src/HOL/Analysis/Caratheodory.thy
author haftmann
Sun Nov 18 18:07:51 2018 +0000 (8 months ago)
changeset 69313 b021008c5397
parent 69260 0a9688695a1b
child 69325 4b6ddc5989fc
permissions -rw-r--r--
removed legacy input syntax
     1 (*  Title:      HOL/Analysis/Caratheodory.thy
     2     Author:     Lawrence C Paulson
     3     Author:     Johannes Hölzl, TU München
     4 *)
     5 
     6 section%important \<open>Caratheodory Extension Theorem\<close>
     7 
     8 theory Caratheodory
     9   imports Measure_Space
    10 begin
    11 
    12 text \<open>
    13   Originally from the Hurd/Coble measure theory development, translated by Lawrence Paulson.
    14 \<close>
    15 
    16 lemma%unimportant suminf_ennreal_2dimen:
    17   fixes f:: "nat \<times> nat \<Rightarrow> ennreal"
    18   assumes "\<And>m. g m = (\<Sum>n. f (m,n))"
    19   shows "(\<Sum>i. f (prod_decode i)) = suminf g"
    20 proof -
    21   have g_def: "g = (\<lambda>m. (\<Sum>n. f (m,n)))"
    22     using assms by (simp add: fun_eq_iff)
    23   have reindex: "\<And>B. (\<Sum>x\<in>B. f (prod_decode x)) = sum f (prod_decode ` B)"
    24     by (simp add: sum.reindex[OF inj_prod_decode] comp_def)
    25   have "(SUP n. \<Sum>i<n. f (prod_decode i)) = (SUP p \<in> UNIV \<times> UNIV. \<Sum>i<fst p. \<Sum>n<snd p. f (i, n))"
    26   proof (intro SUP_eq; clarsimp simp: sum.cartesian_product reindex)
    27     fix n
    28     let ?M = "\<lambda>f. Suc (Max (f ` prod_decode ` {..<n}))"
    29     { fix a b x assume "x < n" and [symmetric]: "(a, b) = prod_decode x"
    30       then have "a < ?M fst" "b < ?M snd"
    31         by (auto intro!: Max_ge le_imp_less_Suc image_eqI) }
    32     then have "sum f (prod_decode ` {..<n}) \<le> sum f ({..<?M fst} \<times> {..<?M snd})"
    33       by (auto intro!: sum_mono2)
    34     then show "\<exists>a b. sum f (prod_decode ` {..<n}) \<le> sum f ({..<a} \<times> {..<b})" by auto
    35   next
    36     fix a b
    37     let ?M = "prod_decode ` {..<Suc (Max (prod_encode ` ({..<a} \<times> {..<b})))}"
    38     { fix a' b' assume "a' < a" "b' < b" then have "(a', b') \<in> ?M"
    39         by (auto intro!: Max_ge le_imp_less_Suc image_eqI[where x="prod_encode (a', b')"]) }
    40     then have "sum f ({..<a} \<times> {..<b}) \<le> sum f ?M"
    41       by (auto intro!: sum_mono2)
    42     then show "\<exists>n. sum f ({..<a} \<times> {..<b}) \<le> sum f (prod_decode ` {..<n})"
    43       by auto
    44   qed
    45   also have "\<dots> = (SUP p. \<Sum>i<p. \<Sum>n. f (i, n))"
    46     unfolding suminf_sum[OF summableI, symmetric]
    47     by (simp add: suminf_eq_SUP SUP_pair sum.swap[of _ "{..< fst _}"])
    48   finally show ?thesis unfolding g_def
    49     by (simp add: suminf_eq_SUP)
    50 qed
    51 
    52 subsection%important \<open>Characterizations of Measures\<close>
    53 
    54 definition%important outer_measure_space where
    55   "outer_measure_space M f \<longleftrightarrow> positive M f \<and> increasing M f \<and> countably_subadditive M f"
    56 
    57 subsubsection%important \<open>Lambda Systems\<close>
    58 
    59 definition%important lambda_system :: "'a set \<Rightarrow> 'a set set \<Rightarrow> ('a set \<Rightarrow> ennreal) \<Rightarrow> 'a set set"
    60 where
    61   "lambda_system \<Omega> M f = {l \<in> M. \<forall>x \<in> M. f (l \<inter> x) + f ((\<Omega> - l) \<inter> x) = f x}"
    62 
    63 lemma%unimportant (in algebra) lambda_system_eq:
    64   "lambda_system \<Omega> M f = {l \<in> M. \<forall>x \<in> M. f (x \<inter> l) + f (x - l) = f x}"
    65 proof -
    66   have [simp]: "\<And>l x. l \<in> M \<Longrightarrow> x \<in> M \<Longrightarrow> (\<Omega> - l) \<inter> x = x - l"
    67     by (metis Int_Diff Int_absorb1 Int_commute sets_into_space)
    68   show ?thesis
    69     by (auto simp add: lambda_system_def) (metis Int_commute)+
    70 qed
    71 
    72 lemma%unimportant (in algebra) lambda_system_empty: "positive M f \<Longrightarrow> {} \<in> lambda_system \<Omega> M f"
    73   by (auto simp add: positive_def lambda_system_eq)
    74 
    75 lemma%unimportant lambda_system_sets: "x \<in> lambda_system \<Omega> M f \<Longrightarrow> x \<in> M"
    76   by (simp add: lambda_system_def)
    77 
    78 lemma%unimportant (in algebra) lambda_system_Compl:
    79   fixes f:: "'a set \<Rightarrow> ennreal"
    80   assumes x: "x \<in> lambda_system \<Omega> M f"
    81   shows "\<Omega> - x \<in> lambda_system \<Omega> M f"
    82 proof -
    83   have "x \<subseteq> \<Omega>"
    84     by (metis sets_into_space lambda_system_sets x)
    85   hence "\<Omega> - (\<Omega> - x) = x"
    86     by (metis double_diff equalityE)
    87   with x show ?thesis
    88     by (force simp add: lambda_system_def ac_simps)
    89 qed
    90 
    91 lemma%unimportant (in algebra) lambda_system_Int:
    92   fixes f:: "'a set \<Rightarrow> ennreal"
    93   assumes xl: "x \<in> lambda_system \<Omega> M f" and yl: "y \<in> lambda_system \<Omega> M f"
    94   shows "x \<inter> y \<in> lambda_system \<Omega> M f"
    95 proof -
    96   from xl yl show ?thesis
    97   proof (auto simp add: positive_def lambda_system_eq Int)
    98     fix u
    99     assume x: "x \<in> M" and y: "y \<in> M" and u: "u \<in> M"
   100        and fx: "\<forall>z\<in>M. f (z \<inter> x) + f (z - x) = f z"
   101        and fy: "\<forall>z\<in>M. f (z \<inter> y) + f (z - y) = f z"
   102     have "u - x \<inter> y \<in> M"
   103       by (metis Diff Diff_Int Un u x y)
   104     moreover
   105     have "(u - (x \<inter> y)) \<inter> y = u \<inter> y - x" by blast
   106     moreover
   107     have "u - x \<inter> y - y = u - y" by blast
   108     ultimately
   109     have ey: "f (u - x \<inter> y) = f (u \<inter> y - x) + f (u - y)" using fy
   110       by force
   111     have "f (u \<inter> (x \<inter> y)) + f (u - x \<inter> y)
   112           = (f (u \<inter> (x \<inter> y)) + f (u \<inter> y - x)) + f (u - y)"
   113       by (simp add: ey ac_simps)
   114     also have "... =  (f ((u \<inter> y) \<inter> x) + f (u \<inter> y - x)) + f (u - y)"
   115       by (simp add: Int_ac)
   116     also have "... = f (u \<inter> y) + f (u - y)"
   117       using fx [THEN bspec, of "u \<inter> y"] Int y u
   118       by force
   119     also have "... = f u"
   120       by (metis fy u)
   121     finally show "f (u \<inter> (x \<inter> y)) + f (u - x \<inter> y) = f u" .
   122   qed
   123 qed
   124 
   125 lemma%unimportant (in algebra) lambda_system_Un:
   126   fixes f:: "'a set \<Rightarrow> ennreal"
   127   assumes xl: "x \<in> lambda_system \<Omega> M f" and yl: "y \<in> lambda_system \<Omega> M f"
   128   shows "x \<union> y \<in> lambda_system \<Omega> M f"
   129 proof -
   130   have "(\<Omega> - x) \<inter> (\<Omega> - y) \<in> M"
   131     by (metis Diff_Un Un compl_sets lambda_system_sets xl yl)
   132   moreover
   133   have "x \<union> y = \<Omega> - ((\<Omega> - x) \<inter> (\<Omega> - y))"
   134     by auto (metis subsetD lambda_system_sets sets_into_space xl yl)+
   135   ultimately show ?thesis
   136     by (metis lambda_system_Compl lambda_system_Int xl yl)
   137 qed
   138 
   139 lemma%unimportant (in algebra) lambda_system_algebra:
   140   "positive M f \<Longrightarrow> algebra \<Omega> (lambda_system \<Omega> M f)"
   141   apply (auto simp add: algebra_iff_Un)
   142   apply (metis lambda_system_sets set_mp sets_into_space)
   143   apply (metis lambda_system_empty)
   144   apply (metis lambda_system_Compl)
   145   apply (metis lambda_system_Un)
   146   done
   147 
   148 lemma%unimportant (in algebra) lambda_system_strong_additive:
   149   assumes z: "z \<in> M" and disj: "x \<inter> y = {}"
   150       and xl: "x \<in> lambda_system \<Omega> M f" and yl: "y \<in> lambda_system \<Omega> M f"
   151   shows "f (z \<inter> (x \<union> y)) = f (z \<inter> x) + f (z \<inter> y)"
   152 proof -
   153   have "z \<inter> x = (z \<inter> (x \<union> y)) \<inter> x" using disj by blast
   154   moreover
   155   have "z \<inter> y = (z \<inter> (x \<union> y)) - x" using disj by blast
   156   moreover
   157   have "(z \<inter> (x \<union> y)) \<in> M"
   158     by (metis Int Un lambda_system_sets xl yl z)
   159   ultimately show ?thesis using xl yl
   160     by (simp add: lambda_system_eq)
   161 qed
   162 
   163 lemma%unimportant (in algebra) lambda_system_additive: "additive (lambda_system \<Omega> M f) f"
   164 proof (auto simp add: additive_def)
   165   fix x and y
   166   assume disj: "x \<inter> y = {}"
   167      and xl: "x \<in> lambda_system \<Omega> M f" and yl: "y \<in> lambda_system \<Omega> M f"
   168   hence  "x \<in> M" "y \<in> M" by (blast intro: lambda_system_sets)+
   169   thus "f (x \<union> y) = f x + f y"
   170     using lambda_system_strong_additive [OF top disj xl yl]
   171     by (simp add: Un)
   172 qed
   173 
   174 lemma%unimportant lambda_system_increasing: "increasing M f \<Longrightarrow> increasing (lambda_system \<Omega> M f) f"
   175   by (simp add: increasing_def lambda_system_def)
   176 
   177 lemma%unimportant lambda_system_positive: "positive M f \<Longrightarrow> positive (lambda_system \<Omega> M f) f"
   178   by (simp add: positive_def lambda_system_def)
   179 
   180 lemma%unimportant (in algebra) lambda_system_strong_sum:
   181   fixes A:: "nat \<Rightarrow> 'a set" and f :: "'a set \<Rightarrow> ennreal"
   182   assumes f: "positive M f" and a: "a \<in> M"
   183       and A: "range A \<subseteq> lambda_system \<Omega> M f"
   184       and disj: "disjoint_family A"
   185   shows  "(\<Sum>i = 0..<n. f (a \<inter>A i)) = f (a \<inter> (\<Union>i\<in>{0..<n}. A i))"
   186 proof (induct n)
   187   case 0 show ?case using f by (simp add: positive_def)
   188 next
   189   case (Suc n)
   190   have 2: "A n \<inter> UNION {0..<n} A = {}" using disj
   191     by (force simp add: disjoint_family_on_def neq_iff)
   192   have 3: "A n \<in> lambda_system \<Omega> M f" using A
   193     by blast
   194   interpret l: algebra \<Omega> "lambda_system \<Omega> M f"
   195     using f by (rule lambda_system_algebra)
   196   have 4: "UNION {0..<n} A \<in> lambda_system \<Omega> M f"
   197     using A l.UNION_in_sets by simp
   198   from Suc.hyps show ?case
   199     by (simp add: atLeastLessThanSuc lambda_system_strong_additive [OF a 2 3 4])
   200 qed
   201 
   202 lemma%important (in sigma_algebra) lambda_system_caratheodory:
   203   assumes oms: "outer_measure_space M f"
   204       and A: "range A \<subseteq> lambda_system \<Omega> M f"
   205       and disj: "disjoint_family A"
   206   shows  "(\<Union>i. A i) \<in> lambda_system \<Omega> M f \<and> (\<Sum>i. f (A i)) = f (\<Union>i. A i)"
   207 proof%unimportant -
   208   have pos: "positive M f" and inc: "increasing M f"
   209    and csa: "countably_subadditive M f"
   210     by (metis oms outer_measure_space_def)+
   211   have sa: "subadditive M f"
   212     by (metis countably_subadditive_subadditive csa pos)
   213   have A': "\<And>S. A`S \<subseteq> (lambda_system \<Omega> M f)" using A
   214     by auto
   215   interpret ls: algebra \<Omega> "lambda_system \<Omega> M f"
   216     using pos by (rule lambda_system_algebra)
   217   have A'': "range A \<subseteq> M"
   218      by (metis A image_subset_iff lambda_system_sets)
   219 
   220   have U_in: "(\<Union>i. A i) \<in> M"
   221     by (metis A'' countable_UN)
   222   have U_eq: "f (\<Union>i. A i) = (\<Sum>i. f (A i))"
   223   proof (rule antisym)
   224     show "f (\<Union>i. A i) \<le> (\<Sum>i. f (A i))"
   225       using csa[unfolded countably_subadditive_def] A'' disj U_in by auto
   226     have dis: "\<And>N. disjoint_family_on A {..<N}" by (intro disjoint_family_on_mono[OF _ disj]) auto
   227     show "(\<Sum>i. f (A i)) \<le> f (\<Union>i. A i)"
   228       using ls.additive_sum [OF lambda_system_positive[OF pos] lambda_system_additive _ A' dis] A''
   229       by (intro suminf_le_const[OF summableI]) (auto intro!: increasingD[OF inc] countable_UN)
   230   qed
   231   have "f (a \<inter> (\<Union>i. A i)) + f (a - (\<Union>i. A i)) = f a"
   232     if a [iff]: "a \<in> M" for a
   233   proof (rule antisym)
   234     have "range (\<lambda>i. a \<inter> A i) \<subseteq> M" using A''
   235       by blast
   236     moreover
   237     have "disjoint_family (\<lambda>i. a \<inter> A i)" using disj
   238       by (auto simp add: disjoint_family_on_def)
   239     moreover
   240     have "a \<inter> (\<Union>i. A i) \<in> M"
   241       by (metis Int U_in a)
   242     ultimately
   243     have "f (a \<inter> (\<Union>i. A i)) \<le> (\<Sum>i. f (a \<inter> A i))"
   244       using csa[unfolded countably_subadditive_def, rule_format, of "(\<lambda>i. a \<inter> A i)"]
   245       by (simp add: o_def)
   246     hence "f (a \<inter> (\<Union>i. A i)) + f (a - (\<Union>i. A i)) \<le> (\<Sum>i. f (a \<inter> A i)) + f (a - (\<Union>i. A i))"
   247       by (rule add_right_mono)
   248     also have "\<dots> \<le> f a"
   249     proof (intro ennreal_suminf_bound_add)
   250       fix n
   251       have UNION_in: "(\<Union>i\<in>{0..<n}. A i) \<in> M"
   252         by (metis A'' UNION_in_sets)
   253       have le_fa: "f (UNION {0..<n} A \<inter> a) \<le> f a" using A''
   254         by (blast intro: increasingD [OF inc] A'' UNION_in_sets)
   255       have ls: "(\<Union>i\<in>{0..<n}. A i) \<in> lambda_system \<Omega> M f"
   256         using ls.UNION_in_sets by (simp add: A)
   257       hence eq_fa: "f a = f (a \<inter> (\<Union>i\<in>{0..<n}. A i)) + f (a - (\<Union>i\<in>{0..<n}. A i))"
   258         by (simp add: lambda_system_eq UNION_in)
   259       have "f (a - (\<Union>i. A i)) \<le> f (a - (\<Union>i\<in>{0..<n}. A i))"
   260         by (blast intro: increasingD [OF inc] UNION_in U_in)
   261       thus "(\<Sum>i<n. f (a \<inter> A i)) + f (a - (\<Union>i. A i)) \<le> f a"
   262         by (simp add: lambda_system_strong_sum pos A disj eq_fa add_left_mono atLeast0LessThan[symmetric])
   263     qed
   264     finally show "f (a \<inter> (\<Union>i. A i)) + f (a - (\<Union>i. A i)) \<le> f a"
   265       by simp
   266   next
   267     have "f a \<le> f (a \<inter> (\<Union>i. A i) \<union> (a - (\<Union>i. A i)))"
   268       by (blast intro:  increasingD [OF inc] U_in)
   269     also have "... \<le>  f (a \<inter> (\<Union>i. A i)) + f (a - (\<Union>i. A i))"
   270       by (blast intro: subadditiveD [OF sa] U_in)
   271     finally show "f a \<le> f (a \<inter> (\<Union>i. A i)) + f (a - (\<Union>i. A i))" .
   272   qed
   273   thus  ?thesis
   274     by (simp add: lambda_system_eq sums_iff U_eq U_in)
   275 qed
   276 
   277 lemma%important (in sigma_algebra) caratheodory_lemma:
   278   assumes oms: "outer_measure_space M f"
   279   defines "L \<equiv> lambda_system \<Omega> M f"
   280   shows "measure_space \<Omega> L f"
   281 proof%unimportant -
   282   have pos: "positive M f"
   283     by (metis oms outer_measure_space_def)
   284   have alg: "algebra \<Omega> L"
   285     using lambda_system_algebra [of f, OF pos]
   286     by (simp add: algebra_iff_Un L_def)
   287   then
   288   have "sigma_algebra \<Omega> L"
   289     using lambda_system_caratheodory [OF oms]
   290     by (simp add: sigma_algebra_disjoint_iff L_def)
   291   moreover
   292   have "countably_additive L f" "positive L f"
   293     using pos lambda_system_caratheodory [OF oms]
   294     by (auto simp add: lambda_system_sets L_def countably_additive_def positive_def)
   295   ultimately
   296   show ?thesis
   297     using pos by (simp add: measure_space_def)
   298 qed
   299 
   300 definition%important outer_measure :: "'a set set \<Rightarrow> ('a set \<Rightarrow> ennreal) \<Rightarrow> 'a set \<Rightarrow> ennreal" where
   301    "outer_measure M f X =
   302      (INF A\<in>{A. range A \<subseteq> M \<and> disjoint_family A \<and> X \<subseteq> (\<Union>i. A i)}. \<Sum>i. f (A i))"
   303 
   304 lemma%unimportant (in ring_of_sets) outer_measure_agrees:
   305   assumes posf: "positive M f" and ca: "countably_additive M f" and s: "s \<in> M"
   306   shows "outer_measure M f s = f s"
   307   unfolding outer_measure_def
   308 proof (safe intro!: antisym INF_greatest)
   309   fix A :: "nat \<Rightarrow> 'a set" assume A: "range A \<subseteq> M" and dA: "disjoint_family A" and sA: "s \<subseteq> (\<Union>x. A x)"
   310   have inc: "increasing M f"
   311     by (metis additive_increasing ca countably_additive_additive posf)
   312   have "f s = f (\<Union>i. A i \<inter> s)"
   313     using sA by (auto simp: Int_absorb1)
   314   also have "\<dots> = (\<Sum>i. f (A i \<inter> s))"
   315     using sA dA A s
   316     by (intro ca[unfolded countably_additive_def, rule_format, symmetric])
   317        (auto simp: Int_absorb1 disjoint_family_on_def)
   318   also have "... \<le> (\<Sum>i. f (A i))"
   319     using A s by (auto intro!: suminf_le increasingD[OF inc])
   320   finally show "f s \<le> (\<Sum>i. f (A i))" .
   321 next
   322   have "(\<Sum>i. f (if i = 0 then s else {})) \<le> f s"
   323     using positiveD1[OF posf] by (subst suminf_finite[of "{0}"]) auto
   324   with s show "(INF A\<in>{A. range A \<subseteq> M \<and> disjoint_family A \<and> s \<subseteq> \<Union>(A ` UNIV)}. \<Sum>i. f (A i)) \<le> f s"
   325     by (intro INF_lower2[of "\<lambda>i. if i = 0 then s else {}"])
   326        (auto simp: disjoint_family_on_def)
   327 qed
   328 
   329 lemma%unimportant outer_measure_empty:
   330   "positive M f \<Longrightarrow> {} \<in> M \<Longrightarrow> outer_measure M f {} = 0"
   331   unfolding outer_measure_def
   332   by (intro antisym INF_lower2[of  "\<lambda>_. {}"]) (auto simp: disjoint_family_on_def positive_def)
   333 
   334 lemma%unimportant (in ring_of_sets) positive_outer_measure:
   335   assumes "positive M f" shows "positive (Pow \<Omega>) (outer_measure M f)"
   336   unfolding positive_def by (auto simp: assms outer_measure_empty)
   337 
   338 lemma%unimportant (in ring_of_sets) increasing_outer_measure: "increasing (Pow \<Omega>) (outer_measure M f)"
   339   by (force simp: increasing_def outer_measure_def intro!: INF_greatest intro: INF_lower)
   340 
   341 lemma%unimportant (in ring_of_sets) outer_measure_le:
   342   assumes pos: "positive M f" and inc: "increasing M f" and A: "range A \<subseteq> M" and X: "X \<subseteq> (\<Union>i. A i)"
   343   shows "outer_measure M f X \<le> (\<Sum>i. f (A i))"
   344   unfolding outer_measure_def
   345 proof (safe intro!: INF_lower2[of "disjointed A"] del: subsetI)
   346   show dA: "range (disjointed A) \<subseteq> M"
   347     by (auto intro!: A range_disjointed_sets)
   348   have "\<forall>n. f (disjointed A n) \<le> f (A n)"
   349     by (metis increasingD [OF inc] UNIV_I dA image_subset_iff disjointed_subset A)
   350   then show "(\<Sum>i. f (disjointed A i)) \<le> (\<Sum>i. f (A i))"
   351     by (blast intro!: suminf_le)
   352 qed (auto simp: X UN_disjointed_eq disjoint_family_disjointed)
   353 
   354 lemma%unimportant (in ring_of_sets) outer_measure_close:
   355   "outer_measure M f X < e \<Longrightarrow> \<exists>A. range A \<subseteq> M \<and> disjoint_family A \<and> X \<subseteq> (\<Union>i. A i) \<and> (\<Sum>i. f (A i)) < e"
   356   unfolding outer_measure_def INF_less_iff by auto
   357 
   358 lemma%unimportant (in ring_of_sets) countably_subadditive_outer_measure:
   359   assumes posf: "positive M f" and inc: "increasing M f"
   360   shows "countably_subadditive (Pow \<Omega>) (outer_measure M f)"
   361 proof (simp add: countably_subadditive_def, safe)
   362   fix A :: "nat \<Rightarrow> _" assume A: "range A \<subseteq> Pow (\<Omega>)" and sb: "(\<Union>i. A i) \<subseteq> \<Omega>"
   363   let ?O = "outer_measure M f"
   364   show "?O (\<Union>i. A i) \<le> (\<Sum>n. ?O (A n))"
   365   proof (rule ennreal_le_epsilon)
   366     fix b and e :: real assume "0 < e" "(\<Sum>n. outer_measure M f (A n)) < top"
   367     then have *: "\<And>n. outer_measure M f (A n) < outer_measure M f (A n) + e * (1/2)^Suc n"
   368       by (auto simp add: less_top dest!: ennreal_suminf_lessD)
   369     obtain B
   370       where B: "\<And>n. range (B n) \<subseteq> M"
   371       and sbB: "\<And>n. A n \<subseteq> (\<Union>i. B n i)"
   372       and Ble: "\<And>n. (\<Sum>i. f (B n i)) \<le> ?O (A n) + e * (1/2)^(Suc n)"
   373       by (metis less_imp_le outer_measure_close[OF *])
   374 
   375     define C where "C = case_prod B o prod_decode"
   376     from B have B_in_M: "\<And>i j. B i j \<in> M"
   377       by (rule range_subsetD)
   378     then have C: "range C \<subseteq> M"
   379       by (auto simp add: C_def split_def)
   380     have A_C: "(\<Union>i. A i) \<subseteq> (\<Union>i. C i)"
   381       using sbB by (auto simp add: C_def subset_eq) (metis prod.case prod_encode_inverse)
   382 
   383     have "?O (\<Union>i. A i) \<le> ?O (\<Union>i. C i)"
   384       using A_C A C by (intro increasing_outer_measure[THEN increasingD]) (auto dest!: sets_into_space)
   385     also have "\<dots> \<le> (\<Sum>i. f (C i))"
   386       using C by (intro outer_measure_le[OF posf inc]) auto
   387     also have "\<dots> = (\<Sum>n. \<Sum>i. f (B n i))"
   388       using B_in_M unfolding C_def comp_def by (intro suminf_ennreal_2dimen) auto
   389     also have "\<dots> \<le> (\<Sum>n. ?O (A n) + e * (1/2) ^ Suc n)"
   390       using B_in_M by (intro suminf_le suminf_nonneg allI Ble) auto
   391     also have "... = (\<Sum>n. ?O (A n)) + (\<Sum>n. ennreal e * ennreal ((1/2) ^ Suc n))"
   392       using \<open>0 < e\<close> by (subst suminf_add[symmetric])
   393                        (auto simp del: ennreal_suminf_cmult simp add: ennreal_mult[symmetric])
   394     also have "\<dots> = (\<Sum>n. ?O (A n)) + e"
   395       unfolding ennreal_suminf_cmult
   396       by (subst suminf_ennreal_eq[OF zero_le_power power_half_series]) auto
   397     finally show "?O (\<Union>i. A i) \<le> (\<Sum>n. ?O (A n)) + e" .
   398   qed
   399 qed
   400 
   401 lemma%unimportant (in ring_of_sets) outer_measure_space_outer_measure:
   402   "positive M f \<Longrightarrow> increasing M f \<Longrightarrow> outer_measure_space (Pow \<Omega>) (outer_measure M f)"
   403   by (simp add: outer_measure_space_def
   404     positive_outer_measure increasing_outer_measure countably_subadditive_outer_measure)
   405 
   406 lemma%unimportant (in ring_of_sets) algebra_subset_lambda_system:
   407   assumes posf: "positive M f" and inc: "increasing M f"
   408       and add: "additive M f"
   409   shows "M \<subseteq> lambda_system \<Omega> (Pow \<Omega>) (outer_measure M f)"
   410 proof (auto dest: sets_into_space
   411             simp add: algebra.lambda_system_eq [OF algebra_Pow])
   412   fix x s assume x: "x \<in> M" and s: "s \<subseteq> \<Omega>"
   413   have [simp]: "\<And>x. x \<in> M \<Longrightarrow> s \<inter> (\<Omega> - x) = s - x" using s
   414     by blast
   415   have "outer_measure M f (s \<inter> x) + outer_measure M f (s - x) \<le> outer_measure M f s"
   416     unfolding outer_measure_def[of M f s]
   417   proof (safe intro!: INF_greatest)
   418     fix A :: "nat \<Rightarrow> 'a set" assume A: "disjoint_family A" "range A \<subseteq> M" "s \<subseteq> (\<Union>i. A i)"
   419     have "outer_measure M f (s \<inter> x) \<le> (\<Sum>i. f (A i \<inter> x))"
   420       unfolding outer_measure_def
   421     proof (safe intro!: INF_lower2[of "\<lambda>i. A i \<inter> x"])
   422       from A(1) show "disjoint_family (\<lambda>i. A i \<inter> x)"
   423         by (rule disjoint_family_on_bisimulation) auto
   424     qed (insert x A, auto)
   425     moreover
   426     have "outer_measure M f (s - x) \<le> (\<Sum>i. f (A i - x))"
   427       unfolding outer_measure_def
   428     proof (safe intro!: INF_lower2[of "\<lambda>i. A i - x"])
   429       from A(1) show "disjoint_family (\<lambda>i. A i - x)"
   430         by (rule disjoint_family_on_bisimulation) auto
   431     qed (insert x A, auto)
   432     ultimately have "outer_measure M f (s \<inter> x) + outer_measure M f (s - x) \<le>
   433         (\<Sum>i. f (A i \<inter> x)) + (\<Sum>i. f (A i - x))" by (rule add_mono)
   434     also have "\<dots> = (\<Sum>i. f (A i \<inter> x) + f (A i - x))"
   435       using A(2) x posf by (subst suminf_add) (auto simp: positive_def)
   436     also have "\<dots> = (\<Sum>i. f (A i))"
   437       using A x
   438       by (subst add[THEN additiveD, symmetric])
   439          (auto intro!: arg_cong[where f=suminf] arg_cong[where f=f])
   440     finally show "outer_measure M f (s \<inter> x) + outer_measure M f (s - x) \<le> (\<Sum>i. f (A i))" .
   441   qed
   442   moreover
   443   have "outer_measure M f s \<le> outer_measure M f (s \<inter> x) + outer_measure M f (s - x)"
   444   proof -
   445     have "outer_measure M f s = outer_measure M f ((s \<inter> x) \<union> (s - x))"
   446       by (metis Un_Diff_Int Un_commute)
   447     also have "... \<le> outer_measure M f (s \<inter> x) + outer_measure M f (s - x)"
   448       apply (rule subadditiveD)
   449       apply (rule ring_of_sets.countably_subadditive_subadditive [OF ring_of_sets_Pow])
   450       apply (simp add: positive_def outer_measure_empty[OF posf])
   451       apply (rule countably_subadditive_outer_measure)
   452       using s by (auto intro!: posf inc)
   453     finally show ?thesis .
   454   qed
   455   ultimately
   456   show "outer_measure M f (s \<inter> x) + outer_measure M f (s - x) = outer_measure M f s"
   457     by (rule order_antisym)
   458 qed
   459 
   460 lemma%unimportant measure_down: "measure_space \<Omega> N \<mu> \<Longrightarrow> sigma_algebra \<Omega> M \<Longrightarrow> M \<subseteq> N \<Longrightarrow> measure_space \<Omega> M \<mu>"
   461   by (auto simp add: measure_space_def positive_def countably_additive_def subset_eq)
   462 
   463 subsection%important \<open>Caratheodory's theorem\<close>
   464 
   465 theorem%important (in ring_of_sets) caratheodory':
   466   assumes posf: "positive M f" and ca: "countably_additive M f"
   467   shows "\<exists>\<mu> :: 'a set \<Rightarrow> ennreal. (\<forall>s \<in> M. \<mu> s = f s) \<and> measure_space \<Omega> (sigma_sets \<Omega> M) \<mu>"
   468 proof%unimportant -
   469   have inc: "increasing M f"
   470     by (metis additive_increasing ca countably_additive_additive posf)
   471   let ?O = "outer_measure M f"
   472   define ls where "ls = lambda_system \<Omega> (Pow \<Omega>) ?O"
   473   have mls: "measure_space \<Omega> ls ?O"
   474     using sigma_algebra.caratheodory_lemma
   475             [OF sigma_algebra_Pow outer_measure_space_outer_measure [OF posf inc]]
   476     by (simp add: ls_def)
   477   hence sls: "sigma_algebra \<Omega> ls"
   478     by (simp add: measure_space_def)
   479   have "M \<subseteq> ls"
   480     by (simp add: ls_def)
   481        (metis ca posf inc countably_additive_additive algebra_subset_lambda_system)
   482   hence sgs_sb: "sigma_sets (\<Omega>) (M) \<subseteq> ls"
   483     using sigma_algebra.sigma_sets_subset [OF sls, of "M"]
   484     by simp
   485   have "measure_space \<Omega> (sigma_sets \<Omega> M) ?O"
   486     by (rule measure_down [OF mls], rule sigma_algebra_sigma_sets)
   487        (simp_all add: sgs_sb space_closed)
   488   thus ?thesis using outer_measure_agrees [OF posf ca]
   489     by (intro exI[of _ ?O]) auto
   490 qed
   491 
   492 lemma%important (in ring_of_sets) caratheodory_empty_continuous:
   493   assumes f: "positive M f" "additive M f" and fin: "\<And>A. A \<in> M \<Longrightarrow> f A \<noteq> \<infinity>"
   494   assumes cont: "\<And>A. range A \<subseteq> M \<Longrightarrow> decseq A \<Longrightarrow> (\<Inter>i. A i) = {} \<Longrightarrow> (\<lambda>i. f (A i)) \<longlonglongrightarrow> 0"
   495   shows "\<exists>\<mu> :: 'a set \<Rightarrow> ennreal. (\<forall>s \<in> M. \<mu> s = f s) \<and> measure_space \<Omega> (sigma_sets \<Omega> M) \<mu>"
   496 proof%unimportant (intro caratheodory' empty_continuous_imp_countably_additive f)
   497   show "\<forall>A\<in>M. f A \<noteq> \<infinity>" using fin by auto
   498 qed (rule cont)
   499 
   500 subsection%important \<open>Volumes\<close>
   501 
   502 definition%important volume :: "'a set set \<Rightarrow> ('a set \<Rightarrow> ennreal) \<Rightarrow> bool" where
   503   "volume M f \<longleftrightarrow>
   504   (f {} = 0) \<and> (\<forall>a\<in>M. 0 \<le> f a) \<and>
   505   (\<forall>C\<subseteq>M. disjoint C \<longrightarrow> finite C \<longrightarrow> \<Union>C \<in> M \<longrightarrow> f (\<Union>C) = (\<Sum>c\<in>C. f c))"
   506 
   507 lemma%unimportant volumeI:
   508   assumes "f {} = 0"
   509   assumes "\<And>a. a \<in> M \<Longrightarrow> 0 \<le> f a"
   510   assumes "\<And>C. C \<subseteq> M \<Longrightarrow> disjoint C \<Longrightarrow> finite C \<Longrightarrow> \<Union>C \<in> M \<Longrightarrow> f (\<Union>C) = (\<Sum>c\<in>C. f c)"
   511   shows "volume M f"
   512   using assms by (auto simp: volume_def)
   513 
   514 lemma%unimportant volume_positive:
   515   "volume M f \<Longrightarrow> a \<in> M \<Longrightarrow> 0 \<le> f a"
   516   by (auto simp: volume_def)
   517 
   518 lemma%unimportant volume_empty:
   519   "volume M f \<Longrightarrow> f {} = 0"
   520   by (auto simp: volume_def)
   521 
   522 lemma%unimportant volume_finite_additive:
   523   assumes "volume M f"
   524   assumes A: "\<And>i. i \<in> I \<Longrightarrow> A i \<in> M" "disjoint_family_on A I" "finite I" "\<Union>(A ` I) \<in> M"
   525   shows "f (\<Union>(A ` I)) = (\<Sum>i\<in>I. f (A i))"
   526 proof -
   527   have "A`I \<subseteq> M" "disjoint (A`I)" "finite (A`I)" "\<Union>(A`I) \<in> M"
   528     using A by (auto simp: disjoint_family_on_disjoint_image)
   529   with \<open>volume M f\<close> have "f (\<Union>(A`I)) = (\<Sum>a\<in>A`I. f a)"
   530     unfolding volume_def by blast
   531   also have "\<dots> = (\<Sum>i\<in>I. f (A i))"
   532   proof (subst sum.reindex_nontrivial)
   533     fix i j assume "i \<in> I" "j \<in> I" "i \<noteq> j" "A i = A j"
   534     with \<open>disjoint_family_on A I\<close> have "A i = {}"
   535       by (auto simp: disjoint_family_on_def)
   536     then show "f (A i) = 0"
   537       using volume_empty[OF \<open>volume M f\<close>] by simp
   538   qed (auto intro: \<open>finite I\<close>)
   539   finally show "f (\<Union>(A ` I)) = (\<Sum>i\<in>I. f (A i))"
   540     by simp
   541 qed
   542 
   543 lemma%unimportant (in ring_of_sets) volume_additiveI:
   544   assumes pos: "\<And>a. a \<in> M \<Longrightarrow> 0 \<le> \<mu> a"
   545   assumes [simp]: "\<mu> {} = 0"
   546   assumes add: "\<And>a b. a \<in> M \<Longrightarrow> b \<in> M \<Longrightarrow> a \<inter> b = {} \<Longrightarrow> \<mu> (a \<union> b) = \<mu> a + \<mu> b"
   547   shows "volume M \<mu>"
   548 proof (unfold volume_def, safe)
   549   fix C assume "finite C" "C \<subseteq> M" "disjoint C"
   550   then show "\<mu> (\<Union>C) = sum \<mu> C"
   551   proof (induct C)
   552     case (insert c C)
   553     from insert(1,2,4,5) have "\<mu> (\<Union>insert c C) = \<mu> c + \<mu> (\<Union>C)"
   554       by (auto intro!: add simp: disjoint_def)
   555     with insert show ?case
   556       by (simp add: disjoint_def)
   557   qed simp
   558 qed fact+
   559 
   560 lemma%important (in semiring_of_sets) extend_volume:
   561   assumes "volume M \<mu>"
   562   shows "\<exists>\<mu>'. volume generated_ring \<mu>' \<and> (\<forall>a\<in>M. \<mu>' a = \<mu> a)"
   563 proof%unimportant -
   564   let ?R = generated_ring
   565   have "\<forall>a\<in>?R. \<exists>m. \<exists>C\<subseteq>M. a = \<Union>C \<and> finite C \<and> disjoint C \<and> m = (\<Sum>c\<in>C. \<mu> c)"
   566     by (auto simp: generated_ring_def)
   567   from bchoice[OF this] guess \<mu>' .. note \<mu>'_spec = this
   568 
   569   { fix C assume C: "C \<subseteq> M" "finite C" "disjoint C"
   570     fix D assume D: "D \<subseteq> M" "finite D" "disjoint D"
   571     assume "\<Union>C = \<Union>D"
   572     have "(\<Sum>d\<in>D. \<mu> d) = (\<Sum>d\<in>D. \<Sum>c\<in>C. \<mu> (c \<inter> d))"
   573     proof (intro sum.cong refl)
   574       fix d assume "d \<in> D"
   575       have Un_eq_d: "(\<Union>c\<in>C. c \<inter> d) = d"
   576         using \<open>d \<in> D\<close> \<open>\<Union>C = \<Union>D\<close> by auto
   577       moreover have "\<mu> (\<Union>c\<in>C. c \<inter> d) = (\<Sum>c\<in>C. \<mu> (c \<inter> d))"
   578       proof (rule volume_finite_additive)
   579         { fix c assume "c \<in> C" then show "c \<inter> d \<in> M"
   580             using C D \<open>d \<in> D\<close> by auto }
   581         show "(\<Union>a\<in>C. a \<inter> d) \<in> M"
   582           unfolding Un_eq_d using \<open>d \<in> D\<close> D by auto
   583         show "disjoint_family_on (\<lambda>a. a \<inter> d) C"
   584           using \<open>disjoint C\<close> by (auto simp: disjoint_family_on_def disjoint_def)
   585       qed fact+
   586       ultimately show "\<mu> d = (\<Sum>c\<in>C. \<mu> (c \<inter> d))" by simp
   587     qed }
   588   note split_sum = this
   589 
   590   { fix C assume C: "C \<subseteq> M" "finite C" "disjoint C"
   591     fix D assume D: "D \<subseteq> M" "finite D" "disjoint D"
   592     assume "\<Union>C = \<Union>D"
   593     with split_sum[OF C D] split_sum[OF D C]
   594     have "(\<Sum>d\<in>D. \<mu> d) = (\<Sum>c\<in>C. \<mu> c)"
   595       by (simp, subst sum.swap, simp add: ac_simps) }
   596   note sum_eq = this
   597 
   598   { fix C assume C: "C \<subseteq> M" "finite C" "disjoint C"
   599     then have "\<Union>C \<in> ?R" by (auto simp: generated_ring_def)
   600     with \<mu>'_spec[THEN bspec, of "\<Union>C"]
   601     obtain D where
   602       D: "D \<subseteq> M" "finite D" "disjoint D" "\<Union>C = \<Union>D" and "\<mu>' (\<Union>C) = (\<Sum>d\<in>D. \<mu> d)"
   603       by auto
   604     with sum_eq[OF C D] have "\<mu>' (\<Union>C) = (\<Sum>c\<in>C. \<mu> c)" by simp }
   605   note \<mu>' = this
   606 
   607   show ?thesis
   608   proof (intro exI conjI ring_of_sets.volume_additiveI[OF generating_ring] ballI)
   609     fix a assume "a \<in> M" with \<mu>'[of "{a}"] show "\<mu>' a = \<mu> a"
   610       by (simp add: disjoint_def)
   611   next
   612     fix a assume "a \<in> ?R" then guess Ca .. note Ca = this
   613     with \<mu>'[of Ca] \<open>volume M \<mu>\<close>[THEN volume_positive]
   614     show "0 \<le> \<mu>' a"
   615       by (auto intro!: sum_nonneg)
   616   next
   617     show "\<mu>' {} = 0" using \<mu>'[of "{}"] by auto
   618   next
   619     fix a assume "a \<in> ?R" then guess Ca .. note Ca = this
   620     fix b assume "b \<in> ?R" then guess Cb .. note Cb = this
   621     assume "a \<inter> b = {}"
   622     with Ca Cb have "Ca \<inter> Cb \<subseteq> {{}}" by auto
   623     then have C_Int_cases: "Ca \<inter> Cb = {{}} \<or> Ca \<inter> Cb = {}" by auto
   624 
   625     from \<open>a \<inter> b = {}\<close> have "\<mu>' (\<Union>(Ca \<union> Cb)) = (\<Sum>c\<in>Ca \<union> Cb. \<mu> c)"
   626       using Ca Cb by (intro \<mu>') (auto intro!: disjoint_union)
   627     also have "\<dots> = (\<Sum>c\<in>Ca \<union> Cb. \<mu> c) + (\<Sum>c\<in>Ca \<inter> Cb. \<mu> c)"
   628       using C_Int_cases volume_empty[OF \<open>volume M \<mu>\<close>] by (elim disjE) simp_all
   629     also have "\<dots> = (\<Sum>c\<in>Ca. \<mu> c) + (\<Sum>c\<in>Cb. \<mu> c)"
   630       using Ca Cb by (simp add: sum.union_inter)
   631     also have "\<dots> = \<mu>' a + \<mu>' b"
   632       using Ca Cb by (simp add: \<mu>')
   633     finally show "\<mu>' (a \<union> b) = \<mu>' a + \<mu>' b"
   634       using Ca Cb by simp
   635   qed
   636 qed
   637 
   638 subsubsection%important \<open>Caratheodory on semirings\<close>
   639 
   640 theorem%important (in semiring_of_sets) caratheodory:
   641   assumes pos: "positive M \<mu>" and ca: "countably_additive M \<mu>"
   642   shows "\<exists>\<mu>' :: 'a set \<Rightarrow> ennreal. (\<forall>s \<in> M. \<mu>' s = \<mu> s) \<and> measure_space \<Omega> (sigma_sets \<Omega> M) \<mu>'"
   643 proof%unimportant -
   644   have "volume M \<mu>"
   645   proof (rule volumeI)
   646     { fix a assume "a \<in> M" then show "0 \<le> \<mu> a"
   647         using pos unfolding positive_def by auto }
   648     note p = this
   649 
   650     fix C assume sets_C: "C \<subseteq> M" "\<Union>C \<in> M" and "disjoint C" "finite C"
   651     have "\<exists>F'. bij_betw F' {..<card C} C"
   652       by (rule finite_same_card_bij[OF _ \<open>finite C\<close>]) auto
   653     then guess F' .. note F' = this
   654     then have F': "C = F' ` {..< card C}" "inj_on F' {..< card C}"
   655       by (auto simp: bij_betw_def)
   656     { fix i j assume *: "i < card C" "j < card C" "i \<noteq> j"
   657       with F' have "F' i \<in> C" "F' j \<in> C" "F' i \<noteq> F' j"
   658         unfolding inj_on_def by auto
   659       with \<open>disjoint C\<close>[THEN disjointD]
   660       have "F' i \<inter> F' j = {}"
   661         by auto }
   662     note F'_disj = this
   663     define F where "F i = (if i < card C then F' i else {})" for i
   664     then have "disjoint_family F"
   665       using F'_disj by (auto simp: disjoint_family_on_def)
   666     moreover from F' have "(\<Union>i. F i) = \<Union>C"
   667       by (auto simp add: F_def split: if_split_asm) blast
   668     moreover have sets_F: "\<And>i. F i \<in> M"
   669       using F' sets_C by (auto simp: F_def)
   670     moreover note sets_C
   671     ultimately have "\<mu> (\<Union>C) = (\<Sum>i. \<mu> (F i))"
   672       using ca[unfolded countably_additive_def, THEN spec, of F] by auto
   673     also have "\<dots> = (\<Sum>i<card C. \<mu> (F' i))"
   674     proof -
   675       have "(\<lambda>i. if i \<in> {..< card C} then \<mu> (F' i) else 0) sums (\<Sum>i<card C. \<mu> (F' i))"
   676         by (rule sums_If_finite_set) auto
   677       also have "(\<lambda>i. if i \<in> {..< card C} then \<mu> (F' i) else 0) = (\<lambda>i. \<mu> (F i))"
   678         using pos by (auto simp: positive_def F_def)
   679       finally show "(\<Sum>i. \<mu> (F i)) = (\<Sum>i<card C. \<mu> (F' i))"
   680         by (simp add: sums_iff)
   681     qed
   682     also have "\<dots> = (\<Sum>c\<in>C. \<mu> c)"
   683       using F'(2) by (subst (2) F') (simp add: sum.reindex)
   684     finally show "\<mu> (\<Union>C) = (\<Sum>c\<in>C. \<mu> c)" .
   685   next
   686     show "\<mu> {} = 0"
   687       using \<open>positive M \<mu>\<close> by (rule positiveD1)
   688   qed
   689   from extend_volume[OF this] obtain \<mu>_r where
   690     V: "volume generated_ring \<mu>_r" "\<And>a. a \<in> M \<Longrightarrow> \<mu> a = \<mu>_r a"
   691     by auto
   692 
   693   interpret G: ring_of_sets \<Omega> generated_ring
   694     by (rule generating_ring)
   695 
   696   have pos: "positive generated_ring \<mu>_r"
   697     using V unfolding positive_def by (auto simp: positive_def intro!: volume_positive volume_empty)
   698 
   699   have "countably_additive generated_ring \<mu>_r"
   700   proof (rule countably_additiveI)
   701     fix A' :: "nat \<Rightarrow> 'a set" assume A': "range A' \<subseteq> generated_ring" "disjoint_family A'"
   702       and Un_A: "(\<Union>i. A' i) \<in> generated_ring"
   703 
   704     from generated_ringE[OF Un_A] guess C' . note C' = this
   705 
   706     { fix c assume "c \<in> C'"
   707       moreover define A where [abs_def]: "A i = A' i \<inter> c" for i
   708       ultimately have A: "range A \<subseteq> generated_ring" "disjoint_family A"
   709         and Un_A: "(\<Union>i. A i) \<in> generated_ring"
   710         using A' C'
   711         by (auto intro!: G.Int G.finite_Union intro: generated_ringI_Basic simp: disjoint_family_on_def)
   712       from A C' \<open>c \<in> C'\<close> have UN_eq: "(\<Union>i. A i) = c"
   713         by (auto simp: A_def)
   714 
   715       have "\<forall>i::nat. \<exists>f::nat \<Rightarrow> 'a set. \<mu>_r (A i) = (\<Sum>j. \<mu>_r (f j)) \<and> disjoint_family f \<and> \<Union>range f = A i \<and> (\<forall>j. f j \<in> M)"
   716         (is "\<forall>i. ?P i")
   717       proof
   718         fix i
   719         from A have Ai: "A i \<in> generated_ring" by auto
   720         from generated_ringE[OF this] guess C . note C = this
   721 
   722         have "\<exists>F'. bij_betw F' {..<card C} C"
   723           by (rule finite_same_card_bij[OF _ \<open>finite C\<close>]) auto
   724         then guess F .. note F = this
   725         define f where [abs_def]: "f i = (if i < card C then F i else {})" for i
   726         then have f: "bij_betw f {..< card C} C"
   727           by (intro bij_betw_cong[THEN iffD1, OF _ F]) auto
   728         with C have "\<forall>j. f j \<in> M"
   729           by (auto simp: Pi_iff f_def dest!: bij_betw_imp_funcset)
   730         moreover
   731         from f C have d_f: "disjoint_family_on f {..<card C}"
   732           by (intro disjoint_image_disjoint_family_on) (auto simp: bij_betw_def)
   733         then have "disjoint_family f"
   734           by (auto simp: disjoint_family_on_def f_def)
   735         moreover
   736         have Ai_eq: "A i = (\<Union>x<card C. f x)"
   737           using f C Ai unfolding bij_betw_def by auto
   738         then have "\<Union>range f = A i"
   739           using f C Ai unfolding bij_betw_def
   740             by (auto simp add: f_def cong del: SUP_cong_strong)
   741         moreover
   742         { have "(\<Sum>j. \<mu>_r (f j)) = (\<Sum>j. if j \<in> {..< card C} then \<mu>_r (f j) else 0)"
   743             using volume_empty[OF V(1)] by (auto intro!: arg_cong[where f=suminf] simp: f_def)
   744           also have "\<dots> = (\<Sum>j<card C. \<mu>_r (f j))"
   745             by (rule sums_If_finite_set[THEN sums_unique, symmetric]) simp
   746           also have "\<dots> = \<mu>_r (A i)"
   747             using C f[THEN bij_betw_imp_funcset] unfolding Ai_eq
   748             by (intro volume_finite_additive[OF V(1) _ d_f, symmetric])
   749                (auto simp: Pi_iff Ai_eq intro: generated_ringI_Basic)
   750           finally have "\<mu>_r (A i) = (\<Sum>j. \<mu>_r (f j))" .. }
   751         ultimately show "?P i"
   752           by blast
   753       qed
   754       from choice[OF this] guess f .. note f = this
   755       then have UN_f_eq: "(\<Union>i. case_prod f (prod_decode i)) = (\<Union>i. A i)"
   756         unfolding UN_extend_simps surj_prod_decode by (auto simp: set_eq_iff)
   757 
   758       have d: "disjoint_family (\<lambda>i. case_prod f (prod_decode i))"
   759         unfolding disjoint_family_on_def
   760       proof (intro ballI impI)
   761         fix m n :: nat assume "m \<noteq> n"
   762         then have neq: "prod_decode m \<noteq> prod_decode n"
   763           using inj_prod_decode[of UNIV] by (auto simp: inj_on_def)
   764         show "case_prod f (prod_decode m) \<inter> case_prod f (prod_decode n) = {}"
   765         proof cases
   766           assume "fst (prod_decode m) = fst (prod_decode n)"
   767           then show ?thesis
   768             using neq f by (fastforce simp: disjoint_family_on_def)
   769         next
   770           assume neq: "fst (prod_decode m) \<noteq> fst (prod_decode n)"
   771           have "case_prod f (prod_decode m) \<subseteq> A (fst (prod_decode m))"
   772             "case_prod f (prod_decode n) \<subseteq> A (fst (prod_decode n))"
   773             using f[THEN spec, of "fst (prod_decode m)"]
   774             using f[THEN spec, of "fst (prod_decode n)"]
   775             by (auto simp: set_eq_iff)
   776           with f A neq show ?thesis
   777             by (fastforce simp: disjoint_family_on_def subset_eq set_eq_iff)
   778         qed
   779       qed
   780       from f have "(\<Sum>n. \<mu>_r (A n)) = (\<Sum>n. \<mu>_r (case_prod f (prod_decode n)))"
   781         by (intro suminf_ennreal_2dimen[symmetric] generated_ringI_Basic)
   782          (auto split: prod.split)
   783       also have "\<dots> = (\<Sum>n. \<mu> (case_prod f (prod_decode n)))"
   784         using f V(2) by (auto intro!: arg_cong[where f=suminf] split: prod.split)
   785       also have "\<dots> = \<mu> (\<Union>i. case_prod f (prod_decode i))"
   786         using f \<open>c \<in> C'\<close> C'
   787         by (intro ca[unfolded countably_additive_def, rule_format])
   788            (auto split: prod.split simp: UN_f_eq d UN_eq)
   789       finally have "(\<Sum>n. \<mu>_r (A' n \<inter> c)) = \<mu> c"
   790         using UN_f_eq UN_eq by (simp add: A_def) }
   791     note eq = this
   792 
   793     have "(\<Sum>n. \<mu>_r (A' n)) = (\<Sum>n. \<Sum>c\<in>C'. \<mu>_r (A' n \<inter> c))"
   794       using C' A'
   795       by (subst volume_finite_additive[symmetric, OF V(1)])
   796          (auto simp: disjoint_def disjoint_family_on_def
   797                intro!: G.Int G.finite_Union arg_cong[where f="\<lambda>X. suminf (\<lambda>i. \<mu>_r (X i))"] ext
   798                intro: generated_ringI_Basic)
   799     also have "\<dots> = (\<Sum>c\<in>C'. \<Sum>n. \<mu>_r (A' n \<inter> c))"
   800       using C' A'
   801       by (intro suminf_sum G.Int G.finite_Union) (auto intro: generated_ringI_Basic)
   802     also have "\<dots> = (\<Sum>c\<in>C'. \<mu>_r c)"
   803       using eq V C' by (auto intro!: sum.cong)
   804     also have "\<dots> = \<mu>_r (\<Union>C')"
   805       using C' Un_A
   806       by (subst volume_finite_additive[symmetric, OF V(1)])
   807          (auto simp: disjoint_family_on_def disjoint_def
   808                intro: generated_ringI_Basic)
   809     finally show "(\<Sum>n. \<mu>_r (A' n)) = \<mu>_r (\<Union>i. A' i)"
   810       using C' by simp
   811   qed
   812   from G.caratheodory'[OF \<open>positive generated_ring \<mu>_r\<close> \<open>countably_additive generated_ring \<mu>_r\<close>]
   813   guess \<mu>' ..
   814   with V show ?thesis
   815     unfolding sigma_sets_generated_ring_eq
   816     by (intro exI[of _ \<mu>']) (auto intro: generated_ringI_Basic)
   817 qed
   818 
   819 lemma%important extend_measure_caratheodory:
   820   fixes G :: "'i \<Rightarrow> 'a set"
   821   assumes M: "M = extend_measure \<Omega> I G \<mu>"
   822   assumes "i \<in> I"
   823   assumes "semiring_of_sets \<Omega> (G ` I)"
   824   assumes empty: "\<And>i. i \<in> I \<Longrightarrow> G i = {} \<Longrightarrow> \<mu> i = 0"
   825   assumes inj: "\<And>i j. i \<in> I \<Longrightarrow> j \<in> I \<Longrightarrow> G i = G j \<Longrightarrow> \<mu> i = \<mu> j"
   826   assumes nonneg: "\<And>i. i \<in> I \<Longrightarrow> 0 \<le> \<mu> i"
   827   assumes add: "\<And>A::nat \<Rightarrow> 'i. \<And>j. A \<in> UNIV \<rightarrow> I \<Longrightarrow> j \<in> I \<Longrightarrow> disjoint_family (G \<circ> A) \<Longrightarrow>
   828     (\<Union>i. G (A i)) = G j \<Longrightarrow> (\<Sum>n. \<mu> (A n)) = \<mu> j"
   829   shows "emeasure M (G i) = \<mu> i"
   830 
   831 proof%unimportant -
   832   interpret semiring_of_sets \<Omega> "G ` I"
   833     by fact
   834   have "\<forall>g\<in>G`I. \<exists>i\<in>I. g = G i"
   835     by auto
   836   then obtain sel where sel: "\<And>g. g \<in> G ` I \<Longrightarrow> sel g \<in> I" "\<And>g. g \<in> G ` I \<Longrightarrow> G (sel g) = g"
   837     by metis
   838 
   839   have "\<exists>\<mu>'. (\<forall>s\<in>G ` I. \<mu>' s = \<mu> (sel s)) \<and> measure_space \<Omega> (sigma_sets \<Omega> (G ` I)) \<mu>'"
   840   proof (rule caratheodory)
   841     show "positive (G ` I) (\<lambda>s. \<mu> (sel s))"
   842       by (auto simp: positive_def intro!: empty sel nonneg)
   843     show "countably_additive (G ` I) (\<lambda>s. \<mu> (sel s))"
   844     proof (rule countably_additiveI)
   845       fix A :: "nat \<Rightarrow> 'a set" assume "range A \<subseteq> G ` I" "disjoint_family A" "(\<Union>i. A i) \<in> G ` I"
   846       then show "(\<Sum>i. \<mu> (sel (A i))) = \<mu> (sel (\<Union>i. A i))"
   847         by (intro add) (auto simp: sel image_subset_iff_funcset comp_def Pi_iff intro!: sel)
   848     qed
   849   qed
   850   then obtain \<mu>' where \<mu>': "\<forall>s\<in>G ` I. \<mu>' s = \<mu> (sel s)" "measure_space \<Omega> (sigma_sets \<Omega> (G ` I)) \<mu>'"
   851     by metis
   852 
   853   show ?thesis
   854   proof (rule emeasure_extend_measure[OF M])
   855     { fix i assume "i \<in> I" then show "\<mu>' (G i) = \<mu> i"
   856       using \<mu>' by (auto intro!: inj sel) }
   857     show "G ` I \<subseteq> Pow \<Omega>"
   858       by (rule space_closed)
   859     then show "positive (sets M) \<mu>'" "countably_additive (sets M) \<mu>'"
   860       using \<mu>' by (simp_all add: M sets_extend_measure measure_space_def)
   861   qed fact
   862 qed
   863 
   864 lemma%important extend_measure_caratheodory_pair:
   865   fixes G :: "'i \<Rightarrow> 'j \<Rightarrow> 'a set"
   866   assumes M: "M = extend_measure \<Omega> {(a, b). P a b} (\<lambda>(a, b). G a b) (\<lambda>(a, b). \<mu> a b)"
   867   assumes "P i j"
   868   assumes semiring: "semiring_of_sets \<Omega> {G a b | a b. P a b}"
   869   assumes empty: "\<And>i j. P i j \<Longrightarrow> G i j = {} \<Longrightarrow> \<mu> i j = 0"
   870   assumes inj: "\<And>i j k l. P i j \<Longrightarrow> P k l \<Longrightarrow> G i j = G k l \<Longrightarrow> \<mu> i j = \<mu> k l"
   871   assumes nonneg: "\<And>i j. P i j \<Longrightarrow> 0 \<le> \<mu> i j"
   872   assumes add: "\<And>A::nat \<Rightarrow> 'i. \<And>B::nat \<Rightarrow> 'j. \<And>j k.
   873     (\<And>n. P (A n) (B n)) \<Longrightarrow> P j k \<Longrightarrow> disjoint_family (\<lambda>n. G (A n) (B n)) \<Longrightarrow>
   874     (\<Union>i. G (A i) (B i)) = G j k \<Longrightarrow> (\<Sum>n. \<mu> (A n) (B n)) = \<mu> j k"
   875   shows "emeasure M (G i j) = \<mu> i j"
   876 proof%unimportant -
   877   have "emeasure M ((\<lambda>(a, b). G a b) (i, j)) = (\<lambda>(a, b). \<mu> a b) (i, j)"
   878   proof (rule extend_measure_caratheodory[OF M])
   879     show "semiring_of_sets \<Omega> ((\<lambda>(a, b). G a b) ` {(a, b). P a b})"
   880       using semiring by (simp add: image_def conj_commute)
   881   next
   882     fix A :: "nat \<Rightarrow> ('i \<times> 'j)" and j assume "A \<in> UNIV \<rightarrow> {(a, b). P a b}" "j \<in> {(a, b). P a b}"
   883       "disjoint_family ((\<lambda>(a, b). G a b) \<circ> A)"
   884       "(\<Union>i. case A i of (a, b) \<Rightarrow> G a b) = (case j of (a, b) \<Rightarrow> G a b)"
   885     then show "(\<Sum>n. case A n of (a, b) \<Rightarrow> \<mu> a b) = (case j of (a, b) \<Rightarrow> \<mu> a b)"
   886       using add[of "\<lambda>i. fst (A i)" "\<lambda>i. snd (A i)" "fst j" "snd j"]
   887       by (simp add: split_beta' comp_def Pi_iff)
   888   qed (auto split: prod.splits intro: assms)
   889   then show ?thesis by simp
   890 qed
   891 
   892 end