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+++ b/src/HOL/Cardinals/Fun_More_FP.thy Mon Nov 18 18:04:45 2013 +0100
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+(* Title: HOL/Cardinals/Fun_More_FP.thy
+ Author: Andrei Popescu, TU Muenchen
+ Copyright 2012
+
+More on injections, bijections and inverses (FP).
+*)
+
+header {* More on Injections, Bijections and Inverses (FP) *}
+
+theory Fun_More_FP
+imports "~~/src/HOL/Library/Infinite_Set"
+begin
+
+
+text {* This section proves more facts (additional to those in @{text "Fun.thy"},
+@{text "Hilbert_Choice.thy"}, @{text "Finite_Set.thy"} and @{text "Infinite_Set.thy"}),
+mainly concerning injections, bijections, inverses and (numeric) cardinals of
+finite sets. *}
+
+
+subsection {* Purely functional properties *}
+
+
+(*2*)lemma bij_betw_id_iff:
+"(A = B) = (bij_betw id A B)"
+by(simp add: bij_betw_def)
+
+
+(*2*)lemma bij_betw_byWitness:
+assumes LEFT: "\<forall>a \<in> A. f'(f a) = a" and
+ RIGHT: "\<forall>a' \<in> A'. f(f' a') = a'" and
+ IM1: "f ` A \<le> A'" and IM2: "f' ` A' \<le> A"
+shows "bij_betw f A A'"
+using assms
+proof(unfold bij_betw_def inj_on_def, safe)
+ fix a b assume *: "a \<in> A" "b \<in> A" and **: "f a = f b"
+ have "a = f'(f a) \<and> b = f'(f b)" using * LEFT by simp
+ with ** show "a = b" by simp
+next
+ fix a' assume *: "a' \<in> A'"
+ hence "f' a' \<in> A" using IM2 by blast
+ moreover
+ have "a' = f(f' a')" using * RIGHT by simp
+ ultimately show "a' \<in> f ` A" by blast
+qed
+
+
+(*3*)corollary notIn_Un_bij_betw:
+assumes NIN: "b \<notin> A" and NIN': "f b \<notin> A'" and
+ BIJ: "bij_betw f A A'"
+shows "bij_betw f (A \<union> {b}) (A' \<union> {f b})"
+proof-
+ have "bij_betw f {b} {f b}"
+ unfolding bij_betw_def inj_on_def by simp
+ with assms show ?thesis
+ using bij_betw_combine[of f A A' "{b}" "{f b}"] by blast
+qed
+
+
+(*1*)lemma notIn_Un_bij_betw3:
+assumes NIN: "b \<notin> A" and NIN': "f b \<notin> A'"
+shows "bij_betw f A A' = bij_betw f (A \<union> {b}) (A' \<union> {f b})"
+proof
+ assume "bij_betw f A A'"
+ thus "bij_betw f (A \<union> {b}) (A' \<union> {f b})"
+ using assms notIn_Un_bij_betw[of b A f A'] by blast
+next
+ assume *: "bij_betw f (A \<union> {b}) (A' \<union> {f b})"
+ have "f ` A = A'"
+ proof(auto)
+ fix a assume **: "a \<in> A"
+ hence "f a \<in> A' \<union> {f b}" using * unfolding bij_betw_def by blast
+ moreover
+ {assume "f a = f b"
+ hence "a = b" using * ** unfolding bij_betw_def inj_on_def by blast
+ with NIN ** have False by blast
+ }
+ ultimately show "f a \<in> A'" by blast
+ next
+ fix a' assume **: "a' \<in> A'"
+ hence "a' \<in> f`(A \<union> {b})"
+ using * by (auto simp add: bij_betw_def)
+ then obtain a where 1: "a \<in> A \<union> {b} \<and> f a = a'" by blast
+ moreover
+ {assume "a = b" with 1 ** NIN' have False by blast
+ }
+ ultimately have "a \<in> A" by blast
+ with 1 show "a' \<in> f ` A" by blast
+ qed
+ thus "bij_betw f A A'" using * bij_betw_subset[of f "A \<union> {b}" _ A] by blast
+qed
+
+
+subsection {* Properties involving finite and infinite sets *}
+
+
+(*3*)lemma inj_on_finite:
+assumes "inj_on f A" "f ` A \<le> B" "finite B"
+shows "finite A"
+using assms infinite_super by (fast dest: finite_imageD)
+
+
+(*3*)lemma infinite_imp_bij_betw:
+assumes INF: "infinite A"
+shows "\<exists>h. bij_betw h A (A - {a})"
+proof(cases "a \<in> A")
+ assume Case1: "a \<notin> A" hence "A - {a} = A" by blast
+ thus ?thesis using bij_betw_id[of A] by auto
+next
+ assume Case2: "a \<in> A"
+ have "infinite (A - {a})" using INF infinite_remove by auto
+ with infinite_iff_countable_subset[of "A - {a}"] obtain f::"nat \<Rightarrow> 'a"
+ where 1: "inj f" and 2: "f ` UNIV \<le> A - {a}" by blast
+ obtain g where g_def: "g = (\<lambda> n. if n = 0 then a else f (Suc n))" by blast
+ obtain A' where A'_def: "A' = g ` UNIV" by blast
+ have temp: "\<forall>y. f y \<noteq> a" using 2 by blast
+ have 3: "inj_on g UNIV \<and> g ` UNIV \<le> A \<and> a \<in> g ` UNIV"
+ proof(auto simp add: Case2 g_def, unfold inj_on_def, intro ballI impI,
+ case_tac "x = 0", auto simp add: 2)
+ fix y assume "a = (if y = 0 then a else f (Suc y))"
+ thus "y = 0" using temp by (case_tac "y = 0", auto)
+ next
+ fix x y
+ assume "f (Suc x) = (if y = 0 then a else f (Suc y))"
+ thus "x = y" using 1 temp unfolding inj_on_def by (case_tac "y = 0", auto)
+ next
+ fix n show "f (Suc n) \<in> A" using 2 by blast
+ qed
+ hence 4: "bij_betw g UNIV A' \<and> a \<in> A' \<and> A' \<le> A"
+ using inj_on_imp_bij_betw[of g] unfolding A'_def by auto
+ hence 5: "bij_betw (inv g) A' UNIV"
+ by (auto simp add: bij_betw_inv_into)
+ (* *)
+ obtain n where "g n = a" using 3 by auto
+ hence 6: "bij_betw g (UNIV - {n}) (A' - {a})"
+ using 3 4 unfolding A'_def
+ by clarify (rule bij_betw_subset, auto simp: image_set_diff)
+ (* *)
+ obtain v where v_def: "v = (\<lambda> m. if m < n then m else Suc m)" by blast
+ have 7: "bij_betw v UNIV (UNIV - {n})"
+ proof(unfold bij_betw_def inj_on_def, intro conjI, clarify)
+ fix m1 m2 assume "v m1 = v m2"
+ thus "m1 = m2"
+ by(case_tac "m1 < n", case_tac "m2 < n",
+ auto simp add: inj_on_def v_def, case_tac "m2 < n", auto)
+ next
+ show "v ` UNIV = UNIV - {n}"
+ proof(auto simp add: v_def)
+ fix m assume *: "m \<noteq> n" and **: "m \<notin> Suc ` {m'. \<not> m' < n}"
+ {assume "n \<le> m" with * have 71: "Suc n \<le> m" by auto
+ then obtain m' where 72: "m = Suc m'" using Suc_le_D by auto
+ with 71 have "n \<le> m'" by auto
+ with 72 ** have False by auto
+ }
+ thus "m < n" by force
+ qed
+ qed
+ (* *)
+ obtain h' where h'_def: "h' = g o v o (inv g)" by blast
+ hence 8: "bij_betw h' A' (A' - {a})" using 5 7 6
+ by (auto simp add: bij_betw_trans)
+ (* *)
+ obtain h where h_def: "h = (\<lambda> b. if b \<in> A' then h' b else b)" by blast
+ have "\<forall>b \<in> A'. h b = h' b" unfolding h_def by auto
+ hence "bij_betw h A' (A' - {a})" using 8 bij_betw_cong[of A' h] by auto
+ moreover
+ {have "\<forall>b \<in> A - A'. h b = b" unfolding h_def by auto
+ hence "bij_betw h (A - A') (A - A')"
+ using bij_betw_cong[of "A - A'" h id] bij_betw_id[of "A - A'"] by auto
+ }
+ moreover
+ have "(A' Int (A - A') = {} \<and> A' \<union> (A - A') = A) \<and>
+ ((A' - {a}) Int (A - A') = {} \<and> (A' - {a}) \<union> (A - A') = A - {a})"
+ using 4 by blast
+ ultimately have "bij_betw h A (A - {a})"
+ using bij_betw_combine[of h A' "A' - {a}" "A - A'" "A - A'"] by simp
+ thus ?thesis by blast
+qed
+
+
+(*3*)lemma infinite_imp_bij_betw2:
+assumes INF: "infinite A"
+shows "\<exists>h. bij_betw h A (A \<union> {a})"
+proof(cases "a \<in> A")
+ assume Case1: "a \<in> A" hence "A \<union> {a} = A" by blast
+ thus ?thesis using bij_betw_id[of A] by auto
+next
+ let ?A' = "A \<union> {a}"
+ assume Case2: "a \<notin> A" hence "A = ?A' - {a}" by blast
+ moreover have "infinite ?A'" using INF by auto
+ ultimately obtain f where "bij_betw f ?A' A"
+ using infinite_imp_bij_betw[of ?A' a] by auto
+ hence "bij_betw(inv_into ?A' f) A ?A'" using bij_betw_inv_into by blast
+ thus ?thesis by auto
+qed
+
+
+subsection {* Properties involving Hilbert choice *}
+
+
+(*2*)lemma bij_betw_inv_into_left:
+assumes BIJ: "bij_betw f A A'" and IN: "a \<in> A"
+shows "(inv_into A f) (f a) = a"
+using assms unfolding bij_betw_def
+by clarify (rule inv_into_f_f)
+
+(*2*)lemma bij_betw_inv_into_right:
+assumes "bij_betw f A A'" "a' \<in> A'"
+shows "f(inv_into A f a') = a'"
+using assms unfolding bij_betw_def using f_inv_into_f by force
+
+
+(*1*)lemma bij_betw_inv_into_subset:
+assumes BIJ: "bij_betw f A A'" and
+ SUB: "B \<le> A" and IM: "f ` B = B'"
+shows "bij_betw (inv_into A f) B' B"
+using assms unfolding bij_betw_def
+by (auto intro: inj_on_inv_into)
+
+
+subsection {* Other facts *}
+
+
+(*2*)lemma atLeastLessThan_less_eq:
+"({0..<m} \<le> {0..<n}) = ((m::nat) \<le> n)"
+unfolding ivl_subset by arith
+
+
+(*2*)lemma atLeastLessThan_less_eq2:
+assumes "inj_on f {0..<(m::nat)} \<and> f ` {0..<m} \<le> {0..<n}"
+shows "m \<le> n"
+using assms
+using finite_atLeastLessThan[of m] finite_atLeastLessThan[of n]
+ card_atLeastLessThan[of m] card_atLeastLessThan[of n]
+ card_inj_on_le[of f "{0 ..< m}" "{0 ..< n}"] by auto
+
+
+
+end