Impressum Perm.thy

(*  Title:      ZF/Perm.thy
  Author: Lawrence C Paulson, Cambridge University Computer Laboratory
  Copyright 1991 University of Cambridge
 
 The theory underlying permutation groups
  -- Composition of relations, the identity relation
  -- Injections, surjections, bijections
  -- Lemmas for the Schroeder-Bernstein Theorem
*)

section‹Injections, Surjections, Bijections, Composition›

theory Perm imports func begin

definition
  (*composition of relations and functions; NOT Suppes's relative product*)
  comp     :: "[i,i]==>i"      (infixr ‹O› 60)  where
    "r O s ≡ {xz ∈ domain(s)*range(r) .
               ∃x y z. xz=⟨x,z⟩ ∧ ⟨x,y⟩:s ∧ ⟨y,z⟩:r}"

definition
  (*the identity function for A*)
  id    :: "i==>i"  where
    "id(A) ≡ (λx∈A. x)"

definition
  (*one-to-one functions from A to B*)
  inj   :: "[i,i]==>i"  where
    "inj(A,B) ≡ { f ∈ A->B. ∀w∈A. ∀x∈A. f`w=f`x ⟶ w=x}"

definition
  (*onto functions from A to B*)
  surj  :: "[i,i]==>i"  where
    "surj(A,B) ≡ { f ∈ A->B . ∀y∈B. ∃x∈A. f`x=y}"

definition
  (*one-to-one and onto functions*)
  bij   :: "[i,i]==>i"  where
    "bij(A,B) ≡ inj(A,B) ∩ surj(A,B)"


subsection‹Surjective Function Space›

lemma surj_is_fun: "f ∈ surj(A,B) ==> f ∈ A->B"
  unfolding surj_def
apply (erule CollectD1)
done

lemma fun_is_surj: "f ∈ Pi(A,B) ==> f ∈ surj(A,range(f))"
  unfolding surj_def
apply (blast intro: apply_equality range_of_fun domain_type)
done

lemma surj_range: "f ∈ surj(A,B) ==> range(f)=B"
  unfolding surj_def
apply (best intro: apply_Pair elim: range_type)
done

text‹A function with a right inverse is a surjection›

lemma f_imp_surjective:
    "[f ∈ A->B; ∧y. y ∈ B ==> d(y): A; ∧y. y ∈ B ==> f`d(y) = y]
     ==> f ∈ surj(A,B)"
  by (simp add: surj_def, blast)

lemma lam_surjective:
    "[∧x. x ∈ A ==> c(x): B;
        ∧y. y ∈ B ==> d(y): A;
        ∧y. y ∈ B ==> c(d(y)) = y
\ ==> (λx∈A. c(x)) ∈ surj(A,B)"
apply (rule_tac d = d in f_imp_surjective)
apply (simp_all add: lam_type)
done

text‹Cantor's theorem revisited›
lemma cantor_surj: "f ∉ surj(A,Pow(A))"
apply (unfold surj_def, safe)
apply (cut_tac cantor)
apply (best del: subsetI)
done


subsection‹Injective Function Space›

lemma inj_is_fun: "f ∈ inj(A,B) ==> f ∈ A->B"
  unfolding inj_def
apply (erule CollectD1)
done

text‹Good for dealing with sets of pairs, but a bit ugly in use [used in AC]›
lemma inj_equality:
    "[⟨a,b⟩:f; ⟨c,b⟩:f; f ∈ inj(A,B)] ==> a=c"
  unfolding inj_def
apply (blast dest: Pair_mem_PiD)
done

lemma inj_apply_equality: "[f ∈ inj(A,B); f`a=f`b; a ∈ A; b ∈ A] ==> a=b"
by (unfold inj_def, blast)

text‹A function with a left inverse is an injection›

lemma f_imp_injective: "[f ∈ A->B; ∀x∈A. d(f`x)=x] ==> f ∈ inj(A,B)"
apply (simp (no_asm_simp) add: inj_def)
apply (blast intro: subst_context [THEN box_equals])
done

lemma lam_injective:
    "[∧x. x ∈ A ==> c(x): B;
        ∧x. x ∈ A ==> d(c(x)) = x]
     ==> (λx∈A. c(x)) ∈ inj(A,B)"
apply (rule_tac d = d in f_imp_injective)
apply (simp_all add: lam_type)
done

subsection‹Bijections›

lemma bij_is_inj: "f ∈ bij(A,B) ==> f ∈ inj(A,B)"
  unfolding bij_def
apply (erule IntD1)
done

lemma bij_is_surj: "f ∈ bij(A,B) ==> f ∈ surj(A,B)"
  unfolding bij_def
apply (erule IntD2)
done

lemma bij_is_fun: "f ∈ bij(A,B) ==> f ∈ A->B"
  by (rule bij_is_inj [THEN inj_is_fun])

lemma lam_bijective:
    "[∧x. x ∈ A ==> c(x): B;
        ∧y. y ∈ B ==> d(y): A;
        ∧x. x ∈ A ==> d(c(x)) = x;
        ∧y. y ∈ B ==> c(d(y)) = y
\ ==> (λx∈A. c(x)) ∈ bij(A,B)"
  unfolding bij_def
apply (blast intro!: lam_injective lam_surjective)
done

lemma RepFun_bijective: "(∀y∈x. ∃!y'. f(y') = f(y))
      ==> (λz∈{f(y). y ∈ x}. THE y. f(y) = z) ∈ bij({f(y). y ∈ x}, x)"
apply (rule_tac d = f in lam_bijective)
apply (auto simp add: the_equality2)
done


subsection‹Identity Function›

lemma idI [intro!]: "a ∈ A ==> ⟨a,a⟩ ∈ id(A)"
  unfolding id_def
apply (erule lamI)
done

lemma idE [elim!]: "[p ∈ id(A); ∧x.[x ∈ A; p=⟨x,x⟩] ==> P] ==> P"
by (simp add: id_def lam_def, blast)

lemma id_type: "id(A) ∈ A->A"
  unfolding id_def
apply (rule lam_type, assumption)
done

lemma id_conv [simp]: "x ∈ A ==> id(A)`x = x"
  unfolding id_def
apply (simp (no_asm_simp))
done

lemma id_mono: "A<=B ==> id(A) ⊆ id(B)"
  unfolding id_def
apply (erule lam_mono)
done

lemma id_subset_inj: "A<=B ==> id(A): inj(A,B)"
apply (simp add: inj_def id_def)
apply (blast intro: lam_type)
done

lemmas id_inj = subset_refl [THEN id_subset_inj]

lemma id_surj: "id(A): surj(A,A)"
  unfolding id_def surj_def
apply (simp (no_asm_simp))
done

lemma id_bij: "id(A): bij(A,A)"
  unfolding bij_def
apply (blast intro: id_inj id_surj)
done

lemma subset_iff_id: "A ⊆ B ⟷ id(A) ∈ A->B"
  unfolding id_def
apply (force intro!: lam_type dest: apply_type)
done

text‹🍋‹id› as the identity relation›
lemma id_iff [simp]: "⟨x,y⟩ ∈ id(A) ⟷ x=y ∧ y ∈ A"
by auto


subsection‹Converse of a Function›

lemma inj_converse_fun: "f ∈ inj(A,B) ==> converse(f) ∈ range(f)->A"
  unfolding inj_def
apply (simp (no_asm_simp) add: Pi_iff function_def)
apply (erule CollectE)
apply (simp (no_asm_simp) add: apply_iff)
apply (blast dest: fun_is_rel)
done

text‹Equations for converse(f)›

text‹The premises are equivalent to saying that f is injective...›
lemma left_inverse_lemma:
     "[f ∈ A->B; converse(f): C->A; a ∈ A] ==> converse(f)`(f`a) = a"
by (blast intro: apply_Pair apply_equality converseI)

lemma left_inverse [simp]: "[f ∈ inj(A,B); a ∈ A] ==> converse(f)`(f`a) = a"
by (blast intro: left_inverse_lemma inj_converse_fun inj_is_fun)

lemma left_inverse_eq:
     "[f ∈ inj(A,B); f ` x = y; x ∈ A] ==> converse(f) ` y = x"
by auto

lemmas left_inverse_bij = bij_is_inj [THEN left_inverse]

lemma right_inverse_lemma:
     "[f ∈ A->B; converse(f): C->A; b ∈ C] ==> f`(converse(f)`b) = b"
by (rule apply_Pair [THEN converseD [THEN apply_equality]], auto)

(*Should the premises be f \<in> surj(A,B), b \<in> B for symmetry with left_inverse?
  No: they would not imply that converse(f) was a function! *)
lemma right_inverse [simp]:
     "[f ∈ inj(A,B); b ∈ range(f)] ==> f`(converse(f)`b) = b"
by (blast intro: right_inverse_lemma inj_converse_fun inj_is_fun)

lemma right_inverse_bij: "[f ∈ bij(A,B); b ∈ B] ==> f`(converse(f)`b) = b"
by (force simp add: bij_def surj_range)

subsection‹Converses of Injections, Surjections, Bijections›

lemma inj_converse_inj: "f ∈ inj(A,B) ==> converse(f): inj(range(f), A)"
apply (rule f_imp_injective)
apply (erule inj_converse_fun, clarify)
apply (rule right_inverse)
 apply assumption
apply blast
done

lemma inj_converse_surj: "f ∈ inj(A,B) ==> converse(f): surj(range(f), A)"
by (blast intro: f_imp_surjective inj_converse_fun left_inverse inj_is_fun
                 range_of_fun [THEN apply_type])

text‹Adding this as an intro! rule seems to cause looping›
lemma bij_converse_bij [TC]: "f ∈ bij(A,B) ==> converse(f): bij(B,A)"
  unfolding bij_def
apply (fast elim: surj_range [THEN subst] inj_converse_inj inj_converse_surj)
done



subsection‹Composition of Two Relations›

text‹The inductive definition package could derive these theorems for 🍋‹r O s›\›

lemma compI [intro]: "[⟨a,b⟩:s; ⟨b,c⟩:r] ==> ⟨a,c⟩ ∈ r O s"
by (unfold comp_def, blast)

lemma compE [elim!]:
    "[xz ∈ r O s;
        ∧x y z. [xz=⟨x,z⟩; ⟨x,y⟩:s; ⟨y,z⟩:r] ==> P]
     ==> P"
by (unfold comp_def, blast)

lemma compEpair:
    "[⟨a,c⟩ ∈ r O s;
        ∧y. [⟨a,y⟩:s; ⟨y,c⟩:r] ==> P]
     ==> P"
by (erule compE, simp)

lemma converse_comp: "converse(R O S) = converse(S) O converse(R)"
by blast


subsection‹Domain and Range -- see Suppes, Section 3.1›

text‹Boyer et al., Set Theory in First-Order Logic, JAR 2 (1986), 287-327›
lemma range_comp: "range(r O s) ⊆ range(r)"
by blast

lemma range_comp_eq: "domain(r) ⊆ range(s) ==> range(r O s) = range(r)"
by (rule range_comp [THEN equalityI], blast)

lemma domain_comp: "domain(r O s) ⊆ domain(s)"
by blast

lemma domain_comp_eq: "range(s) ⊆ domain(r) ==> domain(r O s) = domain(s)"
by (rule domain_comp [THEN equalityI], blast)

lemma image_comp: "(r O s)``A = r``(s``A)"
by blast

lemma inj_inj_range: "f ∈ inj(A,B) ==> f ∈ inj(A,range(f))"
  by (auto simp add: inj_def Pi_iff function_def)

lemma inj_bij_range: "f ∈ inj(A,B) ==> f ∈ bij(A,range(f))"
  by (auto simp add: bij_def intro: inj_inj_range inj_is_fun fun_is_surj)


subsection‹Other Results›

lemma comp_mono: "[r'<=r; s'<=s] ==> (r' O s') ⊆ (r O s)"
by blast

text‹composition preserves relations›
lemma comp_rel: "[s<=A*B; r<=B*C] ==> (r O s) ⊆ A*C"
by blast

text‹associative law for composition›
lemma comp_assoc: "(r O s) O t = r O (s O t)"
by blast

(*left identity of composition; provable inclusions are
  id(A) O r ⊆ r
  and   \<lbrakk>r<=A*B; B<=C\<rbrakk> \<Longrightarrow> r \<subseteq> id(C) O r *)
lemma left_comp_id: "r<=A*B ==> id(B) O r = r"
by blast

(*right identity of composition; provable inclusions are
  r O id(A) ⊆ r
  and   \<lbrakk>r<=A*B; A<=C\<rbrakk> \<Longrightarrow> r \<subseteq> r O id(C) *)
lemma right_comp_id: "r<=A*B ==> r O id(A) = r"
by blast


subsection‹Composition Preserves Functions, Injections, and Surjections›

lemma comp_function: "[function(g); function(f)] ==> function(f O g)"
by (unfold function_def, blast)

text‹Don't think the premises can be weakened much›
lemma comp_fun: "[g ∈ A->B; f ∈ B->C] ==> (f O g) ∈ A->C"
apply (auto simp add: Pi_def comp_function Pow_iff comp_rel)
apply (subst range_rel_subset [THEN domain_comp_eq], auto)
done

(*Thanks to the new definition of "apply", the premise f \<in> B->C is gone!*)
lemma comp_fun_apply [simp]:
     "[g ∈ A->B; a ∈ A] ==> (f O g)`a = f`(g`a)"
apply (frule apply_Pair, assumption)
apply (simp add: apply_def image_comp)
apply (blast dest: apply_equality)
done

text‹Simplifies compositions of lambda-abstractions›
lemma comp_lam:
    "[∧x. x ∈ A ==> b(x): B]
     ==> (λy∈B. c(y)) O (λx∈A. b(x)) = (λx∈A. c(b(x)))"
apply (subgoal_tac "(λx∈A. b(x)) ∈ A -> B")
 apply (rule fun_extension)
   apply (blast intro: comp_fun lam_funtype)
  apply (rule lam_funtype)
 apply simp
apply (simp add: lam_type)
done

lemma comp_inj:
     "[g ∈ inj(A,B); f ∈ inj(B,C)] ==> (f O g) ∈ inj(A,C)"
apply (frule inj_is_fun [of g])
apply (frule inj_is_fun [of f])
apply (rule_tac d = "λy. converse (g) ` (converse (f) ` y)" in f_imp_injective)
 apply (blast intro: comp_fun, simp)
done

lemma comp_surj:
    "[g ∈ surj(A,B); f ∈ surj(B,C)] ==> (f O g) ∈ surj(A,C)"
  unfolding surj_def
apply (blast intro!: comp_fun comp_fun_apply)
done

lemma comp_bij:
    "[g ∈ bij(A,B); f ∈ bij(B,C)] ==> (f O g) ∈ bij(A,C)"
  unfolding bij_def
apply (blast intro: comp_inj comp_surj)
done


subsection‹Dual Properties of 🍋‹inj› and 🍋‹surj›\<close>

text‹Useful for proofs from
  D Pastre. Automatic theorem proving in set theory.
  Artificial Intelligence, 10:1--27, 1978.›

lemma comp_mem_injD1:
    "[(f O g): inj(A,C); g ∈ A->B; f ∈ B->C] ==> g ∈ inj(A,B)"
by (unfold inj_def, force)

lemma comp_mem_injD2:
    "[(f O g): inj(A,C); g ∈ surj(A,B); f ∈ B->C] ==> f ∈ inj(B,C)"
apply (unfold inj_def surj_def, safe)
apply (rule_tac x1 = x in bspec [THEN bexE])
apply (erule_tac [3] x1 = w in bspec [THEN bexE], assumption+, safe)
apply (rule_tac t = "(`) (g) " in subst_context)
apply (erule asm_rl bspec [THEN bspec, THEN mp])+
apply (simp (no_asm_simp))
done

lemma comp_mem_surjD1:
    "[(f O g): surj(A,C); g ∈ A->B; f ∈ B->C] ==> f ∈ surj(B,C)"
  unfolding surj_def
apply (blast intro!: comp_fun_apply [symmetric] apply_funtype)
done


lemma comp_mem_surjD2:
    "[(f O g): surj(A,C); g ∈ A->B; f ∈ inj(B,C)] ==> g ∈ surj(A,B)"
apply (unfold inj_def surj_def, safe)
apply (drule_tac x = "f`y" in bspec, auto)
apply (blast intro: apply_funtype)
done

subsubsection‹Inverses of Composition›

text‹left inverse of composition; one inclusion is
  🍋‹f ∈ A->B ==> id(A) ⊆ converse(f) O f›\›
lemma left_comp_inverse: "f ∈ inj(A,B) ==> converse(f) O f = id(A)"
apply (unfold inj_def, clarify)
apply (rule equalityI)
 apply (auto simp add: apply_iff, blast)
done

text‹right inverse of composition; one inclusion is
  🍋‹f ∈ A->B ==> f O converse(f) ⊆ id(B)›\›
lemma right_comp_inverse:
    "f ∈ surj(A,B) ==> f O converse(f) = id(B)"
apply (simp add: surj_def, clarify)
apply (rule equalityI)
apply (best elim: domain_type range_type dest: apply_equality2)
apply (blast intro: apply_Pair)
done


subsubsection‹Proving that a Function is a Bijection›

lemma comp_eq_id_iff:
    "[f ∈ A->B; g ∈ B->A] ==> f O g = id(B) ⟷ (∀y∈B. f`(g`y)=y)"
apply (unfold id_def, safe)
 apply (drule_tac t = "λh. h`y " in subst_context)
 apply simp
apply (rule fun_extension)
  apply (blast intro: comp_fun lam_type)
 apply auto
done

lemma fg_imp_bijective:
    "[f ∈ A->B; g ∈ B->A; f O g = id(B); g O f = id(A)] ==> f ∈ bij(A,B)"
  unfolding bij_def
apply (simp add: comp_eq_id_iff)
apply (blast intro: f_imp_injective f_imp_surjective apply_funtype)
done

lemma nilpotent_imp_bijective: "[f ∈ A->A; f O f = id(A)] ==> f ∈ bij(A,A)"
by (blast intro: fg_imp_bijective)

lemma invertible_imp_bijective:
     "[converse(f): B->A; f ∈ A->B] ==> f ∈ bij(A,B)"
by (simp add: fg_imp_bijective comp_eq_id_iff
              left_inverse_lemma right_inverse_lemma)

subsubsection‹Unions of Functions›

text‹See similar theorems in func.thy›

text‹Theorem by KG, proof by LCP›
lemma inj_disjoint_Un:
     "[f ∈ inj(A,B); g ∈ inj(C,D); B ∩ D = 0]
      ==> (λa∈A ∪ C. if a ∈ A then f`a else g`a) ∈ inj(A ∪ C, B ∪ D)"
apply (rule_tac d = "λz. if z ∈ B then converse (f) `z else converse (g) `z"
       in lam_injective)
apply (auto simp add: inj_is_fun [THEN apply_type])
done

lemma surj_disjoint_Un:
    "[f ∈ surj(A,B); g ∈ surj(C,D); A ∩ C = 0]
     ==> (f ∪ g) ∈ surj(A ∪ C, B ∪ D)"
apply (simp add: surj_def fun_disjoint_Un)
apply (blast dest!: domain_of_fun
             intro!: fun_disjoint_apply1 fun_disjoint_apply2)
done

text‹A simple, high-level proof; the version for injections follows from it,
  using 🍋‹f ∈ inj(A,B) ⟷ f ∈ bij(A,range(f))›\›
lemma bij_disjoint_Un:
     "[f ∈ bij(A,B); g ∈ bij(C,D); A ∩ C = 0; B ∩ D = 0]
      ==> (f ∪ g) ∈ bij(A ∪ C, B ∪ D)"
apply (rule invertible_imp_bijective)
apply (subst converse_Un)
apply (auto intro: fun_disjoint_Un bij_is_fun bij_converse_bij)
done


subsubsection‹Restrictions as Surjections and Bijections›

lemma surj_image:
    "f ∈ Pi(A,B) ==> f ∈ surj(A, f``A)"
apply (simp add: surj_def)
apply (blast intro: apply_equality apply_Pair Pi_type)
done

lemma surj_image_eq: "f ∈ surj(A, B) ==> f``A = B"
  by (auto simp add: surj_def image_fun) (blast dest: apply_type) 

lemma restrict_image [simp]: "restrict(f,A) `` B = f `` (A ∩ B)"
by (auto simp add: restrict_def)

lemma restrict_inj:
    "[f ∈ inj(A,B); C<=A] ==> restrict(f,C): inj(C,B)"
  unfolding inj_def
apply (safe elim!: restrict_type2, auto)
done

lemma restrict_surj: "[f ∈ Pi(A,B); C<=A] ==> restrict(f,C): surj(C, f``C)"
apply (insert restrict_type2 [THEN surj_image])
apply (simp add: restrict_image)
done

lemma restrict_bij:
    "[f ∈ inj(A,B); C<=A] ==> restrict(f,C): bij(C, f``C)"
apply (simp add: inj_def bij_def)
apply (blast intro: restrict_surj surj_is_fun)
done


subsubsection‹Lemmas for Ramsey's Theorem›

lemma inj_weaken_type: "[f ∈ inj(A,B); B<=D] ==> f ∈ inj(A,D)"
  unfolding inj_def
apply (blast intro: fun_weaken_type)
done

lemma inj_succ_restrict:
     "[f ∈ inj(succ(m), A)] ==> restrict(f,m) ∈ inj(m, A-{f`m})"
apply (rule restrict_bij [THEN bij_is_inj, THEN inj_weaken_type], assumption, blast)
  unfolding inj_def
apply (fast elim: range_type mem_irrefl dest: apply_equality)
done


lemma inj_extend:
    "[f ∈ inj(A,B); a∉A; b∉B]
     ==> cons(⟨a,b⟩,f) ∈ inj(cons(a,A), cons(b,B))"
  unfolding inj_def
apply (force intro: apply_type  simp add: fun_extend)
done

end