Theory Infinite_Set (Isabelle2005: October 2005)

# Theory Infinite_Set

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theory Infinite_Set
imports Hilbert_Choice Binomial
begin

    (*  Title:      HOL/Infnite_Set.thy
        ID:         $Id: Infinite_Set.thy,v 1.11 2005/07/13 13:06:21 paulson Exp $
        Author:     Stephan Merz
    *)

    header {* Infnite Sets and Related Concepts*}

    theory Infinite_Set
    imports Hilbert_Choice Binomial
    begin

    subsection "Infinite Sets"

    text {* Some elementary facts about infinite sets, by Stefan Merz. *}

    syntax
      infinite :: "'a set => bool"
    translations
      "infinite S" == "S ∉ Finites"

    text {*
      Infinite sets are non-empty, and if we remove some elements
      from an infinite set, the result is still infinite.
    *}

    lemma infinite_nonempty:
      "¬ (infinite {})"
    by simp

    lemma infinite_remove:
      "infinite S ==> infinite (S - {a})"
    by simp

    lemma Diff_infinite_finite:
      assumes T: "finite T" and S: "infinite S"
      shows "infinite (S-T)"
    using T
    proof (induct)
      from S
      show "infinite (S - {})" by auto
    next
      fix T x
      assume ih: "infinite (S-T)"
      have "S - (insert x T) = (S-T) - {x}"
        by (rule Diff_insert)
      with ih
      show "infinite (S - (insert x T))"
        by (simp add: infinite_remove)
    qed

    lemma Un_infinite:
      "infinite S ==> infinite (S ∪ T)"
    by simp

    lemma infinite_super:
      assumes T: "S ⊆ T" and S: "infinite S"
      shows "infinite T"
    proof (rule ccontr)
      assume "¬(infinite T)"
      with T
      have "finite S" by (simp add: finite_subset)
      with S
      show False by simp
    qed

    text {*
      As a concrete example, we prove that the set of natural
      numbers is infinite.
    *}

    lemma finite_nat_bounded:
      assumes S: "finite (S::nat set)"
      shows "∃k. S ⊆ {..<k}" (is "∃k. ?bounded S k")
    using S
    proof (induct)
      have "?bounded {} 0" by simp
      thus "∃k. ?bounded {} k" ..
    next
      fix S x
      assume "∃k. ?bounded S k"
      then obtain k where k: "?bounded S k" ..
      show "∃k. ?bounded (insert x S) k"
      proof (cases "x<k")
        case True
        with k show ?thesis by auto
      next
        case False
        with k have "?bounded S (Suc x)" by auto
        thus ?thesis by auto
      qed
    qed

    lemma finite_nat_iff_bounded:
      "finite (S::nat set) = (∃k. S ⊆ {..<k})" (is "?lhs = ?rhs")
    proof
      assume ?lhs
      thus ?rhs by (rule finite_nat_bounded)
    next
      assume ?rhs
      then obtain k where "S ⊆ {..<k}" ..
      thus "finite S"
        by (rule finite_subset, simp)
    qed

    lemma finite_nat_iff_bounded_le:
      "finite (S::nat set) = (∃k. S ⊆ {..k})" (is "?lhs = ?rhs")
    proof
      assume ?lhs
      then obtain k where "S ⊆ {..<k}"
        by (blast dest: finite_nat_bounded)
      hence "S ⊆ {..k}" by auto
      thus ?rhs ..
    next
      assume ?rhs
      then obtain k where "S ⊆ {..k}" ..
      thus "finite S"
        by (rule finite_subset, simp)
    qed

    lemma infinite_nat_iff_unbounded:
      "infinite (S::nat set) = (∀m. ∃n. m<n ∧ n∈S)"
      (is "?lhs = ?rhs")
    proof
      assume inf: ?lhs
      show ?rhs
      proof (rule ccontr)
        assume "¬ ?rhs"
        then obtain m where m: "∀n. m<n --> n∉S" by blast
        hence "S ⊆ {..m}"
          by (auto simp add: sym[OF linorder_not_less])
        with inf show "False"
          by (simp add: finite_nat_iff_bounded_le)
      qed
    next
      assume unbounded: ?rhs
      show ?lhs
      proof
        assume "finite S"
        then obtain m where "S ⊆ {..m}"
          by (auto simp add: finite_nat_iff_bounded_le)
        hence "∀n. m<n --> n∉S" by auto
        with unbounded show "False" by blast
      qed
    qed

    lemma infinite_nat_iff_unbounded_le:
      "infinite (S::nat set) = (∀m. ∃n. m≤n ∧ n∈S)"
      (is "?lhs = ?rhs")
    proof
      assume inf: ?lhs
      show ?rhs
      proof
        fix m
        from inf obtain n where "m<n ∧ n∈S"
          by (auto simp add: infinite_nat_iff_unbounded)
        hence "m≤n ∧ n∈S" by auto
        thus "∃n. m ≤ n ∧ n ∈ S" ..
      qed
    next
      assume unbounded: ?rhs
      show ?lhs
      proof (auto simp add: infinite_nat_iff_unbounded)
        fix m
        from unbounded obtain n where "(Suc m)≤n ∧ n∈S"
          by blast
        hence "m<n ∧ n∈S" by auto
        thus "∃n. m < n ∧ n ∈ S" ..
      qed
    qed

    text {*
      For a set of natural numbers to be infinite, it is enough
      to know that for any number larger than some @{text k}, there
      is some larger number that is an element of the set.
    *}

    lemma unbounded_k_infinite:
      assumes k: "∀m. k<m --> (∃n. m<n ∧ n∈S)"
      shows "infinite (S::nat set)"
    proof (auto simp add: infinite_nat_iff_unbounded)
      fix m show "∃n. m<n ∧ n∈S"
      proof (cases "k<m")
        case True
        with k show ?thesis by blast
      next
        case False
        from k obtain n where "Suc k < n ∧ n∈S" by auto
        with False have "m<n ∧ n∈S" by auto
        thus ?thesis ..
      qed
    qed

    theorem nat_infinite [simp]:
      "infinite (UNIV :: nat set)"
    by (auto simp add: infinite_nat_iff_unbounded)

    theorem nat_not_finite [elim]:
      "finite (UNIV::nat set) ==> R"
    by simp

    text {*
      Every infinite set contains a countable subset. More precisely
      we show that a set @{text S} is infinite if and only if there exists
      an injective function from the naturals into @{text S}.
    *}

    lemma range_inj_infinite:
      "inj (f::nat => 'a) ==> infinite (range f)"
    proof
      assume "inj f"
        and  "finite (range f)"
      hence "finite (UNIV::nat set)"
        by (auto intro: finite_imageD simp del: nat_infinite)
      thus "False" by simp
    qed

    text {*
      The ``only if'' direction is harder because it requires the
      construction of a sequence of pairwise different elements of
      an infinite set @{text S}. The idea is to construct a sequence of
      non-empty and infinite subsets of @{text S} obtained by successively
      removing elements of @{text S}.
    *}

    lemma linorder_injI:
      assumes hyp: "∀x y. x < (y::'a::linorder) --> f x ≠ f y"
      shows "inj f"
    proof (rule inj_onI)
      fix x y
      assume f_eq: "f x = f y"
      show "x = y"
      proof (rule linorder_cases)
        assume "x < y"
        with hyp have "f x ≠ f y" by blast
        with f_eq show ?thesis by simp
      next
        assume "x = y"
        thus ?thesis .
      next
        assume "y < x"
        with hyp have "f y ≠ f x" by blast
        with f_eq show ?thesis by simp
      qed
    qed

    lemma infinite_countable_subset:
      assumes inf: "infinite (S::'a set)"
      shows "∃f. inj (f::nat => 'a) ∧ range f ⊆ S"
    proof -
      def Sseq ≡ "nat_rec S (λn T. T - {SOME e. e ∈ T})"
      def pick ≡ "λn. (SOME e. e ∈ Sseq n)"
      have Sseq_inf: "!!n. infinite (Sseq n)"
      proof -
        fix n
        show "infinite (Sseq n)"
        proof (induct n)
          from inf show "infinite (Sseq 0)"
            by (simp add: Sseq_def)
        next
          fix n
          assume "infinite (Sseq n)" thus "infinite (Sseq (Suc n))"
            by (simp add: Sseq_def infinite_remove)
        qed
      qed
      have Sseq_S: "!!n. Sseq n ⊆ S"
      proof -
        fix n
        show "Sseq n ⊆ S"
          by (induct n, auto simp add: Sseq_def)
      qed
      have Sseq_pick: "!!n. pick n ∈ Sseq n"
      proof -
        fix n
        show "pick n ∈ Sseq n"
        proof (unfold pick_def, rule someI_ex)
          from Sseq_inf have "infinite (Sseq n)" .
          hence "Sseq n ≠ {}" by auto
          thus "∃x. x ∈ Sseq n" by auto
        qed
      qed
      with Sseq_S have rng: "range pick ⊆ S"
        by auto
      have pick_Sseq_gt: "!!n m. pick n ∉ Sseq (n + Suc m)"
      proof -
        fix n m
        show "pick n ∉ Sseq (n + Suc m)"
          by (induct m, auto simp add: Sseq_def pick_def)
      qed
      have pick_pick: "!!n m. pick n ≠ pick (n + Suc m)"
      proof -
        fix n m
        from Sseq_pick have "pick (n + Suc m) ∈ Sseq (n + Suc m)" .
        moreover from pick_Sseq_gt
        have "pick n ∉ Sseq (n + Suc m)" .
        ultimately show "pick n ≠ pick (n + Suc m)"
          by auto
      qed
      have inj: "inj pick"
      proof (rule linorder_injI)
        show "∀i j. i<(j::nat) --> pick i ≠ pick j"
        proof (clarify)
          fix i j
          assume ij: "i<(j::nat)"
            and eq: "pick i = pick j"
          from ij obtain k where "j = i + (Suc k)"
            by (auto simp add: less_iff_Suc_add)
          with pick_pick have "pick i ≠ pick j" by simp
          with eq show "False" by simp
        qed
      qed
      from rng inj show ?thesis by auto
    qed

    theorem infinite_iff_countable_subset:
      "infinite S = (∃f. inj (f::nat => 'a) ∧ range f ⊆ S)"
      (is "?lhs = ?rhs")
    by (auto simp add: infinite_countable_subset
                       range_inj_infinite infinite_super)

    text {*
      For any function with infinite domain and finite range
      there is some element that is the image of infinitely
      many domain elements. In particular, any infinite sequence
      of elements from a finite set contains some element that
      occurs infinitely often.
    *}

    theorem inf_img_fin_dom:
      assumes img: "finite (f`A)" and dom: "infinite A"
      shows "∃y ∈ f`A. infinite (f -` {y})"
    proof (rule ccontr)
      assume "¬ (∃y∈f ` A. infinite (f -` {y}))"
      with img have "finite (UN y:f`A. f -` {y})"
        by (blast intro: finite_UN_I)
      moreover have "A ⊆ (UN y:f`A. f -` {y})" by auto
      moreover note dom
      ultimately show "False"
        by (simp add: infinite_super)
    qed

    theorems inf_img_fin_domE = inf_img_fin_dom[THEN bexE]

    subsection "Infinitely Many and Almost All"

    text {*
      We often need to reason about the existence of infinitely many
      (resp., all but finitely many) objects satisfying some predicate,
      so we introduce corresponding binders and their proof rules.
    *}

    consts
      Inf_many :: "('a => bool) => bool"      (binder "INF " 10)
      Alm_all  :: "('a => bool) => bool"      (binder "MOST " 10)

    defs
      INF_def:  "Inf_many P ≡ infinite {x. P x}"
      MOST_def: "Alm_all P ≡ ¬(INF x. ¬ P x)"

    syntax (xsymbols)
      "MOST " :: "[idts, bool] => bool"       ("(3∀∞_./ _)" [0,10] 10)
      "INF "    :: "[idts, bool] => bool"     ("(3∃∞_./ _)" [0,10] 10)

    syntax (HTML output)
      "MOST " :: "[idts, bool] => bool"       ("(3∀∞_./ _)" [0,10] 10)
      "INF "    :: "[idts, bool] => bool"     ("(3∃∞_./ _)" [0,10] 10)

    lemma INF_EX:
      "(∃∞x. P x) ==> (∃x. P x)"
    proof (unfold INF_def, rule ccontr)
      assume inf: "infinite {x. P x}"
        and notP: "¬(∃x. P x)"
      from notP have "{x. P x} = {}" by simp
      hence "finite {x. P x}" by simp
      with inf show "False" by simp
    qed

    lemma MOST_iff_finiteNeg:
      "(∀∞x. P x) = finite {x. ¬ P x}"
    by (simp add: MOST_def INF_def)

    lemma ALL_MOST:
      "∀x. P x ==> ∀∞x. P x"
    by (simp add: MOST_iff_finiteNeg)

    lemma INF_mono:
      assumes inf: "∃∞x. P x" and q: "!!x. P x ==> Q x"
      shows "∃∞x. Q x"
    proof -
      from inf have "infinite {x. P x}" by (unfold INF_def)
      moreover from q have "{x. P x} ⊆ {x. Q x}" by auto
      ultimately show ?thesis
        by (simp add: INF_def infinite_super)
    qed

    lemma MOST_mono:
      "[| ∀∞x. P x; !!x. P x ==> Q x |] ==> ∀∞x. Q x"
    by (unfold MOST_def, blast intro: INF_mono)

    lemma INF_nat: "(∃∞n. P (n::nat)) = (∀m. ∃n. m<n ∧ P n)"
    by (simp add: INF_def infinite_nat_iff_unbounded)

    lemma INF_nat_le: "(∃∞n. P (n::nat)) = (∀m. ∃n. m≤n ∧ P n)"
    by (simp add: INF_def infinite_nat_iff_unbounded_le)

    lemma MOST_nat: "(∀∞n. P (n::nat)) = (∃m. ∀n. m<n --> P n)"
    by (simp add: MOST_def INF_nat)

    lemma MOST_nat_le: "(∀∞n. P (n::nat)) = (∃m. ∀n. m≤n --> P n)"
    by (simp add: MOST_def INF_nat_le)

    subsection "Miscellaneous"

    text {*
      A few trivial lemmas about sets that contain at most one element.
      These simplify the reasoning about deterministic automata.
    *}

    constdefs
      atmost_one :: "'a set => bool"
      "atmost_one S ≡ ∀x y. x∈S ∧ y∈S --> x=y"

    lemma atmost_one_empty: "S={} ==> atmost_one S"
    by (simp add: atmost_one_def)

    lemma atmost_one_singleton: "S = {x} ==> atmost_one S"
    by (simp add: atmost_one_def)

    lemma atmost_one_unique [elim]: "[| atmost_one S; x ∈ S; y ∈ S |] ==> y=x"
    by (simp add: atmost_one_def)

    end

### Infinite Sets

lemma infinite_nonempty:

      ¬ infinite {}

lemma infinite_remove:

      infinite S ==> infinite (S - {a})

lemma Diff_infinite_finite:

      [| finite T; infinite S |] ==> infinite (S - T)

lemma Un_infinite:

      infinite S ==> infinite (S ∪ T)

lemma infinite_super:

      [| S ⊆ T; infinite S |] ==> infinite T

lemma finite_nat_bounded:

      finite S ==> ∃k. S ⊆ {..<k}

lemma finite_nat_iff_bounded:

      finite S = (∃k. S ⊆ {..<k})

lemma finite_nat_iff_bounded_le:

      finite S = (∃k. S ⊆ {..k})

lemma infinite_nat_iff_unbounded:

      infinite S = (∀m. ∃n. m < n ∧ n ∈ S)

lemma infinite_nat_iff_unbounded_le:

      infinite S = (∀m. ∃n. m ≤ n ∧ n ∈ S)

lemma unbounded_k_infinite:

      ∀m>k. ∃n. m < n ∧ n ∈ S ==> infinite S

theorem nat_infinite:

      infinite UNIV

theorem nat_not_finite:

      finite UNIV ==> R

lemma range_inj_infinite:

      inj f ==> infinite (range f)

lemma linorder_injI:

      ∀x y. x < y --> f x ≠ f y ==> inj f

lemma infinite_countable_subset:

      infinite S ==> ∃f. inj f ∧ range f ⊆ S

theorem infinite_iff_countable_subset:

      infinite S = (∃f. inj f ∧ range f ⊆ S)

theorem inf_img_fin_dom:

      [| finite (f ` A); infinite A |] ==> ∃y∈f ` A. infinite (f -` {y})

theorems inf_img_fin_domE:

      [| finite (f1 ` A1); infinite A1;
         !!x. [| x ∈ f1 ` A1; infinite (f1 -` {x}) |] ==> Q |]
      ==> Q

theorems inf_img_fin_domE:

      [| finite (f1 ` A1); infinite A1;
         !!x. [| x ∈ f1 ` A1; infinite (f1 -` {x}) |] ==> Q |]
      ==> Q

### Infinitely Many and Almost All

lemma INF_EX:

      ∃∞x. P x ==> ∃x. P x

lemma MOST_iff_finiteNeg:

      (∀∞x. P x) = finite {x. ¬ P x}

lemma ALL_MOST:

      ∀x. P x ==> ∀∞x. P x

lemma INF_mono:

      [| ∃∞x. P x; !!x. P x ==> Q x |] ==> ∃∞x. Q x

lemma MOST_mono:

      [| ∀∞x. P x; !!x. P x ==> Q x |] ==> ∀∞x. Q x

lemma INF_nat:

      (∃∞n. P n) = (∀m. ∃n. m < n ∧ P n)

lemma INF_nat_le:

      (∃∞n. P n) = (∀m. ∃n. m ≤ n ∧ P n)

lemma MOST_nat:

      (∀∞n. P n) = (∃m. ∀n. m < n --> P n)

lemma MOST_nat_le:

      (∀∞n. P n) = (∃m. ∀n. m ≤ n --> P n)

### Miscellaneous

lemma atmost_one_empty:

      S = {} ==> atmost_one S

lemma atmost_one_singleton:

      S = {x} ==> atmost_one S

lemma atmost_one_unique:

      [| atmost_one S; x ∈ S; y ∈ S |] ==> y = x