src/ZF/Perm.thy
 author wenzelm Tue, 01 Dec 2020 20:47:19 +0100 changeset 72801 51683cd9d7fa parent 69593 3dda49e08b9d permissions -rw-r--r--
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```
(*  Title:      ZF/Perm.thy
Author:     Lawrence C Paulson, Cambridge University Computer Laboratory

The theory underlying permutation groups
-- Composition of relations, the identity relation
-- Injections, surjections, bijections
-- Lemmas for the Schroeder-Bernstein Theorem
*)

section\<open>Injections, Surjections, Bijections, Composition\<close>

theory Perm imports func begin

definition
(*composition of relations and functions; NOT Suppes's relative product*)
comp     :: "[i,i]=>i"      (infixr \<open>O\<close> 60)  where
"r O s == {xz \<in> domain(s)*range(r) .
\<exists>x y z. xz=<x,z> & <x,y>:s & <y,z>:r}"

definition
(*the identity function for A*)
id    :: "i=>i"  where
"id(A) == (\<lambda>x\<in>A. x)"

definition
(*one-to-one functions from A to B*)
inj   :: "[i,i]=>i"  where
"inj(A,B) == { f \<in> A->B. \<forall>w\<in>A. \<forall>x\<in>A. f`w=f`x \<longrightarrow> w=x}"

definition
(*onto functions from A to B*)
surj  :: "[i,i]=>i"  where
"surj(A,B) == { f \<in> A->B . \<forall>y\<in>B. \<exists>x\<in>A. f`x=y}"

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

subsection\<open>Surjective Function Space\<close>

lemma surj_is_fun: "f \<in> surj(A,B) ==> f \<in> A->B"
apply (unfold surj_def)
apply (erule CollectD1)
done

lemma fun_is_surj: "f \<in> Pi(A,B) ==> f \<in> surj(A,range(f))"
apply (unfold surj_def)
apply (blast intro: apply_equality range_of_fun domain_type)
done

lemma surj_range: "f \<in> surj(A,B) ==> range(f)=B"
apply (unfold surj_def)
apply (best intro: apply_Pair elim: range_type)
done

text\<open>A function with a right inverse is a surjection\<close>

lemma f_imp_surjective:
"[| f \<in> A->B;  !!y. y \<in> B ==> d(y): A;  !!y. y \<in> B ==> f`d(y) = y |]
==> f \<in> surj(A,B)"

lemma lam_surjective:
"[| !!x. x \<in> A ==> c(x): B;
!!y. y \<in> B ==> d(y): A;
!!y. y \<in> B ==> c(d(y)) = y
|] ==> (\<lambda>x\<in>A. c(x)) \<in> surj(A,B)"
apply (rule_tac d = d in f_imp_surjective)
done

text\<open>Cantor's theorem revisited\<close>
lemma cantor_surj: "f \<notin> surj(A,Pow(A))"
apply (unfold surj_def, safe)
apply (cut_tac cantor)
apply (best del: subsetI)
done

subsection\<open>Injective Function Space\<close>

lemma inj_is_fun: "f \<in> inj(A,B) ==> f \<in> A->B"
apply (unfold inj_def)
apply (erule CollectD1)
done

text\<open>Good for dealing with sets of pairs, but a bit ugly in use [used in AC]\<close>
lemma inj_equality:
"[| <a,b>:f;  <c,b>:f;  f \<in> inj(A,B) |] ==> a=c"
apply (unfold inj_def)
apply (blast dest: Pair_mem_PiD)
done

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

text\<open>A function with a left inverse is an injection\<close>

lemma f_imp_injective: "[| f \<in> A->B;  \<forall>x\<in>A. d(f`x)=x |] ==> f \<in> inj(A,B)"
apply (blast intro: subst_context [THEN box_equals])
done

lemma lam_injective:
"[| !!x. x \<in> A ==> c(x): B;
!!x. x \<in> A ==> d(c(x)) = x |]
==> (\<lambda>x\<in>A. c(x)) \<in> inj(A,B)"
apply (rule_tac d = d in f_imp_injective)
done

subsection\<open>Bijections\<close>

lemma bij_is_inj: "f \<in> bij(A,B) ==> f \<in> inj(A,B)"
apply (unfold bij_def)
apply (erule IntD1)
done

lemma bij_is_surj: "f \<in> bij(A,B) ==> f \<in> surj(A,B)"
apply (unfold bij_def)
apply (erule IntD2)
done

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

lemma lam_bijective:
"[| !!x. x \<in> A ==> c(x): B;
!!y. y \<in> B ==> d(y): A;
!!x. x \<in> A ==> d(c(x)) = x;
!!y. y \<in> B ==> c(d(y)) = y
|] ==> (\<lambda>x\<in>A. c(x)) \<in> bij(A,B)"
apply (unfold bij_def)
apply (blast intro!: lam_injective lam_surjective)
done

lemma RepFun_bijective: "(\<forall>y\<in>x. \<exists>!y'. f(y') = f(y))
==> (\<lambda>z\<in>{f(y). y \<in> x}. THE y. f(y) = z) \<in> bij({f(y). y \<in> x}, x)"
apply (rule_tac d = f in lam_bijective)
done

subsection\<open>Identity Function\<close>

lemma idI [intro!]: "a \<in> A ==> <a,a> \<in> id(A)"
apply (unfold id_def)
apply (erule lamI)
done

lemma idE [elim!]: "[| p \<in> id(A);  !!x.[| x \<in> A; p=<x,x> |] ==> P |] ==>  P"
by (simp add: id_def lam_def, blast)

lemma id_type: "id(A) \<in> A->A"
apply (unfold id_def)
apply (rule lam_type, assumption)
done

lemma id_conv [simp]: "x \<in> A ==> id(A)`x = x"
apply (unfold id_def)
apply (simp (no_asm_simp))
done

lemma id_mono: "A<=B ==> id(A) \<subseteq> id(B)"
apply (unfold id_def)
apply (erule lam_mono)
done

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

lemmas id_inj = subset_refl [THEN id_subset_inj]

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

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

lemma subset_iff_id: "A \<subseteq> B \<longleftrightarrow> id(A) \<in> A->B"
apply (unfold id_def)
apply (force intro!: lam_type dest: apply_type)
done

text\<open>\<^term>\<open>id\<close> as the identity relation\<close>
lemma id_iff [simp]: "<x,y> \<in> id(A) \<longleftrightarrow> x=y & y \<in> A"
by auto

subsection\<open>Converse of a Function\<close>

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

text\<open>Equations for converse(f)\<close>

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

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

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

lemmas left_inverse_bij = bij_is_inj [THEN left_inverse]

lemma right_inverse_lemma:
"[| f \<in> A->B;  converse(f): C->A;  b \<in> 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 \<in> inj(A,B);  b \<in> range(f) |] ==> f`(converse(f)`b) = b"
by (blast intro: right_inverse_lemma inj_converse_fun inj_is_fun)

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

subsection\<open>Converses of Injections, Surjections, Bijections\<close>

lemma inj_converse_inj: "f \<in> 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 \<in> 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\<open>Adding this as an intro! rule seems to cause looping\<close>
lemma bij_converse_bij [TC]: "f \<in> bij(A,B) ==> converse(f): bij(B,A)"
apply (unfold bij_def)
apply (fast elim: surj_range [THEN subst] inj_converse_inj inj_converse_surj)
done

subsection\<open>Composition of Two Relations\<close>

text\<open>The inductive definition package could derive these theorems for \<^term>\<open>r O s\<close>\<close>

lemma compI [intro]: "[| <a,b>:s; <b,c>:r |] ==> <a,c> \<in> r O s"
by (unfold comp_def, blast)

lemma compE [elim!]:
"[| xz \<in> 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> \<in> 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\<open>Domain and Range -- see Suppes, Section 3.1\<close>

text\<open>Boyer et al., Set Theory in First-Order Logic, JAR 2 (1986), 287-327\<close>
lemma range_comp: "range(r O s) \<subseteq> range(r)"
by blast

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

lemma domain_comp: "domain(r O s) \<subseteq> domain(s)"
by blast

lemma domain_comp_eq: "range(s) \<subseteq> 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 \<in> inj(A,B) ==> f \<in> inj(A,range(f))"
by (auto simp add: inj_def Pi_iff function_def)

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

subsection\<open>Other Results\<close>

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

text\<open>composition preserves relations\<close>
lemma comp_rel: "[| s<=A*B;  r<=B*C |] ==> (r O s) \<subseteq> A*C"
by blast

text\<open>associative law for composition\<close>
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 \<subseteq> r
and   [| r<=A*B; B<=C |] ==> 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) \<subseteq> r
and   [| r<=A*B; A<=C |] ==> r \<subseteq> r O id(C) *)
lemma right_comp_id: "r<=A*B ==> r O id(A) = r"
by blast

subsection\<open>Composition Preserves Functions, Injections, and Surjections\<close>

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

text\<open>Don't think the premises can be weakened much\<close>
lemma comp_fun: "[| g \<in> A->B;  f \<in> B->C |] ==> (f O g) \<in> 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 \<in> A->B;  a \<in> A |] ==> (f O g)`a = f`(g`a)"
apply (frule apply_Pair, assumption)
apply (blast dest: apply_equality)
done

text\<open>Simplifies compositions of lambda-abstractions\<close>
lemma comp_lam:
"[| !!x. x \<in> A ==> b(x): B |]
==> (\<lambda>y\<in>B. c(y)) O (\<lambda>x\<in>A. b(x)) = (\<lambda>x\<in>A. c(b(x)))"
apply (subgoal_tac "(\<lambda>x\<in>A. b(x)) \<in> A -> B")
apply (rule fun_extension)
apply (blast intro: comp_fun lam_funtype)
apply (rule lam_funtype)
apply simp
done

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

lemma comp_surj:
"[| g \<in> surj(A,B);  f \<in> surj(B,C) |] ==> (f O g) \<in> surj(A,C)"
apply (unfold surj_def)
apply (blast intro!: comp_fun comp_fun_apply)
done

lemma comp_bij:
"[| g \<in> bij(A,B);  f \<in> bij(B,C) |] ==> (f O g) \<in> bij(A,C)"
apply (unfold bij_def)
apply (blast intro: comp_inj comp_surj)
done

subsection\<open>Dual Properties of \<^term>\<open>inj\<close> and \<^term>\<open>surj\<close>\<close>

text\<open>Useful for proofs from
D Pastre.  Automatic theorem proving in set theory.
Artificial Intelligence, 10:1--27, 1978.\<close>

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

lemma comp_mem_injD2:
"[| (f O g): inj(A,C);  g \<in> surj(A,B);  f \<in> B->C |] ==> f \<in> 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 \<in> A->B;  f \<in> B->C |] ==> f \<in> surj(B,C)"
apply (unfold surj_def)
apply (blast intro!: comp_fun_apply [symmetric] apply_funtype)
done

lemma comp_mem_surjD2:
"[| (f O g): surj(A,C);  g \<in> A->B;  f \<in> inj(B,C) |] ==> g \<in> 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\<open>Inverses of Composition\<close>

text\<open>left inverse of composition; one inclusion is
\<^term>\<open>f \<in> A->B ==> id(A) \<subseteq> converse(f) O f\<close>\<close>
lemma left_comp_inverse: "f \<in> 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\<open>right inverse of composition; one inclusion is
\<^term>\<open>f \<in> A->B ==> f O converse(f) \<subseteq> id(B)\<close>\<close>
lemma right_comp_inverse:
"f \<in> surj(A,B) ==> f O converse(f) = id(B)"
apply (rule equalityI)
apply (best elim: domain_type range_type dest: apply_equality2)
apply (blast intro: apply_Pair)
done

subsubsection\<open>Proving that a Function is a Bijection\<close>

lemma comp_eq_id_iff:
"[| f \<in> A->B;  g \<in> B->A |] ==> f O g = id(B) \<longleftrightarrow> (\<forall>y\<in>B. f`(g`y)=y)"
apply (unfold id_def, safe)
apply (drule_tac t = "%h. 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 \<in> A->B;  g \<in> B->A;  f O g = id(B);  g O f = id(A) |] ==> f \<in> bij(A,B)"
apply (unfold bij_def)
apply (blast intro: f_imp_injective f_imp_surjective apply_funtype)
done

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

lemma invertible_imp_bijective:
"[| converse(f): B->A;  f \<in> A->B |] ==> f \<in> bij(A,B)"
left_inverse_lemma right_inverse_lemma)

subsubsection\<open>Unions of Functions\<close>

text\<open>See similar theorems in func.thy\<close>

text\<open>Theorem by KG, proof by LCP\<close>
lemma inj_disjoint_Un:
"[| f \<in> inj(A,B);  g \<in> inj(C,D);  B \<inter> D = 0 |]
==> (\<lambda>a\<in>A \<union> C. if a \<in> A then f`a else g`a) \<in> inj(A \<union> C, B \<union> D)"
apply (rule_tac d = "%z. if z \<in> 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 \<in> surj(A,B);  g \<in> surj(C,D);  A \<inter> C = 0 |]
==> (f \<union> g) \<in> surj(A \<union> C, B \<union> D)"
apply (blast dest!: domain_of_fun
intro!: fun_disjoint_apply1 fun_disjoint_apply2)
done

text\<open>A simple, high-level proof; the version for injections follows from it,
using  \<^term>\<open>f \<in> inj(A,B) \<longleftrightarrow> f \<in> bij(A,range(f))\<close>\<close>
lemma bij_disjoint_Un:
"[| f \<in> bij(A,B);  g \<in> bij(C,D);  A \<inter> C = 0;  B \<inter> D = 0 |]
==> (f \<union> g) \<in> bij(A \<union> C, B \<union> D)"
apply (rule invertible_imp_bijective)
apply (subst converse_Un)
apply (auto intro: fun_disjoint_Un bij_is_fun bij_converse_bij)
done

subsubsection\<open>Restrictions as Surjections and Bijections\<close>

lemma surj_image:
"f \<in> Pi(A,B) ==> f \<in> surj(A, f``A)"
apply (blast intro: apply_equality apply_Pair Pi_type)
done

lemma surj_image_eq: "f \<in> 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 \<inter> B)"

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

lemma restrict_surj: "[| f \<in> Pi(A,B);  C<=A |] ==> restrict(f,C): surj(C, f``C)"
apply (insert restrict_type2 [THEN surj_image])
done

lemma restrict_bij:
"[| f \<in> inj(A,B);  C<=A |] ==> restrict(f,C): bij(C, f``C)"
apply (blast intro: restrict_surj surj_is_fun)
done

subsubsection\<open>Lemmas for Ramsey's Theorem\<close>

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

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

lemma inj_extend:
"[| f \<in> inj(A,B);  a\<notin>A;  b\<notin>B |]
==> cons(<a,b>,f) \<in> inj(cons(a,A), cons(b,B))"
apply (unfold inj_def)
apply (force intro: apply_type  simp add: fun_extend)
done

end
```