src/HOL/Infinite_Set.thy
 author haftmann Mon, 24 Apr 2006 16:37:07 +0200 changeset 19457 b6eb4b4546fa parent 19363 667b5ea637dd child 19537 213ff8b0c60c permissions -rw-r--r--
fixed typo
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(*  Title:      HOL/Infnite_Set.thy
ID:         \$Id\$
Author:     Stephan Merz
*)

header {* Infinite 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. *}

abbreviation
infinite :: "'a set \<Rightarrow> bool"
"infinite S == \<not> finite S"

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

lemma infinite_nonempty:
"\<not> (infinite {})"
by simp

lemma infinite_remove:
"infinite S \<Longrightarrow> 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))"
qed

lemma Un_infinite:
"infinite S \<Longrightarrow> infinite (S \<union> T)"
by simp

lemma infinite_super:
assumes T: "S \<subseteq> T" and S: "infinite S"
shows "infinite T"
proof (rule ccontr)
assume "\<not>(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 "\<exists>k. S \<subseteq> {..<k}" (is "\<exists>k. ?bounded S k")
using S
proof (induct)
have "?bounded {} 0" by simp
thus "\<exists>k. ?bounded {} k" ..
next
fix S x
assume "\<exists>k. ?bounded S k"
then obtain k where k: "?bounded S k" ..
show "\<exists>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) = (\<exists>k. S \<subseteq> {..<k})" (is "?lhs = ?rhs")
proof
assume ?lhs
thus ?rhs by (rule finite_nat_bounded)
next
assume ?rhs
then obtain k where "S \<subseteq> {..<k}" ..
thus "finite S"
by (rule finite_subset, simp)
qed

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

lemma infinite_nat_iff_unbounded:
"infinite (S::nat set) = (\<forall>m. \<exists>n. m<n \<and> n\<in>S)"
(is "?lhs = ?rhs")
proof
assume inf: ?lhs
show ?rhs
proof (rule ccontr)
assume "\<not> ?rhs"
then obtain m where m: "\<forall>n. m<n \<longrightarrow> n\<notin>S" by blast
hence "S \<subseteq> {..m}"
by (auto simp add: sym[OF linorder_not_less])
with inf show "False"
qed
next
assume unbounded: ?rhs
show ?lhs
proof
assume "finite S"
then obtain m where "S \<subseteq> {..m}"
hence "\<forall>n. m<n \<longrightarrow> n\<notin>S" by auto
with unbounded show "False" by blast
qed
qed

lemma infinite_nat_iff_unbounded_le:
"infinite (S::nat set) = (\<forall>m. \<exists>n. m\<le>n \<and> n\<in>S)"
(is "?lhs = ?rhs")
proof
assume inf: ?lhs
show ?rhs
proof
fix m
from inf obtain n where "m<n \<and> n\<in>S"
hence "m\<le>n \<and> n\<in>S" by auto
thus "\<exists>n. m \<le> n \<and> n \<in> S" ..
qed
next
assume unbounded: ?rhs
show ?lhs
fix m
from unbounded obtain n where "(Suc m)\<le>n \<and> n\<in>S"
by blast
hence "m<n \<and> n\<in>S" by auto
thus "\<exists>n. m < n \<and> n \<in> 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: "\<forall>m. k<m \<longrightarrow> (\<exists>n. m<n \<and> n\<in>S)"
shows "infinite (S::nat set)"
fix m show "\<exists>n. m<n \<and> n\<in>S"
proof (cases "k<m")
case True
with k show ?thesis by blast
next
case False
from k obtain n where "Suc k < n \<and> n\<in>S" by auto
with False have "m<n \<and> n\<in>S" by auto
thus ?thesis ..
qed
qed

theorem nat_infinite [simp]:
"infinite (UNIV :: nat set)"

theorem nat_not_finite [elim]:
"finite (UNIV::nat set) \<Longrightarrow> 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 \<Rightarrow> 'a) \<Longrightarrow> 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: "\<forall>x y. x < (y::'a::linorder) \<longrightarrow> f x \<noteq> 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 \<noteq> 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 \<noteq> f x" by blast
with f_eq show ?thesis by simp
qed
qed

lemma infinite_countable_subset:
assumes inf: "infinite (S::'a set)"
shows "\<exists>f. inj (f::nat \<Rightarrow> 'a) \<and> range f \<subseteq> S"
proof -
def Sseq \<equiv> "nat_rec S (\<lambda>n T. T - {SOME e. e \<in> T})"
def pick \<equiv> "\<lambda>n. (SOME e. e \<in> Sseq n)"
have Sseq_inf: "\<And>n. infinite (Sseq n)"
proof -
fix n
show "infinite (Sseq n)"
proof (induct n)
from inf show "infinite (Sseq 0)"
next
fix n
assume "infinite (Sseq n)" thus "infinite (Sseq (Suc n))"
qed
qed
have Sseq_S: "\<And>n. Sseq n \<subseteq> S"
proof -
fix n
show "Sseq n \<subseteq> S"
by (induct n, auto simp add: Sseq_def)
qed
have Sseq_pick: "\<And>n. pick n \<in> Sseq n"
proof -
fix n
show "pick n \<in> Sseq n"
proof (unfold pick_def, rule someI_ex)
from Sseq_inf have "infinite (Sseq n)" .
hence "Sseq n \<noteq> {}" by auto
thus "\<exists>x. x \<in> Sseq n" by auto
qed
qed
with Sseq_S have rng: "range pick \<subseteq> S"
by auto
have pick_Sseq_gt: "\<And>n m. pick n \<notin> Sseq (n + Suc m)"
proof -
fix n m
show "pick n \<notin> Sseq (n + Suc m)"
by (induct m, auto simp add: Sseq_def pick_def)
qed
have pick_pick: "\<And>n m. pick n \<noteq> pick (n + Suc m)"
proof -
fix n m
from Sseq_pick have "pick (n + Suc m) \<in> Sseq (n + Suc m)" .
moreover from pick_Sseq_gt
have "pick n \<notin> Sseq (n + Suc m)" .
ultimately show "pick n \<noteq> pick (n + Suc m)"
by auto
qed
have inj: "inj pick"
proof (rule linorder_injI)
show "\<forall>i j. i<(j::nat) \<longrightarrow> pick i \<noteq> 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)"
with pick_pick have "pick i \<noteq> 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 = (\<exists>f. inj (f::nat \<Rightarrow> 'a) \<and> range f \<subseteq> S)"
(is "?lhs = ?rhs")
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 "\<exists>y \<in> f`A. infinite (f -` {y})"
proof (rule ccontr)
assume "\<not> (\<exists>y\<in>f ` A. infinite (f -` {y}))"
with img have "finite (UN y:f`A. f -` {y})"
by (blast intro: finite_UN_I)
moreover have "A \<subseteq> (UN y:f`A. f -` {y})" by auto
moreover note dom
ultimately show "False"
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 \<Rightarrow> bool) \<Rightarrow> bool"      (binder "INF " 10)
Alm_all  :: "('a \<Rightarrow> bool) \<Rightarrow> bool"      (binder "MOST " 10)

defs
INF_def:  "Inf_many P \<equiv> infinite {x. P x}"
MOST_def: "Alm_all P \<equiv> \<not>(INF x. \<not> P x)"

syntax (xsymbols)
"MOST " :: "[idts, bool] \<Rightarrow> bool"       ("(3\<forall>\<^sub>\<infinity>_./ _)" [0,10] 10)
"INF "    :: "[idts, bool] \<Rightarrow> bool"     ("(3\<exists>\<^sub>\<infinity>_./ _)" [0,10] 10)

syntax (HTML output)
"MOST " :: "[idts, bool] \<Rightarrow> bool"       ("(3\<forall>\<^sub>\<infinity>_./ _)" [0,10] 10)
"INF "    :: "[idts, bool] \<Rightarrow> bool"     ("(3\<exists>\<^sub>\<infinity>_./ _)" [0,10] 10)

lemma INF_EX:
"(\<exists>\<^sub>\<infinity>x. P x) \<Longrightarrow> (\<exists>x. P x)"
proof (unfold INF_def, rule ccontr)
assume inf: "infinite {x. P x}"
and notP: "\<not>(\<exists>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:
"(\<forall>\<^sub>\<infinity>x. P x) = finite {x. \<not> P x}"

lemma ALL_MOST:
"\<forall>x. P x \<Longrightarrow> \<forall>\<^sub>\<infinity>x. P x"

lemma INF_mono:
assumes inf: "\<exists>\<^sub>\<infinity>x. P x" and q: "\<And>x. P x \<Longrightarrow> Q x"
shows "\<exists>\<^sub>\<infinity>x. Q x"
proof -
from inf have "infinite {x. P x}" by (unfold INF_def)
moreover from q have "{x. P x} \<subseteq> {x. Q x}" by auto
ultimately show ?thesis
qed

lemma MOST_mono:
"\<lbrakk> \<forall>\<^sub>\<infinity>x. P x; \<And>x. P x \<Longrightarrow> Q x \<rbrakk> \<Longrightarrow> \<forall>\<^sub>\<infinity>x. Q x"
by (unfold MOST_def, blast intro: INF_mono)

lemma INF_nat: "(\<exists>\<^sub>\<infinity>n. P (n::nat)) = (\<forall>m. \<exists>n. m<n \<and> P n)"

lemma INF_nat_le: "(\<exists>\<^sub>\<infinity>n. P (n::nat)) = (\<forall>m. \<exists>n. m\<le>n \<and> P n)"

lemma MOST_nat: "(\<forall>\<^sub>\<infinity>n. P (n::nat)) = (\<exists>m. \<forall>n. m<n \<longrightarrow> P n)"

lemma MOST_nat_le: "(\<forall>\<^sub>\<infinity>n. P (n::nat)) = (\<exists>m. \<forall>n. m\<le>n \<longrightarrow> P n)"

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 \<Rightarrow> bool"
"atmost_one S \<equiv> \<forall>x y. x\<in>S \<and> y\<in>S \<longrightarrow> x=y"

lemma atmost_one_empty: "S={} \<Longrightarrow> atmost_one S"