author  wenzelm 
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changeset 53381  355a4cac5440 
parent 51542  738598beeb26 
child 54410  0a578fb7fb73 
permissions  rwrr 
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(* Title: HOL/Library/Countable_Set.thy 
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Author: Johannes Hölzl 

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Author: Andrei Popescu 

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*) 

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header {* Countable sets *} 

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theory Countable_Set 

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imports Countable Infinite_Set 
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begin 
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subsection {* Predicate for countable sets *} 

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definition countable :: "'a set \<Rightarrow> bool" where 

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"countable S \<longleftrightarrow> (\<exists>f::'a \<Rightarrow> nat. inj_on f S)" 

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lemma countableE: 

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assumes S: "countable S" obtains f :: "'a \<Rightarrow> nat" where "inj_on f S" 

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using S by (auto simp: countable_def) 

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lemma countableI: "inj_on (f::'a \<Rightarrow> nat) S \<Longrightarrow> countable S" 

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by (auto simp: countable_def) 

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lemma countableI': "inj_on (f::'a \<Rightarrow> 'b::countable) S \<Longrightarrow> countable S" 

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using comp_inj_on[of f S to_nat] by (auto intro: countableI) 

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lemma countableE_bij: 

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assumes S: "countable S" obtains f :: "nat \<Rightarrow> 'a" and C :: "nat set" where "bij_betw f C S" 

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using S by (blast elim: countableE dest: inj_on_imp_bij_betw bij_betw_inv) 

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lemma countableI_bij: "bij_betw f (C::nat set) S \<Longrightarrow> countable S" 

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by (blast intro: countableI bij_betw_inv_into bij_betw_imp_inj_on) 

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lemma countable_finite: "finite S \<Longrightarrow> countable S" 

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by (blast dest: finite_imp_inj_to_nat_seg countableI) 

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lemma countableI_bij1: "bij_betw f A B \<Longrightarrow> countable A \<Longrightarrow> countable B" 

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by (blast elim: countableE_bij intro: bij_betw_trans countableI_bij) 

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lemma countableI_bij2: "bij_betw f B A \<Longrightarrow> countable A \<Longrightarrow> countable B" 

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by (blast elim: countableE_bij intro: bij_betw_trans bij_betw_inv_into countableI_bij) 

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lemma countable_iff_bij[simp]: "bij_betw f A B \<Longrightarrow> countable A \<longleftrightarrow> countable B" 

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by (blast intro: countableI_bij1 countableI_bij2) 

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lemma countable_subset: "A \<subseteq> B \<Longrightarrow> countable B \<Longrightarrow> countable A" 

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by (auto simp: countable_def intro: subset_inj_on) 

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lemma countableI_type[intro, simp]: "countable (A:: 'a :: countable set)" 

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using countableI[of to_nat A] by auto 

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subsection {* Enumerate a countable set *} 

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lemma countableE_infinite: 

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assumes "countable S" "infinite S" 

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obtains e :: "'a \<Rightarrow> nat" where "bij_betw e S UNIV" 

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proof  

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obtain f :: "'a \<Rightarrow> nat" where "inj_on f S" 
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using `countable S` by (rule countableE) 

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then have "bij_betw f S (f`S)" 
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unfolding bij_betw_def by simp 

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moreover 

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from `inj_on f S` `infinite S` have inf_fS: "infinite (f`S)" 

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by (auto dest: finite_imageD) 

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then have "bij_betw (the_inv_into UNIV (enumerate (f`S))) (f`S) UNIV" 

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by (intro bij_betw_the_inv_into bij_enumerate) 

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ultimately have "bij_betw (the_inv_into UNIV (enumerate (f`S)) \<circ> f) S UNIV" 

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by (rule bij_betw_trans) 

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then show thesis .. 

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qed 

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lemma countable_enum_cases: 

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assumes "countable S" 

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obtains (finite) f :: "'a \<Rightarrow> nat" where "finite S" "bij_betw f S {..<card S}" 

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 (infinite) f :: "'a \<Rightarrow> nat" where "infinite S" "bij_betw f S UNIV" 

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using ex_bij_betw_finite_nat[of S] countableE_infinite `countable S` 

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by (cases "finite S") (auto simp add: atLeast0LessThan) 

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definition to_nat_on :: "'a set \<Rightarrow> 'a \<Rightarrow> nat" where 

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"to_nat_on S = (SOME f. if finite S then bij_betw f S {..< card S} else bij_betw f S UNIV)" 

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definition from_nat_into :: "'a set \<Rightarrow> nat \<Rightarrow> 'a" where 

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"from_nat_into S n = (if n \<in> to_nat_on S ` S then inv_into S (to_nat_on S) n else SOME s. s\<in>S)" 
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lemma to_nat_on_finite: "finite S \<Longrightarrow> bij_betw (to_nat_on S) S {..< card S}" 

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using ex_bij_betw_finite_nat unfolding to_nat_on_def 

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by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S {..<card S}"]) (auto simp add: atLeast0LessThan) 

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lemma to_nat_on_infinite: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (to_nat_on S) S UNIV" 

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using countableE_infinite unfolding to_nat_on_def 

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by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S UNIV"]) auto 

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lemma bij_betw_from_nat_into_finite: "finite S \<Longrightarrow> bij_betw (from_nat_into S) {..< card S} S" 
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unfolding from_nat_into_def[abs_def] 
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using to_nat_on_finite[of S] 
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apply (subst bij_betw_cong) 
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apply (split split_if) 
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apply (simp add: bij_betw_def) 
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apply (auto cong: bij_betw_cong 
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intro: bij_betw_inv_into to_nat_on_finite) 
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done 
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lemma bij_betw_from_nat_into: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (from_nat_into S) UNIV S" 
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unfolding from_nat_into_def[abs_def] 
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using to_nat_on_infinite[of S, unfolded bij_betw_def] 
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by (auto cong: bij_betw_cong intro: bij_betw_inv_into to_nat_on_infinite) 
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lemma inj_on_to_nat_on[intro]: "countable A \<Longrightarrow> inj_on (to_nat_on A) A" 

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using to_nat_on_infinite[of A] to_nat_on_finite[of A] 

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by (cases "finite A") (auto simp: bij_betw_def) 

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lemma to_nat_on_inj[simp]: 

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"countable A \<Longrightarrow> a \<in> A \<Longrightarrow> b \<in> A \<Longrightarrow> to_nat_on A a = to_nat_on A b \<longleftrightarrow> a = b" 

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using inj_on_to_nat_on[of A] by (auto dest: inj_onD) 

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lemma from_nat_into_to_nat_on[simp]: "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> from_nat_into A (to_nat_on A a) = a" 
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by (auto simp: from_nat_into_def intro!: inv_into_f_f) 
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lemma subset_range_from_nat_into: "countable A \<Longrightarrow> A \<subseteq> range (from_nat_into A)" 

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by (auto intro: from_nat_into_to_nat_on[symmetric]) 

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lemma from_nat_into: "A \<noteq> {} \<Longrightarrow> from_nat_into A n \<in> A" 
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lemma range_from_nat_into_subset: "A \<noteq> {} \<Longrightarrow> range (from_nat_into A) \<subseteq> A" 
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using from_nat_into[of A] by auto 
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lemma range_from_nat_into[simp]: "A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> range (from_nat_into A) = A" 
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by (metis equalityI range_from_nat_into_subset subset_range_from_nat_into) 
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lemma image_to_nat_on: "countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A ` A = UNIV" 
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lemma to_nat_on_surj: "countable A \<Longrightarrow> infinite A \<Longrightarrow> \<exists>a\<in>A. to_nat_on A a = n" 
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by (metis (no_types) image_iff iso_tuple_UNIV_I image_to_nat_on) 
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lemma to_nat_on_from_nat_into[simp]: "n \<in> to_nat_on A ` A \<Longrightarrow> to_nat_on A (from_nat_into A n) = n" 
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by (simp add: f_inv_into_f from_nat_into_def) 
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lemma to_nat_on_from_nat_into_infinite[simp]: 
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"countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A (from_nat_into A n) = n" 
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by (metis image_iff to_nat_on_surj to_nat_on_from_nat_into) 
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lemma from_nat_into_inj: 
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"countable A \<Longrightarrow> m \<in> to_nat_on A ` A \<Longrightarrow> n \<in> to_nat_on A ` A \<Longrightarrow> 
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from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n" 
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by (subst to_nat_on_inj[symmetric, of A]) auto 
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lemma from_nat_into_inj_infinite[simp]: 
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"countable A \<Longrightarrow> infinite A \<Longrightarrow> from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n" 
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using image_to_nat_on[of A] from_nat_into_inj[of A m n] by simp 
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lemma eq_from_nat_into_iff: 
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"countable A \<Longrightarrow> x \<in> A \<Longrightarrow> i \<in> to_nat_on A ` A \<Longrightarrow> x = from_nat_into A i \<longleftrightarrow> i = to_nat_on A x" 
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by auto 
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lemma from_nat_into_surj: "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> \<exists>n. from_nat_into A n = a" 
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by (rule exI[of _ "to_nat_on A a"]) simp 
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lemma from_nat_into_inject[simp]: 
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"A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> B \<noteq> {} \<Longrightarrow> countable B \<Longrightarrow> from_nat_into A = from_nat_into B \<longleftrightarrow> A = B" 
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by (metis range_from_nat_into) 
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lemma inj_on_from_nat_into: "inj_on from_nat_into ({A. A \<noteq> {} \<and> countable A})" 
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unfolding inj_on_def by auto 
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subsection {* Closure properties of countability *} 
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lemma countable_SIGMA[intro, simp]: 

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"countable I \<Longrightarrow> (\<And>i. i \<in> I \<Longrightarrow> countable (A i)) \<Longrightarrow> countable (SIGMA i : I. A i)" 

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by (intro countableI'[of "\<lambda>(i, a). (to_nat_on I i, to_nat_on (A i) a)"]) (auto simp: inj_on_def) 

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lemma countable_image[intro, simp]: 
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assumes "countable A" 

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shows "countable (f`A)" 

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proof  
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obtain g :: "'a \<Rightarrow> nat" where "inj_on g A" 
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using assms by (rule countableE) 

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moreover have "inj_on (inv_into A f) (f`A)" "inv_into A f ` f ` A \<subseteq> A" 
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by (auto intro: inj_on_inv_into inv_into_into) 

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ultimately show ?thesis 

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by (blast dest: comp_inj_on subset_inj_on intro: countableI) 

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qed 

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lemma countable_UN[intro, simp]: 

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fixes I :: "'i set" and A :: "'i => 'a set" 

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assumes I: "countable I" 

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assumes A: "\<And>i. i \<in> I \<Longrightarrow> countable (A i)" 

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shows "countable (\<Union>i\<in>I. A i)" 

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proof  

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have "(\<Union>i\<in>I. A i) = snd ` (SIGMA i : I. A i)" by (auto simp: image_iff) 

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then show ?thesis by (simp add: assms) 

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qed 

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lemma countable_Un[intro]: "countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A \<union> B)" 

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by (rule countable_UN[of "{True, False}" "\<lambda>True \<Rightarrow> A  False \<Rightarrow> B", simplified]) 

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(simp split: bool.split) 

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lemma countable_Un_iff[simp]: "countable (A \<union> B) \<longleftrightarrow> countable A \<and> countable B" 

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by (metis countable_Un countable_subset inf_sup_ord(3,4)) 

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lemma countable_Plus[intro, simp]: 

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"countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A <+> B)" 

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by (simp add: Plus_def) 

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lemma countable_empty[intro, simp]: "countable {}" 

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by (blast intro: countable_finite) 

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lemma countable_insert[intro, simp]: "countable A \<Longrightarrow> countable (insert a A)" 

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using countable_Un[of "{a}" A] by (auto simp: countable_finite) 

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lemma countable_Int1[intro, simp]: "countable A \<Longrightarrow> countable (A \<inter> B)" 

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by (force intro: countable_subset) 

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lemma countable_Int2[intro, simp]: "countable B \<Longrightarrow> countable (A \<inter> B)" 

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by (blast intro: countable_subset) 

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lemma countable_INT[intro, simp]: "i \<in> I \<Longrightarrow> countable (A i) \<Longrightarrow> countable (\<Inter>i\<in>I. A i)" 

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by (blast intro: countable_subset) 

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lemma countable_Diff[intro, simp]: "countable A \<Longrightarrow> countable (A  B)" 

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by (blast intro: countable_subset) 

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lemma countable_vimage: "B \<subseteq> range f \<Longrightarrow> countable (f ` B) \<Longrightarrow> countable B" 

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by (metis Int_absorb2 assms countable_image image_vimage_eq) 

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lemma surj_countable_vimage: "surj f \<Longrightarrow> countable (f ` B) \<Longrightarrow> countable B" 

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by (metis countable_vimage top_greatest) 

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lemma countable_Collect[simp]: "countable A \<Longrightarrow> countable {a \<in> A. \<phi> a}" 
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by (metis Collect_conj_eq Int_absorb Int_commute Int_def countable_Int1) 
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lemma countable_lists[intro, simp]: 
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assumes A: "countable A" shows "countable (lists A)" 

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proof  

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have "countable (lists (range (from_nat_into A)))" 

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by (auto simp: lists_image) 

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with A show ?thesis 

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by (auto dest: subset_range_from_nat_into countable_subset lists_mono) 

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qed 

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lemma Collect_finite_eq_lists: "Collect finite = set ` lists UNIV" 
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using finite_list by auto 
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lemma countable_Collect_finite: "countable (Collect (finite::'a::countable set\<Rightarrow>bool))" 
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by (simp add: Collect_finite_eq_lists) 
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lemma countable_rat: "countable \<rat>" 
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unfolding Rats_def by auto 
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lemma Collect_finite_subset_eq_lists: "{A. finite A \<and> A \<subseteq> T} = set ` lists T" 
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using finite_list by (auto simp: lists_eq_set) 
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lemma countable_Collect_finite_subset: 
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"countable T \<Longrightarrow> countable {A. finite A \<and> A \<subseteq> T}" 
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50134  258 
subsection {* Misc lemmas *} 
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lemma countable_all: 

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assumes S: "countable S" 

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shows "(\<forall>s\<in>S. P s) \<longleftrightarrow> (\<forall>n::nat. from_nat_into S n \<in> S \<longrightarrow> P (from_nat_into S n))" 

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using S[THEN subset_range_from_nat_into] by auto 

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end 