src/HOL/Analysis/Regularity.thy
author nipkow
Sat Dec 29 15:43:53 2018 +0100 (6 months ago)
changeset 69529 4ab9657b3257
parent 69260 0a9688695a1b
child 69661 a03a63b81f44
permissions -rw-r--r--
capitalize proper names in lemma names
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(*  Title:      HOL/Analysis/Regularity.thy
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    Author:     Fabian Immler, TU M√ľnchen
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*)
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section \<open>Regularity of Measures\<close>
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theory Regularity (* FIX suggestion to rename to e.g. RegularityMeasures  *)
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imports Measure_Space Borel_Space
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begin
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lemma%important (*FIX needs name *)
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  fixes M::"'a::{second_countable_topology, complete_space} measure"
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  assumes sb: "sets M = sets borel"
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  assumes "emeasure M (space M) \<noteq> \<infinity>"
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  assumes "B \<in> sets borel"
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  shows inner_regular: "emeasure M B =
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    (SUP K \<in> {K. K \<subseteq> B \<and> compact K}. emeasure M K)" (is "?inner B")
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  and outer_regular: "emeasure M B =
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    (INF U \<in> {U. B \<subseteq> U \<and> open U}. emeasure M U)" (is "?outer B")
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proof%unimportant -
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  have Us: "UNIV = space M" by (metis assms(1) sets_eq_imp_space_eq space_borel)
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  hence sU: "space M = UNIV" by simp
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  interpret finite_measure M by rule fact
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  have approx_inner: "\<And>A. A \<in> sets M \<Longrightarrow>
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    (\<And>e. e > 0 \<Longrightarrow> \<exists>K. K \<subseteq> A \<and> compact K \<and> emeasure M A \<le> emeasure M K + ennreal e) \<Longrightarrow> ?inner A"
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    by (rule ennreal_approx_SUP)
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      (force intro!: emeasure_mono simp: compact_imp_closed emeasure_eq_measure)+
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  have approx_outer: "\<And>A. A \<in> sets M \<Longrightarrow>
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    (\<And>e. e > 0 \<Longrightarrow> \<exists>B. A \<subseteq> B \<and> open B \<and> emeasure M B \<le> emeasure M A + ennreal e) \<Longrightarrow> ?outer A"
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    by (rule ennreal_approx_INF)
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       (force intro!: emeasure_mono simp: emeasure_eq_measure sb)+
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  from countable_dense_setE guess X::"'a set"  . note X = this
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  {
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    fix r::real assume "r > 0" hence "\<And>y. open (ball y r)" "\<And>y. ball y r \<noteq> {}" by auto
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    with X(2)[OF this]
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    have x: "space M = (\<Union>x\<in>X. cball x r)"
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      by (auto simp add: sU) (metis dist_commute order_less_imp_le)
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    let ?U = "\<Union>k. (\<Union>n\<in>{0..k}. cball (from_nat_into X n) r)"
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    have "(\<lambda>k. emeasure M (\<Union>n\<in>{0..k}. cball (from_nat_into X n) r)) \<longlonglongrightarrow> M ?U"
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      by (rule Lim_emeasure_incseq) (auto intro!: borel_closed bexI simp: incseq_def Us sb)
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    also have "?U = space M"
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    proof safe
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      fix x from X(2)[OF open_ball[of x r]] \<open>r > 0\<close> obtain d where d: "d\<in>X" "d \<in> ball x r" by auto
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      show "x \<in> ?U"
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        using X(1) d
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        by simp (auto intro!: exI [where x = "to_nat_on X d"] simp: dist_commute Bex_def)
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    qed (simp add: sU)
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    finally have "(\<lambda>k. M (\<Union>n\<in>{0..k}. cball (from_nat_into X n) r)) \<longlonglongrightarrow> M (space M)" .
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  } note M_space = this
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  {
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    fix e ::real and n :: nat assume "e > 0" "n > 0"
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    hence "1/n > 0" "e * 2 powr - n > 0" by (auto)
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    from M_space[OF \<open>1/n>0\<close>]
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    have "(\<lambda>k. measure M (\<Union>i\<in>{0..k}. cball (from_nat_into X i) (1/real n))) \<longlonglongrightarrow> measure M (space M)"
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      unfolding emeasure_eq_measure by (auto simp: measure_nonneg)
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    from metric_LIMSEQ_D[OF this \<open>0 < e * 2 powr -n\<close>]
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    obtain k where "dist (measure M (\<Union>i\<in>{0..k}. cball (from_nat_into X i) (1/real n))) (measure M (space M)) <
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      e * 2 powr -n"
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      by auto
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    hence "measure M (\<Union>i\<in>{0..k}. cball (from_nat_into X i) (1/real n)) \<ge>
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      measure M (space M) - e * 2 powr -real n"
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      by (auto simp: dist_real_def)
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    hence "\<exists>k. measure M (\<Union>i\<in>{0..k}. cball (from_nat_into X i) (1/real n)) \<ge>
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      measure M (space M) - e * 2 powr - real n" ..
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  } note k=this
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  hence "\<forall>e\<in>{0<..}. \<forall>(n::nat)\<in>{0<..}. \<exists>k.
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    measure M (\<Union>i\<in>{0..k}. cball (from_nat_into X i) (1/real n)) \<ge> measure M (space M) - e * 2 powr - real n"
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    by blast
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  then obtain k where k: "\<forall>e\<in>{0<..}. \<forall>n\<in>{0<..}. measure M (space M) - e * 2 powr - real (n::nat)
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    \<le> measure M (\<Union>i\<in>{0..k e n}. cball (from_nat_into X i) (1 / n))"
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    by metis
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  hence k: "\<And>e n. e > 0 \<Longrightarrow> n > 0 \<Longrightarrow> measure M (space M) - e * 2 powr - n
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    \<le> measure M (\<Union>i\<in>{0..k e n}. cball (from_nat_into X i) (1 / n))"
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    unfolding Ball_def by blast
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  have approx_space:
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    "\<exists>K \<in> {K. K \<subseteq> space M \<and> compact K}. emeasure M (space M) \<le> emeasure M K + ennreal e"
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    (is "?thesis e") if "0 < e" for e :: real
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  proof -
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    define B where [abs_def]:
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      "B n = (\<Union>i\<in>{0..k e (Suc n)}. cball (from_nat_into X i) (1 / Suc n))" for n
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    have "\<And>n. closed (B n)" by (auto simp: B_def)
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    hence [simp]: "\<And>n. B n \<in> sets M" by (simp add: sb)
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    from k[OF \<open>e > 0\<close> zero_less_Suc]
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    have "\<And>n. measure M (space M) - measure M (B n) \<le> e * 2 powr - real (Suc n)"
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      by (simp add: algebra_simps B_def finite_measure_compl)
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    hence B_compl_le: "\<And>n::nat. measure M (space M - B n) \<le> e * 2 powr - real (Suc n)"
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      by (simp add: finite_measure_compl)
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    define K where "K = (\<Inter>n. B n)"
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    from \<open>closed (B _)\<close> have "closed K" by (auto simp: K_def)
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    hence [simp]: "K \<in> sets M" by (simp add: sb)
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    have "measure M (space M) - measure M K = measure M (space M - K)"
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      by (simp add: finite_measure_compl)
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    also have "\<dots> = emeasure M (\<Union>n. space M - B n)" by (auto simp: K_def emeasure_eq_measure)
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    also have "\<dots> \<le> (\<Sum>n. emeasure M (space M - B n))"
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      by (rule emeasure_subadditive_countably) (auto simp: summable_def)
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    also have "\<dots> \<le> (\<Sum>n. ennreal (e*2 powr - real (Suc n)))"
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      using B_compl_le by (intro suminf_le) (simp_all add: measure_nonneg emeasure_eq_measure ennreal_leI)
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    also have "\<dots> \<le> (\<Sum>n. ennreal (e * (1 / 2) ^ Suc n))"
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      by (simp add: powr_minus powr_realpow field_simps del: of_nat_Suc)
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    also have "\<dots> = ennreal e * (\<Sum>n. ennreal ((1 / 2) ^ Suc n))"
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      unfolding ennreal_power[symmetric]
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      using \<open>0 < e\<close>
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      by (simp add: ac_simps ennreal_mult' divide_ennreal[symmetric] divide_ennreal_def
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                    ennreal_power[symmetric])
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    also have "\<dots> = e"
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      by (subst suminf_ennreal_eq[OF zero_le_power power_half_series]) auto
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    finally have "measure M (space M) \<le> measure M K + e"
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      using \<open>0 < e\<close> by simp
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    hence "emeasure M (space M) \<le> emeasure M K + e"
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      using \<open>0 < e\<close> by (simp add: emeasure_eq_measure flip: ennreal_plus)
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    moreover have "compact K"
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      unfolding compact_eq_totally_bounded
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    proof safe
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      show "complete K" using \<open>closed K\<close> by (simp add: complete_eq_closed)
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      fix e'::real assume "0 < e'"
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      from nat_approx_posE[OF this] guess n . note n = this
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      let ?k = "from_nat_into X ` {0..k e (Suc n)}"
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      have "finite ?k" by simp
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      moreover have "K \<subseteq> (\<Union>x\<in>?k. ball x e')" unfolding K_def B_def using n by force
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      ultimately show "\<exists>k. finite k \<and> K \<subseteq> (\<Union>x\<in>k. ball x e')" by blast
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    qed
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    ultimately
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    show ?thesis by (auto simp: sU)
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  qed
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  { fix A::"'a set" assume "closed A" hence "A \<in> sets borel" by (simp add: compact_imp_closed)
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    hence [simp]: "A \<in> sets M" by (simp add: sb)
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    have "?inner A"
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    proof (rule approx_inner)
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      fix e::real assume "e > 0"
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      from approx_space[OF this] obtain K where
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        K: "K \<subseteq> space M" "compact K" "emeasure M (space M) \<le> emeasure M K + e"
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        by (auto simp: emeasure_eq_measure)
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      hence [simp]: "K \<in> sets M" by (simp add: sb compact_imp_closed)
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      have "measure M A - measure M (A \<inter> K) = measure M (A - A \<inter> K)"
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        by (subst finite_measure_Diff) auto
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      also have "A - A \<inter> K = A \<union> K - K" by auto
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      also have "measure M \<dots> = measure M (A \<union> K) - measure M K"
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        by (subst finite_measure_Diff) auto
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      also have "\<dots> \<le> measure M (space M) - measure M K"
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        by (simp add: emeasure_eq_measure sU sb finite_measure_mono)
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      also have "\<dots> \<le> e"
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        using K \<open>0 < e\<close> by (simp add: emeasure_eq_measure flip: ennreal_plus)
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      finally have "emeasure M A \<le> emeasure M (A \<inter> K) + ennreal e"
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        using \<open>0<e\<close> by (simp add: emeasure_eq_measure algebra_simps flip: ennreal_plus)
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      moreover have "A \<inter> K \<subseteq> A" "compact (A \<inter> K)" using \<open>closed A\<close> \<open>compact K\<close> by auto
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      ultimately show "\<exists>K \<subseteq> A. compact K \<and> emeasure M A \<le> emeasure M K + ennreal e"
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        by blast
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    qed simp
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    have "?outer A"
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    proof cases
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      assume "A \<noteq> {}"
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      let ?G = "\<lambda>d. {x. infdist x A < d}"
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      {
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        fix d
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        have "?G d = (\<lambda>x. infdist x A) -` {..<d}" by auto
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        also have "open \<dots>"
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          by (intro continuous_open_vimage) (auto intro!: continuous_infdist continuous_ident)
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        finally have "open (?G d)" .
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      } note open_G = this
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      from in_closed_iff_infdist_zero[OF \<open>closed A\<close> \<open>A \<noteq> {}\<close>]
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      have "A = {x. infdist x A = 0}" by auto
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      also have "\<dots> = (\<Inter>i. ?G (1/real (Suc i)))"
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      proof (auto simp del: of_nat_Suc, rule ccontr)
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        fix x
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        assume "infdist x A \<noteq> 0"
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        hence pos: "infdist x A > 0" using infdist_nonneg[of x A] by simp
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        from nat_approx_posE[OF this] guess n .
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        moreover
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        assume "\<forall>i. infdist x A < 1 / real (Suc i)"
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        hence "infdist x A < 1 / real (Suc n)" by auto
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        ultimately show False by simp
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      qed
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      also have "M \<dots> = (INF n. emeasure M (?G (1 / real (Suc n))))"
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      proof (rule INF_emeasure_decseq[symmetric], safe)
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        fix i::nat
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        from open_G[of "1 / real (Suc i)"]
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        show "?G (1 / real (Suc i)) \<in> sets M" by (simp add: sb borel_open)
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      next
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        show "decseq (\<lambda>i. {x. infdist x A < 1 / real (Suc i)})"
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          by (auto intro: less_trans intro!: divide_strict_left_mono
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            simp: decseq_def le_eq_less_or_eq)
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      qed simp
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      finally
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      have "emeasure M A = (INF n. emeasure M {x. infdist x A < 1 / real (Suc n)})" .
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      moreover
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      have "\<dots> \<ge> (INF U\<in>{U. A \<subseteq> U \<and> open U}. emeasure M U)"
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      proof (intro INF_mono)
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        fix m
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        have "?G (1 / real (Suc m)) \<in> {U. A \<subseteq> U \<and> open U}" using open_G by auto
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        moreover have "M (?G (1 / real (Suc m))) \<le> M (?G (1 / real (Suc m)))" by simp
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        ultimately show "\<exists>U\<in>{U. A \<subseteq> U \<and> open U}.
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          emeasure M U \<le> emeasure M {x. infdist x A < 1 / real (Suc m)}"
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          by blast
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      qed
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      moreover
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      have "emeasure M A \<le> (INF U\<in>{U. A \<subseteq> U \<and> open U}. emeasure M U)"
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        by (rule INF_greatest) (auto intro!: emeasure_mono simp: sb)
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      ultimately show ?thesis by simp
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    qed (auto intro!: INF_eqI)
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    note \<open>?inner A\<close> \<open>?outer A\<close> }
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  note closed_in_D = this
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  from \<open>B \<in> sets borel\<close>
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  have "Int_stable (Collect closed)" "Collect closed \<subseteq> Pow UNIV" "B \<in> sigma_sets UNIV (Collect closed)"
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    by (auto simp: Int_stable_def borel_eq_closed)
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  then show "?inner B" "?outer B"
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  proof (induct B rule: sigma_sets_induct_disjoint)
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    case empty
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    { case 1 show ?case by (intro SUP_eqI[symmetric]) auto }
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    { case 2 show ?case by (intro INF_eqI[symmetric]) (auto elim!: meta_allE[of _ "{}"]) }
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  next
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    case (basic B)
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    { case 1 from basic closed_in_D show ?case by auto }
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    { case 2 from basic closed_in_D show ?case by auto }
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  next
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    case (compl B)
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    note inner = compl(2) and outer = compl(3)
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    from compl have [simp]: "B \<in> sets M" by (auto simp: sb borel_eq_closed)
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    case 2
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    have "M (space M - B) = M (space M) - emeasure M B" by (auto simp: emeasure_compl)
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    also have "\<dots> = (INF K\<in>{K. K \<subseteq> B \<and> compact K}. M (space M) -  M K)"
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      by (subst ennreal_SUP_const_minus) (auto simp: less_top[symmetric] inner)
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    also have "\<dots> = (INF U\<in>{U. U \<subseteq> B \<and> compact U}. M (space M - U))"
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      by (rule INF_cong) (auto simp add: emeasure_compl sb compact_imp_closed)
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    also have "\<dots> \<ge> (INF U\<in>{U. U \<subseteq> B \<and> closed U}. M (space M - U))"
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      by (rule INF_superset_mono) (auto simp add: compact_imp_closed)
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    also have "(INF U\<in>{U. U \<subseteq> B \<and> closed U}. M (space M - U)) =
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        (INF U\<in>{U. space M - B \<subseteq> U \<and> open U}. emeasure M U)"
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      unfolding INF_image [of _ "\<lambda>u. space M - u" _, symmetric, unfolded comp_def]
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        by (rule INF_cong) (auto simp add: sU Compl_eq_Diff_UNIV [symmetric, simp])
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    finally have
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      "(INF U\<in>{U. space M - B \<subseteq> U \<and> open U}. emeasure M U) \<le> emeasure M (space M - B)" .
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    moreover have
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      "(INF U\<in>{U. space M - B \<subseteq> U \<and> open U}. emeasure M U) \<ge> emeasure M (space M - B)"
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      by (auto simp: sb sU intro!: INF_greatest emeasure_mono)
hoelzl@50125
   235
    ultimately show ?case by (auto intro!: antisym simp: sets_eq_imp_space_eq[OF sb])
hoelzl@50125
   236
hoelzl@50125
   237
    case 1
hoelzl@50125
   238
    have "M (space M - B) = M (space M) - emeasure M B" by (auto simp: emeasure_compl)
haftmann@69260
   239
    also have "\<dots> = (SUP U\<in> {U. B \<subseteq> U \<and> open U}. M (space M) -  M U)"
hoelzl@62975
   240
      unfolding outer by (subst ennreal_INF_const_minus) auto
haftmann@69260
   241
    also have "\<dots> = (SUP U\<in>{U. B \<subseteq> U \<and> open U}. M (space M - U))"
hoelzl@50125
   242
      by (rule SUP_cong) (auto simp add: emeasure_compl sb compact_imp_closed)
haftmann@69260
   243
    also have "\<dots> = (SUP K\<in>{K. K \<subseteq> space M - B \<and> closed K}. emeasure M K)"
haftmann@62343
   244
      unfolding SUP_image [of _ "\<lambda>u. space M - u" _, symmetric, unfolded comp_def]
haftmann@62343
   245
        by (rule SUP_cong) (auto simp add: sU)
haftmann@69260
   246
    also have "\<dots> = (SUP K\<in>{K. K \<subseteq> space M - B \<and> compact K}. emeasure M K)"
hoelzl@50125
   247
    proof (safe intro!: antisym SUP_least)
hoelzl@50125
   248
      fix K assume "closed K" "K \<subseteq> space M - B"
wenzelm@61808
   249
      from closed_in_D[OF \<open>closed K\<close>]
haftmann@69260
   250
      have K_inner: "emeasure M K = (SUP K\<in>{Ka. Ka \<subseteq> K \<and> compact Ka}. emeasure M K)" by simp
haftmann@69260
   251
      show "emeasure M K \<le> (SUP K\<in>{K. K \<subseteq> space M - B \<and> compact K}. emeasure M K)"
wenzelm@61808
   252
        unfolding K_inner using \<open>K \<subseteq> space M - B\<close>
hoelzl@50125
   253
        by (auto intro!: SUP_upper SUP_least)
hoelzl@50125
   254
    qed (fastforce intro!: SUP_least SUP_upper simp: compact_imp_closed)
hoelzl@50125
   255
    finally show ?case by (auto intro!: antisym simp: sets_eq_imp_space_eq[OF sb])
immler@50087
   256
  next
hoelzl@50125
   257
    case (union D)
hoelzl@50125
   258
    then have "range D \<subseteq> sets M" by (auto simp: sb borel_eq_closed)
hoelzl@50125
   259
    with union have M[symmetric]: "(\<Sum>i. M (D i)) = M (\<Union>i. D i)" by (intro suminf_emeasure)
wenzelm@61969
   260
    also have "(\<lambda>n. \<Sum>i<n. M (D i)) \<longlonglongrightarrow> (\<Sum>i. M (D i))"
hoelzl@62975
   261
      by (intro summable_LIMSEQ) auto
wenzelm@61969
   262
    finally have measure_LIMSEQ: "(\<lambda>n. \<Sum>i<n. measure M (D i)) \<longlonglongrightarrow> measure M (\<Union>i. D i)"
nipkow@64267
   263
      by (simp add: emeasure_eq_measure measure_nonneg sum_nonneg)
wenzelm@61808
   264
    have "(\<Union>i. D i) \<in> sets M" using \<open>range D \<subseteq> sets M\<close> by auto
lp15@61609
   265
hoelzl@50125
   266
    case 1
hoelzl@50125
   267
    show ?case
immler@50087
   268
    proof (rule approx_inner)
immler@50087
   269
      fix e::real assume "e > 0"
immler@50087
   270
      with measure_LIMSEQ
hoelzl@56193
   271
      have "\<exists>no. \<forall>n\<ge>no. \<bar>(\<Sum>i<n. measure M (D i)) -measure M (\<Union>x. D x)\<bar> < e/2"
lp15@60017
   272
        by (auto simp: lim_sequentially dist_real_def simp del: less_divide_eq_numeral1)
hoelzl@56193
   273
      hence "\<exists>n0. \<bar>(\<Sum>i<n0. measure M (D i)) - measure M (\<Union>x. D x)\<bar> < e/2" by auto
hoelzl@56193
   274
      then obtain n0 where n0: "\<bar>(\<Sum>i<n0. measure M (D i)) - measure M (\<Union>i. D i)\<bar> < e/2"
immler@50087
   275
        unfolding choice_iff by blast
hoelzl@62975
   276
      have "ennreal (\<Sum>i<n0. measure M (D i)) = (\<Sum>i<n0. M (D i))"
nipkow@64267
   277
        by (auto simp add: emeasure_eq_measure sum_nonneg measure_nonneg)
nipkow@64267
   278
      also have "\<dots> \<le> (\<Sum>i. M (D i))" by (rule sum_le_suminf) auto
immler@50087
   279
      also have "\<dots> = M (\<Union>i. D i)" by (simp add: M)
immler@50087
   280
      also have "\<dots> = measure M (\<Union>i. D i)" by (simp add: emeasure_eq_measure)
hoelzl@56193
   281
      finally have n0: "measure M (\<Union>i. D i) - (\<Sum>i<n0. measure M (D i)) < e/2"
nipkow@64267
   282
        using n0 by (auto simp: measure_nonneg sum_nonneg)
immler@50087
   283
      have "\<forall>i. \<exists>K. K \<subseteq> D i \<and> compact K \<and> emeasure M (D i) \<le> emeasure M K + e/(2*Suc n0)"
immler@50087
   284
      proof
immler@50087
   285
        fix i
wenzelm@61808
   286
        from \<open>0 < e\<close> have "0 < e/(2*Suc n0)" by simp
haftmann@69260
   287
        have "emeasure M (D i) = (SUP K\<in>{K. K \<subseteq> (D i) \<and> compact K}. emeasure M K)"
hoelzl@50125
   288
          using union by blast
hoelzl@62975
   289
        from SUP_approx_ennreal[OF \<open>0 < e/(2*Suc n0)\<close> _ this]
immler@50087
   290
        show "\<exists>K. K \<subseteq> D i \<and> compact K \<and> emeasure M (D i) \<le> emeasure M K + e/(2*Suc n0)"
hoelzl@62975
   291
          by (auto simp: emeasure_eq_measure intro: less_imp_le compact_empty)
immler@50087
   292
      qed
immler@50087
   293
      then obtain K where K: "\<And>i. K i \<subseteq> D i" "\<And>i. compact (K i)"
immler@50087
   294
        "\<And>i. emeasure M (D i) \<le> emeasure M (K i) + e/(2*Suc n0)"
immler@50087
   295
        unfolding choice_iff by blast
hoelzl@56193
   296
      let ?K = "\<Union>i\<in>{..<n0}. K i"
wenzelm@61808
   297
      have "disjoint_family_on K {..<n0}" using K \<open>disjoint_family D\<close>
immler@50087
   298
        unfolding disjoint_family_on_def by blast
hoelzl@56193
   299
      hence mK: "measure M ?K = (\<Sum>i<n0. measure M (K i))" using K
immler@50087
   300
        by (intro finite_measure_finite_Union) (auto simp: sb compact_imp_closed)
hoelzl@56193
   301
      have "measure M (\<Union>i. D i) < (\<Sum>i<n0. measure M (D i)) + e/2" using n0 by simp
hoelzl@56193
   302
      also have "(\<Sum>i<n0. measure M (D i)) \<le> (\<Sum>i<n0. measure M (K i) + e/(2*Suc n0))"
hoelzl@62975
   303
        using K \<open>0 < e\<close>
nipkow@68403
   304
        by (auto intro: sum_mono simp: emeasure_eq_measure simp flip: ennreal_plus)
hoelzl@56193
   305
      also have "\<dots> = (\<Sum>i<n0. measure M (K i)) + (\<Sum>i<n0. e/(2*Suc n0))"
nipkow@64267
   306
        by (simp add: sum.distrib)
wenzelm@61808
   307
      also have "\<dots> \<le> (\<Sum>i<n0. measure M (K i)) +  e / 2" using \<open>0 < e\<close>
lp15@61609
   308
        by (auto simp: field_simps intro!: mult_left_mono)
immler@50087
   309
      finally
hoelzl@56193
   310
      have "measure M (\<Union>i. D i) < (\<Sum>i<n0. measure M (K i)) + e / 2 + e / 2"
immler@50087
   311
        by auto
hoelzl@62975
   312
      hence "M (\<Union>i. D i) < M ?K + e"
nipkow@68403
   313
        using \<open>0<e\<close> by (auto simp: mK emeasure_eq_measure sum_nonneg ennreal_less_iff simp flip: ennreal_plus)
immler@50087
   314
      moreover
immler@50087
   315
      have "?K \<subseteq> (\<Union>i. D i)" using K by auto
immler@50087
   316
      moreover
immler@50087
   317
      have "compact ?K" using K by auto
immler@50087
   318
      ultimately
hoelzl@62975
   319
      have "?K\<subseteq>(\<Union>i. D i) \<and> compact ?K \<and> emeasure M (\<Union>i. D i) \<le> emeasure M ?K + ennreal e" by simp
hoelzl@62975
   320
      thus "\<exists>K\<subseteq>\<Union>i. D i. compact K \<and> emeasure M (\<Union>i. D i) \<le> emeasure M K + ennreal e" ..
hoelzl@50125
   321
    qed fact
hoelzl@50125
   322
    case 2
hoelzl@50125
   323
    show ?case
wenzelm@61808
   324
    proof (rule approx_outer[OF \<open>(\<Union>i. D i) \<in> sets M\<close>])
immler@50087
   325
      fix e::real assume "e > 0"
immler@50087
   326
      have "\<forall>i::nat. \<exists>U. D i \<subseteq> U \<and> open U \<and> e/(2 powr Suc i) > emeasure M U - emeasure M (D i)"
immler@50087
   327
      proof
immler@50087
   328
        fix i::nat
wenzelm@61808
   329
        from \<open>0 < e\<close> have "0 < e/(2 powr Suc i)" by simp
haftmann@69260
   330
        have "emeasure M (D i) = (INF U\<in>{U. (D i) \<subseteq> U \<and> open U}. emeasure M U)"
hoelzl@50125
   331
          using union by blast
hoelzl@62975
   332
        from INF_approx_ennreal[OF \<open>0 < e/(2 powr Suc i)\<close> this]
immler@50087
   333
        show "\<exists>U. D i \<subseteq> U \<and> open U \<and> e/(2 powr Suc i) > emeasure M U - emeasure M (D i)"
hoelzl@62975
   334
          using \<open>0<e\<close>
nipkow@68046
   335
          by (auto simp: emeasure_eq_measure sum_nonneg ennreal_less_iff ennreal_minus
hoelzl@62975
   336
                         finite_measure_mono sb
nipkow@68403
   337
                   simp flip: ennreal_plus)
immler@50087
   338
      qed
immler@50087
   339
      then obtain U where U: "\<And>i. D i \<subseteq> U i" "\<And>i. open (U i)"
immler@50087
   340
        "\<And>i. e/(2 powr Suc i) > emeasure M (U i) - emeasure M (D i)"
immler@50087
   341
        unfolding choice_iff by blast
immler@50087
   342
      let ?U = "\<Union>i. U i"
hoelzl@62975
   343
      have "ennreal (measure M ?U - measure M (\<Union>i. D i)) = M ?U - M (\<Union>i. D i)"
hoelzl@62975
   344
        using U(1,2)
hoelzl@62975
   345
        by (subst ennreal_minus[symmetric])
hoelzl@62975
   346
           (auto intro!: finite_measure_mono simp: sb measure_nonneg emeasure_eq_measure)
hoelzl@62975
   347
      also have "\<dots> = M (?U - (\<Union>i. D i))" using U  \<open>(\<Union>i. D i) \<in> sets M\<close>
immler@50087
   348
        by (subst emeasure_Diff) (auto simp: sb)
wenzelm@61808
   349
      also have "\<dots> \<le> M (\<Union>i. U i - D i)" using U  \<open>range D \<subseteq> sets M\<close>
immler@50244
   350
        by (intro emeasure_mono) (auto simp: sb intro!: sets.countable_nat_UN sets.Diff)
wenzelm@61808
   351
      also have "\<dots> \<le> (\<Sum>i. M (U i - D i))" using U  \<open>range D \<subseteq> sets M\<close>
immler@50244
   352
        by (intro emeasure_subadditive_countably) (auto intro!: sets.Diff simp: sb)
hoelzl@62975
   353
      also have "\<dots> \<le> (\<Sum>i. ennreal e/(2 powr Suc i))" using U \<open>range D \<subseteq> sets M\<close>
hoelzl@62975
   354
        using \<open>0<e\<close>
hoelzl@62975
   355
        by (intro suminf_le, subst emeasure_Diff)
hoelzl@62975
   356
           (auto simp: emeasure_Diff emeasure_eq_measure sb measure_nonneg ennreal_minus
hoelzl@62975
   357
                       finite_measure_mono divide_ennreal ennreal_less_iff
hoelzl@62975
   358
                 intro: less_imp_le)
hoelzl@62975
   359
      also have "\<dots> \<le> (\<Sum>n. ennreal (e * (1 / 2) ^ Suc n))"
hoelzl@62975
   360
        using \<open>0<e\<close>
hoelzl@62975
   361
        by (simp add: powr_minus powr_realpow field_simps divide_ennreal del: of_nat_Suc)
hoelzl@62975
   362
      also have "\<dots> = ennreal e * (\<Sum>n. ennreal ((1 / 2) ^  Suc n))"
hoelzl@62975
   363
        unfolding ennreal_power[symmetric]
hoelzl@62975
   364
        using \<open>0 < e\<close>
hoelzl@62975
   365
        by (simp add: ac_simps ennreal_mult' divide_ennreal[symmetric] divide_ennreal_def
hoelzl@62975
   366
                      ennreal_power[symmetric])
hoelzl@62975
   367
      also have "\<dots> = ennreal e"
hoelzl@62975
   368
        by (subst suminf_ennreal_eq[OF zero_le_power power_half_series]) auto
hoelzl@62975
   369
      finally have "emeasure M ?U \<le> emeasure M (\<Union>i. D i) + ennreal e"
nipkow@68403
   370
        using \<open>0<e\<close> by (simp add: emeasure_eq_measure flip: ennreal_plus)
immler@50087
   371
      moreover
immler@50087
   372
      have "(\<Union>i. D i) \<subseteq> ?U" using U by auto
immler@50087
   373
      moreover
immler@50087
   374
      have "open ?U" using U by auto
immler@50087
   375
      ultimately
hoelzl@62975
   376
      have "(\<Union>i. D i) \<subseteq> ?U \<and> open ?U \<and> emeasure M ?U \<le> emeasure M (\<Union>i. D i) + ennreal e" by simp
hoelzl@62975
   377
      thus "\<exists>B. (\<Union>i. D i) \<subseteq> B \<and> open B \<and> emeasure M B \<le> emeasure M (\<Union>i. D i) + ennreal e" ..
immler@50087
   378
    qed
immler@50087
   379
  qed
immler@50087
   380
qed
immler@50087
   381
immler@50087
   382
end
immler@50087
   383