Theory Fun_More

(*  Title:      HOL/Cardinals/Fun_More.thy
    Author:     Andrei Popescu, TU Muenchen
    Copyright   2012

More on injections, bijections and inverses.
*)

section ‹More on Injections, Bijections and Inverses›

theory Fun_More
imports Main
begin

subsection ‹Purely functional properties›

(* unused *)
(*1*)lemma notIn_Un_bij_betw2:
assumes NIN: "b ∉ A" and NIN': "b' ∉ A'" and
        BIJ: "bij_betw f A A'"
shows "bij_betw f (A ∪ {b}) (A' ∪ {b'}) = (f b = b')"
proof
  assume "f b = b'"
  thus "bij_betw f (A ∪ {b}) (A' ∪ {b'})"
  using assms notIn_Un_bij_betw[of b A f A'] by auto
next
  assume *: "bij_betw f (A ∪ {b}) (A' ∪ {b'})"
  hence "f b ∈ A' ∪ {b'}"
  unfolding bij_betw_def by auto
  moreover
  {assume "f b ∈ A'"
   then obtain b1 where 1: "b1 ∈ A" and 2: "f b1 = f b" using BIJ
   by (auto simp add: bij_betw_def)
   hence "b = b1" using *
   by (auto simp add: bij_betw_def inj_on_def)
   with 1 NIN have False by auto
  }
  ultimately show "f b = b'" by blast
qed

(* unused *)
(*1*)lemma bij_betw_ball:
assumes BIJ: "bij_betw f A B"
shows "(∀b ∈ B. phi b) = (∀a ∈ A. phi(f a))"
using assms unfolding bij_betw_def inj_on_def by blast

(* unused *)
(*1*)lemma bij_betw_diff_singl:
assumes BIJ: "bij_betw f A A'" and IN: "a ∈ A"
shows "bij_betw f (A - {a}) (A' - {f a})"
proof-
  let ?B = "A - {a}"   let ?B' = "A' - {f a}"
  have "f a ∈ A'" using IN BIJ unfolding bij_betw_def by blast
  hence "a ∉ ?B ∧ f a ∉ ?B' ∧ A = ?B ∪ {a} ∧ A' = ?B' ∪ {f a}"
  using IN by blast
  thus ?thesis using notIn_Un_bij_betw3[of a ?B f ?B'] BIJ by simp
qed


subsection ‹Properties involving finite and infinite sets›

(* unused *)
(*1*)lemma bij_betw_inv_into_RIGHT:
assumes BIJ: "bij_betw f A A'" and SUB: "B' ≤ A'"
shows "f `((inv_into A f)`B') = B'"
using assms
proof(auto simp add: bij_betw_inv_into_right)
  let ?f' = "(inv_into A f)"
  fix a' assume *: "a' ∈ B'"
  hence "a' ∈ A'" using SUB by auto
  hence "a' = f (?f' a')"
  using BIJ by (auto simp add: bij_betw_inv_into_right)
  thus "a' ∈ f ` (?f' ` B')" using * by blast
qed

(* unused *)
(*1*)lemma bij_betw_inv_into_RIGHT_LEFT:
assumes BIJ: "bij_betw f A A'" and SUB: "B' ≤ A'" and
        IM: "(inv_into A f) ` B' = B"
shows "f ` B = B'"
proof-
  have "f`((inv_into A f)` B') = B'"
  using assms bij_betw_inv_into_RIGHT[of f A A' B'] by auto
  thus ?thesis using IM by auto
qed

(* unused *)
(*2*)lemma bij_betw_inv_into_twice:
assumes "bij_betw f A A'"
shows "∀a ∈ A. inv_into A' (inv_into A f) a = f a"
proof
  let ?f' = "inv_into A f"   let ?f'' = "inv_into A' ?f'"
  have 1: "bij_betw ?f' A' A" using assms
  by (auto simp add: bij_betw_inv_into)
  fix a assume *: "a ∈ A"
  then obtain a' where 2: "a' ∈ A'" and 3: "?f' a' = a"
  using 1 unfolding bij_betw_def by force
  hence "?f'' a = a'"
  using * 1 3 by (auto simp add: bij_betw_inv_into_left)
  moreover have "f a = a'" using assms 2 3
  by (auto simp add: bij_betw_inv_into_right)
  ultimately show "?f'' a = f a" by simp
qed


subsection ‹Properties involving Hilbert choice›

(*1*)lemma bij_betw_inv_into_LEFT:
assumes BIJ: "bij_betw f A A'" and SUB: "B ≤ A"
shows "(inv_into A f)`(f ` B) = B"
using assms unfolding bij_betw_def using inv_into_image_cancel by force

(*1*)lemma bij_betw_inv_into_LEFT_RIGHT:
assumes BIJ: "bij_betw f A A'" and SUB: "B ≤ A" and
        IM: "f ` B = B'"
shows "(inv_into A f) ` B' = B"
using assms bij_betw_inv_into_LEFT[of f A A' B] by fast


subsection ‹Other facts›

(*3*)lemma atLeastLessThan_injective:
assumes "{0 ..< m::nat} = {0 ..< n}"
shows "m = n"
proof-
  {assume "m < n"
   hence "m ∈ {0 ..< n}" by auto
   hence "{0 ..< m} < {0 ..< n}" by auto
   hence False using assms by blast
  }
  moreover
  {assume "n < m"
   hence "n ∈ {0 ..< m}" by auto
   hence "{0 ..< n} < {0 ..< m}" by auto
   hence False using assms by blast
  }
  ultimately show ?thesis by force
qed

(*2*)lemma atLeastLessThan_injective2:
"bij_betw f {0 ..< m::nat} {0 ..< n} ⟹ m = n"
using finite_atLeastLessThan[of m] finite_atLeastLessThan[of n]
      card_atLeastLessThan[of m] card_atLeastLessThan[of n]
      bij_betw_iff_card[of "{0 ..< m}" "{0 ..< n}"] by auto

(*2*)lemma atLeastLessThan_less_eq:
"({0..<m} ≤ {0..<n}) = ((m::nat) ≤ n)"
unfolding ivl_subset by arith

(*2*)lemma atLeastLessThan_less_eq2:
assumes "inj_on f {0..<(m::nat)} ∧ f ` {0..<m} ≤ {0..<n}"
shows "m ≤ 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 fastforce

(* unused *)
(*2*)lemma atLeastLessThan_less_eq3:
"(∃f. inj_on f {0..<(m::nat)} ∧ f ` {0..<m} ≤ {0..<n}) = (m ≤ n)"
using atLeastLessThan_less_eq2
proof(auto)
  assume "m ≤ n"
  hence "inj_on id {0..<m} ∧ id ` {0..<m} ⊆ {0..<n}" unfolding inj_on_def by force
  thus "∃f. inj_on f {0..<m} ∧ f ` {0..<m} ⊆ {0..<n}" by blast
qed

(* unused *)
(*3*)lemma atLeastLessThan_less:
"({0..<m} < {0..<n}) = ((m::nat) < n)"
proof-
  have "({0..<m} < {0..<n}) = ({0..<m} ≤ {0..<n} ∧ {0..<m} ~= {0..<n})"
  using subset_iff_psubset_eq by blast
  also have "… = (m ≤ n ∧ m ~= n)"
  using atLeastLessThan_less_eq atLeastLessThan_injective by blast
  also have "… = (m < n)" by auto
  finally show ?thesis .
qed

end