329   fix a' assume *: "a' \<in> B'"
```
```   330   then have "a' \<in> A'" using SUB by auto
```
```   331   then have "a' = f (?f' a')"
```
```   332     using SURJ by (auto simp add: f_inv_into_f)
```
```   333   then show "a' \<in> f ` (?f' ` B')" using * by blast
```
```   334 qed
```
```   335
```
```   336 lemma inv_into_inv_into_eq:
```
```   337   assumes "bij_betw f A A'" "a \<in> A"
```
```   338   shows "inv_into A' (inv_into A f) a = f a"
```
```   339 proof -
```
```   340   let ?f' = "inv_into A f"   let ?f'' = "inv_into A' ?f'"
```
```   341   have 1: "bij_betw ?f' A' A" using assms
```
```   342   by (auto simp add: bij_betw_inv_into)
```
```   343   obtain a' where 2: "a' \<in> A'" and 3: "?f' a' = a"
```
```   344     using 1 `a \<in> A` unfolding bij_betw_def by force
```
```   345   hence "?f'' a = a'"
```
```   346     using `a \<in> A` 1 3 by (auto simp add: f_inv_into_f bij_betw_def)
```
```   347   moreover have "f a = a'" using assms 2 3
```
```   348     by (auto simp add: bij_betw_def)
```
```   349   ultimately show "?f'' a = f a" by simp
```
```   350 qed
```
```   351
```
```   352 lemma inj_on_iff_surj:
```
```   353   assumes "A \<noteq> {}"
```
```   354   shows "(\<exists>f. inj_on f A \<and> f ` A \<le> A') \<longleftrightarrow> (\<exists>g. g ` A' = A)"
```
```   355 proof safe
```
```   356   fix f assume INJ: "inj_on f A" and INCL: "f ` A \<le> A'"
```
```   357   let ?phi = "\<lambda>a' a. a \<in> A \<and> f a = a'"  let ?csi = "\<lambda>a. a \<in> A"
```
```   358   let ?g = "\<lambda>a'. if a' \<in> f ` A then (SOME a. ?phi a' a) else (SOME a. ?csi a)"
```
```   359   have "?g ` A' = A"
```
```   360   proof
```
```   361     show "?g ` A' \<le> A"
```
```   362     proof clarify
```
```   363       fix a' assume *: "a' \<in> A'"
```
```   364       show "?g a' \<in> A"
```
```   365       proof cases
```
```   366         assume Case1: "a' \<in> f ` A"
```
```   367         then obtain a where "?phi a' a" by blast
```
```   368         hence "?phi a' (SOME a. ?phi a' a)" using someI[of "?phi a'" a] by blast
```
```   369         with Case1 show ?thesis by auto
```
```   370       next
```
```   371         assume Case2: "a' \<notin> f ` A"
```
```   372         hence "?csi (SOME a. ?csi a)" using assms someI_ex[of ?csi] by blast
```
```   373         with Case2 show ?thesis by auto
```
```   374       qed
```
```   375     qed
```
```   376   next
```
```   377     show "A \<le> ?g ` A'"
```
```   378     proof-
```
```   379       {fix a assume *: "a \<in> A"
```
```   380        let ?b = "SOME aa. ?phi (f a) aa"
```
```   381        have "?phi (f a) a" using * by auto
```
```   382        hence 1: "?phi (f a) ?b" using someI[of "?phi(f a)" a] by blast
```
```   383        hence "?g(f a) = ?b" using * by auto
```
```   384        moreover have "a = ?b" using 1 INJ * by (auto simp add: inj_on_def)
```
```   385        ultimately have "?g(f a) = a" by simp
```
```   386        with INCL * have "?g(f a) = a \<and> f a \<in> A'" by auto
```
```   387       }
```
```   388       thus ?thesis by force
```
```   389     qed
```
```   390   qed
```
```   391   thus "\<exists>g. g ` A' = A" by blast
```
```   392 next
```
```   393   fix g  let ?f = "inv_into A' g"
```
```   394   have "inj_on ?f (g ` A')"
```
```   395     by (auto simp add: inj_on_inv_into)
```
```   396   moreover
```
```   397   {fix a' assume *: "a' \<in> A'"
```
```   398    let ?phi = "\<lambda> b'. b' \<in> A' \<and> g b' = g a'"
```
```   399    have "?phi a'" using * by auto
```
```   400    hence "?phi(SOME b'. ?phi b')" using someI[of ?phi] by blast
```
```   401    hence "?f(g a') \<in> A'" unfolding inv_into_def by auto
```
```   402   }
```
```   403   ultimately show "\<exists>f. inj_on f (g ` A') \<and> f ` g ` A' \<subseteq> A'" by auto
```
```   404 qed
```
```   405
```
```   406 lemma Ex_inj_on_UNION_Sigma:
```
```   407   "\<exists>f. (inj_on f (\<Union> i \<in> I. A i) \<and> f ` (\<Union> i \<in> I. A i) \<le> (SIGMA i : I. A i))"
```
```   408 proof
```
```   409   let ?phi = "\<lambda> a i. i \<in> I \<and> a \<in> A i"
```
```   410   let ?sm = "\<lambda> a. SOME i. ?phi a i"
```
```   411   let ?f = "\<lambda>a. (?sm a, a)"
```
```   412   have "inj_on ?f (\<Union> i \<in> I. A i)" unfolding inj_on_def by auto
```
```   413   moreover
```
```   414   { { fix i a assume "i \<in> I" and "a \<in> A i"
```
```   415       hence "?sm a \<in> I \<and> a \<in> A(?sm a)" using someI[of "?phi a" i] by auto
```
```   416     }
```
```   417     hence "?f ` (\<Union> i \<in> I. A i) \<le> (SIGMA i : I. A i)" by auto
```
```   418   }
```
```   419   ultimately
```
```   420   show "inj_on ?f (\<Union> i \<in> I. A i) \<and> ?f ` (\<Union> i \<in> I. A i) \<le> (SIGMA i : I. A i)"
```
```   421   by auto
```
```   422 qed
```
```   423
```
```   424 lemma inv_unique_comp:
```
```   425   assumes fg: "f \<circ> g = id"
```
```   426     and gf: "g \<circ> f = id"
```
```   427   shows "inv f = g"
```
```   428   using fg gf inv_equality[of g f] by (auto simp add: fun_eq_iff)
```
```   429
```
```   430
```
```   431 subsection {* The Cantor-Bernstein Theorem *}
```
```   432
```
```   433 lemma Cantor_Bernstein_aux:
```
```   434   shows "\<exists>A' h. A' \<le> A \<and>
```
```   435                 (\<forall>a \<in> A'. a \<notin> g`(B - f ` A')) \<and>
```
```   436                 (\<forall>a \<in> A'. h a = f a) \<and>
```
```   437                 (\<forall>a \<in> A - A'. h a \<in> B - (f ` A') \<and> a = g(h a))"
```
```   438 proof-
```
```   439   obtain H where H_def: "H = (\<lambda> A'. A - (g`(B - (f ` A'))))" by blast
```
```   440   have 0: "mono H" unfolding mono_def H_def by blast
```
```   441   then obtain A' where 1: "H A' = A'" using lfp_unfold by blast
```
```   442   hence 2: "A' = A - (g`(B - (f ` A')))" unfolding H_def by simp
```
```   443   hence 3: "A' \<le> A" by blast
```
```   444   have 4: "\<forall>a \<in> A'.  a \<notin> g`(B - f ` A')"
```
```   445   using 2 by blast
```
```   446   have 5: "\<forall>a \<in> A - A'. \<exists>b \<in> B - (f ` A'). a = g b"
```
```   447   using 2 by blast
```
```   448   (*  *)
```
```   449   obtain h where h_def:
```
```   450   "h = (\<lambda> a. if a \<in> A' then f a else (SOME b. b \<in> B - (f ` A') \<and> a = g b))" by blast
```
```   451   hence "\<forall>a \<in> A'. h a = f a" by auto
```
```   452   moreover
```
```   453   have "\<forall>a \<in> A - A'. h a \<in> B - (f ` A') \<and> a = g(h a)"
```
```   454   proof
```
```   455     fix a assume *: "a \<in> A - A'"
```
```   456     let ?phi = "\<lambda> b. b \<in> B - (f ` A') \<and> a = g b"
```
```   457     have "h a = (SOME b. ?phi b)" using h_def * by auto
```
```   458     moreover have "\<exists>b. ?phi b" using 5 *  by auto
```
```   459     ultimately show  "?phi (h a)" using someI_ex[of ?phi] by auto
```
```   460   qed
```
```   461   ultimately show ?thesis using 3 4 by blast
```
```   462 qed
```
```   463
```
```   464 theorem Cantor_Bernstein:
```
```   465   assumes INJ1: "inj_on f A" and SUB1: "f ` A \<le> B" and
```
```   466           INJ2: "inj_on g B" and SUB2: "g ` B \<le> A"
```
```   467   shows "\<exists>h. bij_betw h A B"
```
```   468 proof-
```
```   469   obtain A' and h where 0: "A' \<le> A" and
```
```   470   1: "\<forall>a \<in> A'. a \<notin> g`(B - f ` A')" and
```
```   471   2: "\<forall>a \<in> A'. h a = f a" and
```
```   472   3: "\<forall>a \<in> A - A'. h a \<in> B - (f ` A') \<and> a = g(h a)"
```
```   473   using Cantor_Bernstein_aux[of A g B f] by blast
```
```   474   have "inj_on h A"
```
```   475   proof (intro inj_onI)
```
```   476     fix a1 a2
```
```   477     assume 4: "a1 \<in> A" and 5: "a2 \<in> A" and 6: "h a1 = h a2"
```
```   478     show "a1 = a2"
```
```   479     proof(cases "a1 \<in> A'")
```
```   480       assume Case1: "a1 \<in> A'"
```
```   481       show ?thesis
```
```   482       proof(cases "a2 \<in> A'")
```
```   483         assume Case11: "a2 \<in> A'"
```
```   484         hence "f a1 = f a2" using Case1 2 6 by auto
```
```   485         thus ?thesis using INJ1 Case1 Case11 0
```
```   486         unfolding inj_on_def by blast
```
```   487       next
```
```   488         assume Case12: "a2 \<notin> A'"
```
```   489         hence False using 3 5 2 6 Case1 by force
```
```   490         thus ?thesis by simp
```
```   491       qed
```
```   492     next
```
```   493     assume Case2: "a1 \<notin> A'"
```
```   494       show ?thesis
```
```   495       proof(cases "a2 \<in> A'")
```
```   496         assume Case21: "a2 \<in> A'"
```
```   497         hence False using 3 4 2 6 Case2 by auto
```
```   498         thus ?thesis by simp
```
```   499       next
```
```   500         assume Case22: "a2 \<notin> A'"
```
```   501         hence "a1 = g(h a1) \<and> a2 = g(h a2)" using Case2 4 5 3 by auto
```
```   502         thus ?thesis using 6 by simp
```
```   503       qed
```
```   504     qed
```
```   505   qed
```
```   506   (*  *)
```
```   507   moreover
```
```   508   have "h ` A = B"
```
```   509   proof safe
```
```   510     fix a assume "a \<in> A"
```
```   511     thus "h a \<in> B" using SUB1 2 3 by (cases "a \<in> A'") auto
```
```   512   next
```
```   513     fix b assume *: "b \<in> B"
```
```   514     show "b \<in> h ` A"
```
```   515     proof(cases "b \<in> f ` A'")
```
```   516       assume Case1: "b \<in> f ` A'"
```
```   517       then obtain a where "a \<in> A' \<and> b = f a" by blast
```
```   518       thus ?thesis using 2 0 by force
```
```   519     next
```
```   520       assume Case2: "b \<notin> f ` A'"
```
```   521       hence "g b \<notin> A'" using 1 * by auto
```
```   522       hence 4: "g b \<in> A - A'" using * SUB2 by auto
```
```   523       hence "h(g b) \<in> B \<and> g(h(g b)) = g b"
```
```   524       using 3 by auto
```
```   525       hence "h(g b) = b" using * INJ2 unfolding inj_on_def by auto
```
```   526       thus ?thesis using 4 by force
```
```   527     qed
```
```   528   qed
```
```   529   (*  *)
```
```   530   ultimately show ?thesis unfolding bij_betw_def by auto
```
```   531 qed
```
```   532
```
```   533 subsection {*Other Consequences of Hilbert's Epsilon*}
```
```   534
```
```   535 text {*Hilbert's Epsilon and the @{term split} Operator*}
```
```   536
```
```   537 text{*Looping simprule*}
```
```   538 lemma split_paired_Eps: "(SOME x. P x) = (SOME (a,b). P(a,b))"
```
```   539   by simp
```
```   540
```
```   541 lemma Eps_split: "Eps (split P) = (SOME xy. P (fst xy) (snd xy))"
```
```   542   by (simp add: split_def)
```
```   543
```
```   544 lemma Eps_split_eq [simp]: "(@(x',y'). x = x' & y = y') = (x,y)"
```
```   545   by blast
```
```   546
```
```   547
```
```   548 text{*A relation is wellfounded iff it has no infinite descending chain*}
```
```   549 lemma wf_iff_no_infinite_down_chain:
```
```   550   "wf r = (~(\<exists>f. \<forall>i. (f(Suc i),f i) \<in> r))"
```
```   551 apply (simp only: wf_eq_minimal)
```
```   552 apply (rule iffI)
```
```   553  apply (rule notI)
```
```   554  apply (erule exE)
```
```   555  apply (erule_tac x = "{w. \<exists>i. w=f i}" in allE, blast)
```
```   556 apply (erule contrapos_np, simp, clarify)
```
```   557 apply (subgoal_tac "\<forall>n. rec_nat x (%i y. @z. z:Q & (z,y) :r) n \<in> Q")
```
```   558  apply (rule_tac x = "rec_nat x (%i y. @z. z:Q & (z,y) :r)" in exI)
```
```   559  apply (rule allI, simp)
```
```   560  apply (rule someI2_ex, blast, blast)
```
```   561 apply (rule allI)
```
```   562 apply (induct_tac "n", simp_all)
```
```   563 apply (rule someI2_ex, blast+)
```
```   564 done
```
```   565
```
```   566 lemma wf_no_infinite_down_chainE:
```
```   567   assumes "wf r" obtains k where "(f (Suc k), f k) \<notin> r"
```
```   568 using `wf r` wf_iff_no_infinite_down_chain[of r] by blast
```
```   569
```
```   570
```
```   571 text{*A dynamically-scoped fact for TFL *}
```
```   572 lemma tfl_some: "\<forall>P x. P x --> P (Eps P)"
```
```   573   by (blast intro: someI)
```
```   574
```
```   575
```
```   576 subsection {* Least value operator *}
```
```   577
```
```   578 definition
```
```   579   LeastM :: "['a => 'b::ord, 'a => bool] => 'a" where
```
```   580   "LeastM m P == SOME x. P x & (\<forall>y. P y --> m x <= m y)"
```
```   581
```
```   582 syntax
```
```   583   "_LeastM" :: "[pttrn, 'a => 'b::ord, bool] => 'a"    ("LEAST _ WRT _. _" [0, 4, 10] 10)
```
```   584 translations
```
```   585   "LEAST x WRT m. P" == "CONST LeastM m (%x. P)"
```
```   586
```
```   587 lemma LeastMI2:
```
```   588   "P x ==> (!!y. P y ==> m x <= m y)
```
```   589     ==> (!!x. P x ==> \<forall>y. P y --> m x \<le> m y ==> Q x)
```
```   590     ==> Q (LeastM m P)"
```
```   591   apply (simp add: LeastM_def)
```
```   592   apply (rule someI2_ex, blast, blast)
```
```   593   done
```
```   594
```
```   595 lemma LeastM_equality:
```
```   596   "P k ==> (!!x. P x ==> m k <= m x)
```
```   597     ==> m (LEAST x WRT m. P x) = (m k::'a::order)"
```
```   598   apply (rule LeastMI2, assumption, blast)
```
```   599   apply (blast intro!: order_antisym)
```
```   600   done
```
```   601
```
```   602 lemma wf_linord_ex_has_least:
```
```   603   "wf r ==> \<forall>x y. ((x,y):r^+) = ((y,x)~:r^*) ==> P k
```
```   604     ==> \<exists>x. P x & (!y. P y --> (m x,m y):r^*)"
```
```   605   apply (drule wf_trancl [THEN wf_eq_minimal [THEN iffD1]])
```
```   606   apply (drule_tac x = "m`Collect P" in spec, force)
```
```   607   done
```
```   608
```
```   609 lemma ex_has_least_nat:
```
```   610     "P k ==> \<exists>x. P x & (\<forall>y. P y --> m x <= (m y::nat))"
```
```   611   apply (simp only: pred_nat_trancl_eq_le [symmetric])
```
```   612   apply (rule wf_pred_nat [THEN wf_linord_ex_has_least])
```
```   613    apply (simp add: less_eq linorder_not_le pred_nat_trancl_eq_le, assumption)
```
```   614   done
```
```   615
```
```   616 lemma LeastM_nat_lemma:
```
```   617     "P k ==> P (LeastM m P) & (\<forall>y. P y --> m (LeastM m P) <= (m y::nat))"
```
```   618   apply (simp add: LeastM_def)
```
```   619   apply (rule someI_ex)
```
```   620   apply (erule ex_has_least_nat)
```
```   621   done
```
```   622
```
```   623 lemmas LeastM_natI = LeastM_nat_lemma [THEN conjunct1]
```
```   624
```
```   625 lemma LeastM_nat_le: "P x ==> m (LeastM m P) <= (m x::nat)"
```
```   626 by (rule LeastM_nat_lemma [THEN conjunct2, THEN spec, THEN mp], assumption, assumption)
```
```   627
```
```   628
```
```   629 subsection {* Greatest value operator *}
```
```   630
```
```   631 definition
```
```   632   GreatestM :: "['a => 'b::ord, 'a => bool] => 'a" where
```
```   633   "GreatestM m P == SOME x. P x & (\<forall>y. P y --> m y <= m x)"
```
```   634
```
```   635 definition
```
```   636   Greatest :: "('a::ord => bool) => 'a" (binder "GREATEST " 10) where
```
```   637   "Greatest == GreatestM (%x. x)"
```
```   638
```
```   639 syntax
```
```   640   "_GreatestM" :: "[pttrn, 'a => 'b::ord, bool] => 'a"
```
```   641       ("GREATEST _ WRT _. _" [0, 4, 10] 10)
```
```   642 translations
```
```   643   "GREATEST x WRT m. P" == "CONST GreatestM m (%x. P)"
```
```   644
```
```   645 lemma GreatestMI2:
```
```   646   "P x ==> (!!y. P y ==> m y <= m x)
```
```   647     ==> (!!x. P x ==> \<forall>y. P y --> m y \<le> m x ==> Q x)
```
```   648     ==> Q (GreatestM m P)"
```
```   649   apply (simp add: GreatestM_def)
```
```   650   apply (rule someI2_ex, blast, blast)
```
```   651   done
```
```   652
```
```   653 lemma GreatestM_equality:
```
```   654  "P k ==> (!!x. P x ==> m x <= m k)
```
```   655     ==> m (GREATEST x WRT m. P x) = (m k::'a::order)"
```
```   656   apply (rule_tac m = m in GreatestMI2, assumption, blast)
```
```   657   apply (blast intro!: order_antisym)
```
```   658   done
```
```   659
```
```   660 lemma Greatest_equality:
```
```   661   "P (k::'a::order) ==> (!!x. P x ==> x <= k) ==> (GREATEST x. P x) = k"
```
```   662   apply (simp add: Greatest_def)
```
```   663   apply (erule GreatestM_equality, blast)
```
```   664   done
```
```   665
```
```   666 lemma ex_has_greatest_nat_lemma:
```
```   667   "P k ==> \<forall>x. P x --> (\<exists>y. P y & ~ ((m y::nat) <= m x))
```
```   668     ==> \<exists>y. P y & ~ (m y < m k + n)"
```
```   669   apply (induct n, force)
```
```   670   apply (force simp add: le_Suc_eq)
```
```   671   done
```
```   672
```
```   673 lemma ex_has_greatest_nat:
```
```   674   "P k ==> \<forall>y. P y --> m y < b
```
```   675     ==> \<exists>x. P x & (\<forall>y. P y --> (m y::nat) <= m x)"
```
```   676   apply (rule ccontr)
```
```   677   apply (cut_tac P = P and n = "b - m k" in ex_has_greatest_nat_lemma)
```
```   678     apply (subgoal_tac  "m k <= b", auto)
```
```   679   done
```
```   680
```
```   681 lemma GreatestM_nat_lemma:
```
```   682   "P k ==> \<forall>y. P y --> m y < b
```
```   683     ==> P (GreatestM m P) & (\<forall>y. P y --> (m y::nat) <= m (GreatestM m P))"
```
```   684   apply (simp add: GreatestM_def)
```
```   685   apply (rule someI_ex)
```
```   686   apply (erule ex_has_greatest_nat, assumption)
```
```   687   done
```
```   688
```
```   689 lemmas GreatestM_natI = GreatestM_nat_lemma [THEN conjunct1]
```
```   690
```
```   691 lemma GreatestM_nat_le:
```
```   692   "P x ==> \<forall>y. P y --> m y < b
```
```   693     ==> (m x::nat) <= m (GreatestM m P)"
```
```   694   apply (blast dest: GreatestM_nat_lemma [THEN conjunct2, THEN spec, of P])
```
```   695   done
```
```   696
```
```   697
```
```   698 text {* \medskip Specialization to @{text GREATEST}. *}
```
```   699
```
```   700 lemma GreatestI: "P (k::nat) ==> \<forall>y. P y --> y < b ==> P (GREATEST x. P x)"
```
```   701   apply (simp add: Greatest_def)
```
```   702   apply (rule GreatestM_natI, auto)
```
```   703   done
```
```   704
```
```   705 lemma Greatest_le:
```
```   706     "P x ==> \<forall>y. P y --> y < b ==> (x::nat) <= (GREATEST x. P x)"
```
```   707   apply (simp add: Greatest_def)
```
```   708   apply (rule GreatestM_nat_le, auto)
```
```   709   done
```
```   710
```
```   711
```
```   712 subsection {* An aside: bounded accessible part *}
```
```   713
```
```   714 text {* Finite monotone eventually stable sequences *}
```
```   715
```
```   716 lemma finite_mono_remains_stable_implies_strict_prefix:
```
```   717   fixes f :: "nat \<Rightarrow> 'a::order"
```
```   718   assumes S: "finite (range f)" "mono f" and eq: "\<forall>n. f n = f (Suc n) \<longrightarrow> f (Suc n) = f (Suc (Suc n))"
```
```   719   shows "\<exists>N. (\<forall>n\<le>N. \<forall>m\<le>N. m < n \<longrightarrow> f m < f n) \<and> (\<forall>n\<ge>N. f N = f n)"
```
```   720   using assms
```
```   721 proof -
```
```   722   have "\<exists>n. f n = f (Suc n)"
```
```   723   proof (rule ccontr)
```
```   724     assume "\<not> ?thesis"
```
```   725     then have "\<And>n. f n \<noteq> f (Suc n)" by auto
```
```   726     then have "\<And>n. f n < f (Suc n)"
```
```   727       using  `mono f` by (auto simp: le_less mono_iff_le_Suc)
```
```   728     with lift_Suc_mono_less_iff[of f]
```
```   729     have *: "\<And>n m. n < m \<Longrightarrow> f n < f m" by auto
```
```   730     have "inj f"
```
```   731     proof (intro injI)
```
```   732       fix x y
```
```   733       assume "f x = f y"
```
```   734       then show "x = y" by (cases x y rule: linorder_cases) (auto dest: *)
```
```   735     qed
```
```   736     with `finite (range f)` have "finite (UNIV::nat set)"
```
```   737       by (rule finite_imageD)
```
```   738     then show False by simp
```
```   739   qed
```
```   740   then obtain n where n: "f n = f (Suc n)" ..
```
```   741   def N \<equiv> "LEAST n. f n = f (Suc n)"
```
```   742   have N: "f N = f (Suc N)"
```
```   743     unfolding N_def using n by (rule LeastI)
```
```   744   show ?thesis
```
```   745   proof (intro exI[of _ N] conjI allI impI)
```
```   746     fix n assume "N \<le> n"
```
```   747     then have "\<And>m. N \<le> m \<Longrightarrow> m \<le> n \<Longrightarrow> f m = f N"
```
```   748     proof (induct rule: dec_induct)
```
```   749       case (step n) then show ?case
```
```   750         using eq[rule_format, of "n - 1"] N
```
```   751         by (cases n) (auto simp add: le_Suc_eq)
```
```   752     qed simp
```
```   753     from this[of n] `N \<le> n` show "f N = f n" by auto
```
```   754   next
```
```   755     fix n m :: nat assume "m < n" "n \<le> N"
```
```   756     then show "f m < f n"
```
```   757     proof (induct rule: less_Suc_induct[consumes 1])
```
```   758       case (1 i)
```
```   759       then have "i < N" by simp
```
```   760       then have "f i \<noteq> f (Suc i)"
```
```   761         unfolding N_def by (rule not_less_Least)
```
```   762       with `mono f` show ?case by (simp add: mono_iff_le_Suc less_le)
```
```   763     qed auto
```
```   764   qed
```
```   765 qed
```
```   766
```
```   767 lemma finite_mono_strict_prefix_implies_finite_fixpoint:
```
```   768   fixes f :: "nat \<Rightarrow> 'a set"
```
```   769   assumes S: "\<And>i. f i \<subseteq> S" "finite S"
```
```   770     and inj: "\<exists>N. (\<forall>n\<le>N. \<forall>m\<le>N. m < n \<longrightarrow> f m \<subset> f n) \<and> (\<forall>n\<ge>N. f N = f n)"
```
```   771   shows "f (card S) = (\<Union>n. f n)"
```
```   772 proof -
```
```   773   from inj obtain N where inj: "(\<forall>n\<le>N. \<forall>m\<le>N. m < n \<longrightarrow> f m \<subset> f n)" and eq: "(\<forall>n\<ge>N. f N = f n)" by auto
```
```   774
```
```   775   { fix i have "i \<le> N \<Longrightarrow> i \<le> card (f i)"
```
```   776     proof (induct i)
```
```   777       case 0 then show ?case by simp
```
```   778     next
```
```   779       case (Suc i)
```
```   780       with inj[rule_format, of "Suc i" i]
```
```   781       have "(f i) \<subset> (f (Suc i))" by auto
```
```   782       moreover have "finite (f (Suc i))" using S by (rule finite_subset)
```
```   783       ultimately have "card (f i) < card (f (Suc i))" by (intro psubset_card_mono)
```
```   784       with Suc show ?case using inj by auto
```
```   785     qed
```
```   786   }
```
```   787   then have "N \<le> card (f N)" by simp
```
```   788   also have "\<dots> \<le> card S" using S by (intro card_mono)
```
```   789   finally have "f (card S) = f N" using eq by auto
```
```   790   then show ?thesis using eq inj[rule_format, of N]
```
```   791     apply auto
```
```   792     apply (case_tac "n < N")
```
```   793     apply (auto simp: not_less)
```
```   794     done
```
```   795 qed
```
```   796
```
```   797
```
```   798 subsection {* More on injections, bijections, and inverses *}
```
```   799
```
```   800 lemma infinite_imp_bij_betw:
```
```   801 assumes INF: "\<not> finite A"
```
```   802 shows "\<exists>h. bij_betw h A (A - {a})"
```
```   803 proof(cases "a \<in> A")
```
```   804   assume Case1: "a \<notin> A"  hence "A - {a} = A" by blast
```
```   805   thus ?thesis using bij_betw_id[of A] by auto
```
```   806 next
```
```   807   assume Case2: "a \<in> A"
```
```   808 find_theorems "\<not> finite _"
```
```   809   have "\<not> finite (A - {a})" using INF by auto
```
```   810   with infinite_iff_countable_subset[of "A - {a}"] obtain f::"nat \<Rightarrow> 'a"
```
```   811   where 1: "inj f" and 2: "f ` UNIV \<le> A - {a}" by blast
```
```   812   obtain g where g_def: "g = (\<lambda> n. if n = 0 then a else f (Suc n))" by blast
```
```   813   obtain A' where A'_def: "A' = g ` UNIV" by blast
```
```   814   have temp: "\<forall>y. f y \<noteq> a" using 2 by blast
```
```   815   have 3: "inj_on g UNIV \<and> g ` UNIV \<le> A \<and> a \<in> g ` UNIV"
```
```   816   proof(auto simp add: Case2 g_def, unfold inj_on_def, intro ballI impI,
```
```   817         case_tac "x = 0", auto simp add: 2)
```
```   818     fix y  assume "a = (if y = 0 then a else f (Suc y))"
```
```   819     thus "y = 0" using temp by (case_tac "y = 0", auto)
```
```   820   next
```
```   821     fix x y
```
```   822     assume "f (Suc x) = (if y = 0 then a else f (Suc y))"
```
```   823     thus "x = y" using 1 temp unfolding inj_on_def by (case_tac "y = 0", auto)
```
```   824   next
```
```   825     fix n show "f (Suc n) \<in> A" using 2 by blast
```
```   826   qed
```
```   827   hence 4: "bij_betw g UNIV A' \<and> a \<in> A' \<and> A' \<le> A"
```
```   828   using inj_on_imp_bij_betw[of g] unfolding A'_def by auto
```
```   829   hence 5: "bij_betw (inv g) A' UNIV"
```
```   830   by (auto simp add: bij_betw_inv_into)
```
```   831   (*  *)
```
```   832   obtain n where "g n = a" using 3 by auto
```
```   833   hence 6: "bij_betw g (UNIV - {n}) (A' - {a})"
```
```   834   using 3 4 unfolding A'_def
```
```   835   by clarify (rule bij_betw_subset, auto simp: image_set_diff)
```
```   836   (*  *)
```
```   837   obtain v where v_def: "v = (\<lambda> m. if m < n then m else Suc m)" by blast
```
```   838   have 7: "bij_betw v UNIV (UNIV - {n})"
```
```   839   proof(unfold bij_betw_def inj_on_def, intro conjI, clarify)
```
```   840     fix m1 m2 assume "v m1 = v m2"
```
```   841     thus "m1 = m2"
```
```   842     by(case_tac "m1 < n", case_tac "m2 < n",
```
```   843        auto simp add: inj_on_def v_def, case_tac "m2 < n", auto)
```
```   844   next
```
```   845     show "v ` UNIV = UNIV - {n}"
```
```   846     proof(auto simp add: v_def)
```
```   847       fix m assume *: "m \<noteq> n" and **: "m \<notin> Suc ` {m'. \<not> m' < n}"
```
```   848       {assume "n \<le> m" with * have 71: "Suc n \<le> m" by auto
```
```   849        then obtain m' where 72: "m = Suc m'" using Suc_le_D by auto
```
```   850        with 71 have "n \<le> m'" by auto
```
```   851        with 72 ** have False by auto
```
```   852       }
```
```   853       thus "m < n" by force
```
```   854     qed
```
```   855   qed
```
```   856   (*  *)
```
```   857   obtain h' where h'_def: "h' = g o v o (inv g)" by blast
```
```   858   hence 8: "bij_betw h' A' (A' - {a})" using 5 7 6
```
```   859   by (auto simp add: bij_betw_trans)
```
```   860   (*  *)
```
```   861   obtain h where h_def: "h = (\<lambda> b. if b \<in> A' then h' b else b)" by blast
```
```   862   have "\<forall>b \<in> A'. h b = h' b" unfolding h_def by auto
```
```   863   hence "bij_betw h  A' (A' - {a})" using 8 bij_betw_cong[of A' h] by auto
```
```   864   moreover
```
```   865   {have "\<forall>b \<in> A - A'. h b = b" unfolding h_def by auto
```
```   866    hence "bij_betw h  (A - A') (A - A')"
```
```   867    using bij_betw_cong[of "A - A'" h id] bij_betw_id[of "A - A'"] by auto
```
```   868   }
```
```   869   moreover
```
```   870   have "(A' Int (A - A') = {} \<and> A' \<union> (A - A') = A) \<and>
```
```   871         ((A' - {a}) Int (A - A') = {} \<and> (A' - {a}) \<union> (A - A') = A - {a})"
```
```   872   using 4 by blast
```
```   873   ultimately have "bij_betw h A (A - {a})"
```
```   874   using bij_betw_combine[of h A' "A' - {a}" "A - A'" "A - A'"] by simp
```
```   875   thus ?thesis by blast
```
```   876 qed
```
```   877
```
```   878 lemma infinite_imp_bij_betw2:
```
```   879 assumes INF: "\<not> finite A"
```
```   880 shows "\<exists>h. bij_betw h A (A \<union> {a})"
```
```   881 proof(cases "a \<in> A")
```
```   882   assume Case1: "a \<in> A"  hence "A \<union> {a} = A" by blast
```
```   883   thus ?thesis using bij_betw_id[of A] by auto
```
```   884 next
```
```   885   let ?A' = "A \<union> {a}"
```
```   886   assume Case2: "a \<notin> A" hence "A = ?A' - {a}" by blast
```
```   887   moreover have "\<not> finite ?A'" using INF by auto
```
```   888   ultimately obtain f where "bij_betw f ?A' A"
```
```   889   using infinite_imp_bij_betw[of ?A' a] by auto
```
```   890   hence "bij_betw(inv_into ?A' f) A ?A'" using bij_betw_inv_into by blast
```
```   891   thus ?thesis by auto
```
```   892 qed
```
```   893
```
```   894 lemma bij_betw_inv_into_left:
```
```   895 assumes BIJ: "bij_betw f A A'" and IN: "a \<in> A"
```
```   896 shows "(inv_into A f) (f a) = a"
```
```   897 using assms unfolding bij_betw_def
```
```   898 by clarify (rule inv_into_f_f)
```
```   899
```
```   900 lemma bij_betw_inv_into_right:
```
```   901 assumes "bij_betw f A A'" "a' \<in> A'"
```
```   902 shows "f(inv_into A f a') = a'"
```
```   903 using assms unfolding bij_betw_def using f_inv_into_f by force
```
```   904
```
```   905 lemma bij_betw_inv_into_subset:
```
```   906 assumes BIJ: "bij_betw f A A'" and
```
```   907         SUB: "B \<le> A" and IM: "f ` B = B'"
```
```   908 shows "bij_betw (inv_into A f) B' B"
```
```   909 using assms unfolding bij_betw_def
```
```   910 by (auto intro: inj_on_inv_into)
```
```   911
```
```   912
```
```   913 subsection {* Specification package -- Hilbertized version *}
```
```   914
```
```   915 lemma exE_some: "[| Ex P ; c == Eps P |] ==> P c"
```
```   916   by (simp only: someI_ex)
```
```   917
```
```   918 ML_file "Tools/choice_specification.ML"
```
```   919
```
```   920 end
```
```   921
```