src/HOL/Library/Countable_Set.thy

tuned proofs -- clarified flow of facts wrt. calculation;

(*  Title:      HOL/Library/Countable_Set.thy
    Author:     Johannes Hölzl
    Author:     Andrei Popescu
*)

header {* Countable sets *}

theory Countable_Set
imports Countable Infinite_Set
begin

subsection {* Predicate for countable sets *}

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

lemma countableE:
  assumes S: "countable S" obtains f :: "'a \<Rightarrow> nat" where "inj_on f S"
  using S by (auto simp: countable_def)

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

lemma countableI': "inj_on (f::'a \<Rightarrow> 'b::countable) S \<Longrightarrow> countable S"
  using comp_inj_on[of f S to_nat] by (auto intro: countableI)

lemma countableE_bij:
  assumes S: "countable S" obtains f :: "nat \<Rightarrow> 'a" and C :: "nat set" where "bij_betw f C S"
  using S by (blast elim: countableE dest: inj_on_imp_bij_betw bij_betw_inv)

lemma countableI_bij: "bij_betw f (C::nat set) S \<Longrightarrow> countable S"
  by (blast intro: countableI bij_betw_inv_into bij_betw_imp_inj_on)

lemma countable_finite: "finite S \<Longrightarrow> countable S"
  by (blast dest: finite_imp_inj_to_nat_seg countableI)

lemma countableI_bij1: "bij_betw f A B \<Longrightarrow> countable A \<Longrightarrow> countable B"
  by (blast elim: countableE_bij intro: bij_betw_trans countableI_bij)

lemma countableI_bij2: "bij_betw f B A \<Longrightarrow> countable A \<Longrightarrow> countable B"
  by (blast elim: countableE_bij intro: bij_betw_trans bij_betw_inv_into countableI_bij)

lemma countable_iff_bij[simp]: "bij_betw f A B \<Longrightarrow> countable A \<longleftrightarrow> countable B"
  by (blast intro: countableI_bij1 countableI_bij2)

lemma countable_subset: "A \<subseteq> B \<Longrightarrow> countable B \<Longrightarrow> countable A"
  by (auto simp: countable_def intro: subset_inj_on)

lemma countableI_type[intro, simp]: "countable (A:: 'a :: countable set)"
  using countableI[of to_nat A] by auto

subsection {* Enumerate a countable set *}

lemma countableE_infinite:
  assumes "countable S" "infinite S"
  obtains e :: "'a \<Rightarrow> nat" where "bij_betw e S UNIV"
proof -
  from `countable S`[THEN countableE] guess f .
  then have "bij_betw f S (f`S)"
    unfolding bij_betw_def by simp
  moreover
  from `inj_on f S` `infinite S` have inf_fS: "infinite (f`S)"
    by (auto dest: finite_imageD)
  then have "bij_betw (the_inv_into UNIV (enumerate (f`S))) (f`S) UNIV"
    by (intro bij_betw_the_inv_into bij_enumerate)
  ultimately have "bij_betw (the_inv_into UNIV (enumerate (f`S)) \<circ> f) S UNIV"
    by (rule bij_betw_trans)
  then show thesis ..
qed

lemma countable_enum_cases:
  assumes "countable S"
  obtains (finite) f :: "'a \<Rightarrow> nat" where "finite S" "bij_betw f S {..<card S}"
        | (infinite) f :: "'a \<Rightarrow> nat" where "infinite S" "bij_betw f S UNIV"
  using ex_bij_betw_finite_nat[of S] countableE_infinite `countable S`
  by (cases "finite S") (auto simp add: atLeast0LessThan)

definition to_nat_on :: "'a set \<Rightarrow> 'a \<Rightarrow> nat" where
  "to_nat_on S = (SOME f. if finite S then bij_betw f S {..< card S} else bij_betw f S UNIV)"

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

lemma to_nat_on_finite: "finite S \<Longrightarrow> bij_betw (to_nat_on S) S {..< card S}"
  using ex_bij_betw_finite_nat unfolding to_nat_on_def
  by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S {..<card S}"]) (auto simp add: atLeast0LessThan)

lemma to_nat_on_infinite: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (to_nat_on S) S UNIV"
  using countableE_infinite unfolding to_nat_on_def
  by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S UNIV"]) auto

lemma bij_betw_from_nat_into_finite: "finite S \<Longrightarrow> bij_betw (from_nat_into S) {..< card S} S"
  unfolding from_nat_into_def[abs_def]
  using to_nat_on_finite[of S]
  apply (subst bij_betw_cong)
  apply (split split_if)
  apply (simp add: bij_betw_def)
  apply (auto cong: bij_betw_cong
              intro: bij_betw_inv_into to_nat_on_finite)
  done

lemma bij_betw_from_nat_into: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (from_nat_into S) UNIV S"
  unfolding from_nat_into_def[abs_def]
  using to_nat_on_infinite[of S, unfolded bij_betw_def]
  by (auto cong: bij_betw_cong intro: bij_betw_inv_into to_nat_on_infinite)

lemma inj_on_to_nat_on[intro]: "countable A \<Longrightarrow> inj_on (to_nat_on A) A"
  using to_nat_on_infinite[of A] to_nat_on_finite[of A]
  by (cases "finite A") (auto simp: bij_betw_def)

lemma to_nat_on_inj[simp]:
  "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> b \<in> A \<Longrightarrow> to_nat_on A a = to_nat_on A b \<longleftrightarrow> a = b"
  using inj_on_to_nat_on[of A] by (auto dest: inj_onD)

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"
  by (auto simp: from_nat_into_def intro!: inv_into_f_f)

lemma subset_range_from_nat_into: "countable A \<Longrightarrow> A \<subseteq> range (from_nat_into A)"
  by (auto intro: from_nat_into_to_nat_on[symmetric])

lemma from_nat_into: "A \<noteq> {} \<Longrightarrow> from_nat_into A n \<in> A"
  unfolding from_nat_into_def by (metis equals0I inv_into_into someI_ex)

lemma range_from_nat_into_subset: "A \<noteq> {} \<Longrightarrow> range (from_nat_into A) \<subseteq> A"
  using from_nat_into[of A] by auto

lemma range_from_nat_into[simp]: "A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> range (from_nat_into A) = A"
  by (metis equalityI range_from_nat_into_subset subset_range_from_nat_into)

lemma image_to_nat_on: "countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A ` A = UNIV"
  using to_nat_on_infinite[of A] by (simp add: bij_betw_def)

lemma to_nat_on_surj: "countable A \<Longrightarrow> infinite A \<Longrightarrow> \<exists>a\<in>A. to_nat_on A a = n"
  by (metis (no_types) image_iff iso_tuple_UNIV_I image_to_nat_on)

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"
  by (simp add: f_inv_into_f from_nat_into_def)

lemma to_nat_on_from_nat_into_infinite[simp]:
  "countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A (from_nat_into A n) = n"
  by (metis image_iff to_nat_on_surj to_nat_on_from_nat_into)

lemma from_nat_into_inj:
  "countable A \<Longrightarrow> m \<in> to_nat_on A ` A \<Longrightarrow> n \<in> to_nat_on A ` A \<Longrightarrow>
    from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n"
  by (subst to_nat_on_inj[symmetric, of A]) auto

lemma from_nat_into_inj_infinite[simp]:
  "countable A \<Longrightarrow> infinite A \<Longrightarrow> from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n"
  using image_to_nat_on[of A] from_nat_into_inj[of A m n] by simp

lemma eq_from_nat_into_iff:
  "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"
  by auto

lemma from_nat_into_surj: "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> \<exists>n. from_nat_into A n = a"
  by (rule exI[of _ "to_nat_on A a"]) simp

lemma from_nat_into_inject[simp]:
  "A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> B \<noteq> {} \<Longrightarrow> countable B \<Longrightarrow> from_nat_into A = from_nat_into B \<longleftrightarrow> A = B"
  by (metis range_from_nat_into)

lemma inj_on_from_nat_into: "inj_on from_nat_into ({A. A \<noteq> {} \<and> countable A})"
  unfolding inj_on_def by auto

subsection {* Closure properties of countability *}

lemma countable_SIGMA[intro, simp]:
  "countable I \<Longrightarrow> (\<And>i. i \<in> I \<Longrightarrow> countable (A i)) \<Longrightarrow> countable (SIGMA i : I. A i)"
  by (intro countableI'[of "\<lambda>(i, a). (to_nat_on I i, to_nat_on (A i) a)"]) (auto simp: inj_on_def)

lemma countable_image[intro, simp]: assumes A: "countable A" shows "countable (f`A)"
proof -
  from A guess g by (rule countableE)
  moreover have "inj_on (inv_into A f) (f`A)" "inv_into A f ` f ` A \<subseteq> A"
    by (auto intro: inj_on_inv_into inv_into_into)
  ultimately show ?thesis
    by (blast dest: comp_inj_on subset_inj_on intro: countableI)
qed

lemma countable_UN[intro, simp]:
  fixes I :: "'i set" and A :: "'i => 'a set"
  assumes I: "countable I"
  assumes A: "\<And>i. i \<in> I \<Longrightarrow> countable (A i)"
  shows "countable (\<Union>i\<in>I. A i)"
proof -
  have "(\<Union>i\<in>I. A i) = snd ` (SIGMA i : I. A i)" by (auto simp: image_iff)
  then show ?thesis by (simp add: assms)
qed

lemma countable_Un[intro]: "countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A \<union> B)"
  by (rule countable_UN[of "{True, False}" "\<lambda>True \<Rightarrow> A | False \<Rightarrow> B", simplified])
     (simp split: bool.split)

lemma countable_Un_iff[simp]: "countable (A \<union> B) \<longleftrightarrow> countable A \<and> countable B"
  by (metis countable_Un countable_subset inf_sup_ord(3,4))

lemma countable_Plus[intro, simp]:
  "countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A <+> B)"
  by (simp add: Plus_def)

lemma countable_empty[intro, simp]: "countable {}"
  by (blast intro: countable_finite)

lemma countable_insert[intro, simp]: "countable A \<Longrightarrow> countable (insert a A)"
  using countable_Un[of "{a}" A] by (auto simp: countable_finite)

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

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

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

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

lemma countable_vimage: "B \<subseteq> range f \<Longrightarrow> countable (f -` B) \<Longrightarrow> countable B"
  by (metis Int_absorb2 assms countable_image image_vimage_eq)

lemma surj_countable_vimage: "surj f \<Longrightarrow> countable (f -` B) \<Longrightarrow> countable B"
  by (metis countable_vimage top_greatest)

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

lemma countable_lists[intro, simp]:
  assumes A: "countable A" shows "countable (lists A)"
proof -
  have "countable (lists (range (from_nat_into A)))"
    by (auto simp: lists_image)
  with A show ?thesis
    by (auto dest: subset_range_from_nat_into countable_subset lists_mono)
qed

lemma Collect_finite_eq_lists: "Collect finite = set ` lists UNIV"
  using finite_list by auto

lemma countable_Collect_finite: "countable (Collect (finite::'a::countable set\<Rightarrow>bool))"
  by (simp add: Collect_finite_eq_lists)

lemma countable_rat: "countable \<rat>"
  unfolding Rats_def by auto

lemma Collect_finite_subset_eq_lists: "{A. finite A \<and> A \<subseteq> T} = set ` lists T"
  using finite_list by (auto simp: lists_eq_set)

lemma countable_Collect_finite_subset:
  "countable T \<Longrightarrow> countable {A. finite A \<and> A \<subseteq> T}"
  unfolding Collect_finite_subset_eq_lists by auto

subsection {* Misc lemmas *}

lemma countable_all:
  assumes S: "countable S"
  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))"
  using S[THEN subset_range_from_nat_into] by auto

end