src/HOL/Algebra/Bij.thy
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(*  Title:      HOL/Algebra/Bij.thy
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    ID:         $Id$
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    Author:     Florian Kammueller, with new proofs by L C Paulson
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*)
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theory Bij imports Group begin
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section {* Bijections of a Set, Permutation and Automorphism Groups *}
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constdefs
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  Bij :: "'a set \<Rightarrow> ('a \<Rightarrow> 'a) set"
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    --{*Only extensional functions, since otherwise we get too many.*}
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  "Bij S \<equiv> extensional S \<inter> {f. bij_betw f S S}"
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  BijGroup :: "'a set \<Rightarrow> ('a \<Rightarrow> 'a) monoid"
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  "BijGroup S \<equiv>
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    \<lparr>carrier = Bij S,
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     mult = \<lambda>g \<in> Bij S. \<lambda>f \<in> Bij S. compose S g f,
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     one = \<lambda>x \<in> S. x\<rparr>"
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declare Id_compose [simp] compose_Id [simp]
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lemma Bij_imp_extensional: "f \<in> Bij S \<Longrightarrow> f \<in> extensional S"
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  by (simp add: Bij_def)
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lemma Bij_imp_funcset: "f \<in> Bij S \<Longrightarrow> f \<in> S \<rightarrow> S"
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  by (auto simp add: Bij_def bij_betw_imp_funcset)
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subsection {*Bijections Form a Group *}
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lemma restrict_inv_into_Bij: "f \<in> Bij S \<Longrightarrow> (\<lambda>x \<in> S. (inv_into S f) x) \<in> Bij S"
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  by (simp add: Bij_def bij_betw_inv_into)
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lemma id_Bij: "(\<lambda>x\<in>S. x) \<in> Bij S "
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  by (auto simp add: Bij_def bij_betw_def inj_on_def)
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lemma compose_Bij: "\<lbrakk>x \<in> Bij S; y \<in> Bij S\<rbrakk> \<Longrightarrow> compose S x y \<in> Bij S"
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  by (auto simp add: Bij_def bij_betw_compose) 
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lemma Bij_compose_restrict_eq:
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     "f \<in> Bij S \<Longrightarrow> compose S (restrict (inv_into S f) S) f = (\<lambda>x\<in>S. x)"
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  by (simp add: Bij_def compose_inv_into_id)
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theorem group_BijGroup: "group (BijGroup S)"
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apply (simp add: BijGroup_def)
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apply (rule groupI)
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    apply (simp add: compose_Bij)
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   apply (simp add: id_Bij)
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  apply (simp add: compose_Bij)
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  apply (blast intro: compose_assoc [symmetric] dest: Bij_imp_funcset)
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 apply (simp add: id_Bij Bij_imp_funcset Bij_imp_extensional, simp)
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apply (blast intro: Bij_compose_restrict_eq restrict_inv_into_Bij)
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done
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subsection{*Automorphisms Form a Group*}
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lemma Bij_inv_into_mem: "\<lbrakk> f \<in> Bij S;  x \<in> S\<rbrakk> \<Longrightarrow> inv_into S f x \<in> S"
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by (simp add: Bij_def bij_betw_def inv_into_into)
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lemma Bij_inv_into_lemma:
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 assumes eq: "\<And>x y. \<lbrakk>x \<in> S; y \<in> S\<rbrakk> \<Longrightarrow> h(g x y) = g (h x) (h y)"
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 shows "\<lbrakk>h \<in> Bij S;  g \<in> S \<rightarrow> S \<rightarrow> S;  x \<in> S;  y \<in> S\<rbrakk>
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        \<Longrightarrow> inv_into S h (g x y) = g (inv_into S h x) (inv_into S h y)"
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apply (simp add: Bij_def bij_betw_def)
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apply (subgoal_tac "\<exists>x'\<in>S. \<exists>y'\<in>S. x = h x' & y = h y'", clarify)
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 apply (simp add: eq [symmetric] inv_f_f funcset_mem [THEN funcset_mem], blast)
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done
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constdefs
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  auto :: "('a, 'b) monoid_scheme \<Rightarrow> ('a \<Rightarrow> 'a) set"
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  "auto G \<equiv> hom G G \<inter> Bij (carrier G)"
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  AutoGroup :: "('a, 'c) monoid_scheme \<Rightarrow> ('a \<Rightarrow> 'a) monoid"
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  "AutoGroup G \<equiv> BijGroup (carrier G) \<lparr>carrier := auto G\<rparr>"
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lemma (in group) id_in_auto: "(\<lambda>x \<in> carrier G. x) \<in> auto G"
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  by (simp add: auto_def hom_def restrictI group.axioms id_Bij)
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lemma (in group) mult_funcset: "mult G \<in> carrier G \<rightarrow> carrier G \<rightarrow> carrier G"
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  by (simp add:  Pi_I group.axioms)
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lemma (in group) restrict_inv_into_hom:
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      "\<lbrakk>h \<in> hom G G; h \<in> Bij (carrier G)\<rbrakk>
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       \<Longrightarrow> restrict (inv_into (carrier G) h) (carrier G) \<in> hom G G"
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  by (simp add: hom_def Bij_inv_into_mem restrictI mult_funcset
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                group.axioms Bij_inv_into_lemma)
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lemma inv_BijGroup:
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     "f \<in> Bij S \<Longrightarrow> m_inv (BijGroup S) f = (\<lambda>x \<in> S. (inv_into S f) x)"
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apply (rule group.inv_equality)
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apply (rule group_BijGroup)
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apply (simp_all add:BijGroup_def restrict_inv_into_Bij Bij_compose_restrict_eq)
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done
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lemma (in group) subgroup_auto:
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      "subgroup (auto G) (BijGroup (carrier G))"
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proof (rule subgroup.intro)
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  show "auto G \<subseteq> carrier (BijGroup (carrier G))"
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    by (force simp add: auto_def BijGroup_def)
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next
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  fix x y
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  assume "x \<in> auto G" "y \<in> auto G" 
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  thus "x \<otimes>\<^bsub>BijGroup (carrier G)\<^esub> y \<in> auto G"
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    by (force simp add: BijGroup_def is_group auto_def Bij_imp_funcset 
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                        group.hom_compose compose_Bij)
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next
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  show "\<one>\<^bsub>BijGroup (carrier G)\<^esub> \<in> auto G" by (simp add:  BijGroup_def id_in_auto)
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next
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  fix x 
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  assume "x \<in> auto G" 
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  thus "inv\<^bsub>BijGroup (carrier G)\<^esub> x \<in> auto G"
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    by (simp del: restrict_apply
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        add: inv_BijGroup auto_def restrict_inv_into_Bij restrict_inv_into_hom)
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qed
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theorem (in group) AutoGroup: "group (AutoGroup G)"
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by (simp add: AutoGroup_def subgroup.subgroup_is_group subgroup_auto 
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              group_BijGroup)
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end