src/HOL/Algebra/Group.thy
author wenzelm
Sun Sep 13 22:56:52 2015 +0200 (2015-09-13)
changeset 61169 4de9ff3ea29a
parent 58622 aa99568f56de
child 61382 efac889fccbc
permissions -rw-r--r--
tuned proofs -- less legacy;
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(*  Title:      HOL/Algebra/Group.thy
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    Author:     Clemens Ballarin, started 4 February 2003
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Based on work by Florian Kammueller, L C Paulson and Markus Wenzel.
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*)
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theory Group
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imports Lattice "~~/src/HOL/Library/FuncSet"
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begin
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section {* Monoids and Groups *}
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subsection {* Definitions *}
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text {*
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  Definitions follow @{cite "Jacobson:1985"}.
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*}
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record 'a monoid =  "'a partial_object" +
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  mult    :: "['a, 'a] \<Rightarrow> 'a" (infixl "\<otimes>\<index>" 70)
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  one     :: 'a ("\<one>\<index>")
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definition
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  m_inv :: "('a, 'b) monoid_scheme => 'a => 'a" ("inv\<index> _" [81] 80)
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  where "inv\<^bsub>G\<^esub> x = (THE y. y \<in> carrier G & x \<otimes>\<^bsub>G\<^esub> y = \<one>\<^bsub>G\<^esub> & y \<otimes>\<^bsub>G\<^esub> x = \<one>\<^bsub>G\<^esub>)"
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definition
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  Units :: "_ => 'a set"
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  --{*The set of invertible elements*}
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  where "Units G = {y. y \<in> carrier G & (\<exists>x \<in> carrier G. x \<otimes>\<^bsub>G\<^esub> y = \<one>\<^bsub>G\<^esub> & y \<otimes>\<^bsub>G\<^esub> x = \<one>\<^bsub>G\<^esub>)}"
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consts
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  pow :: "[('a, 'm) monoid_scheme, 'a, 'b::semiring_1] => 'a"  (infixr "'(^')\<index>" 75)
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overloading nat_pow == "pow :: [_, 'a, nat] => 'a"
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begin
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  definition "nat_pow G a n = rec_nat \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a) n"
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end
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overloading int_pow == "pow :: [_, 'a, int] => 'a"
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begin
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  definition "int_pow G a z =
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   (let p = rec_nat \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a)
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    in if z < 0 then inv\<^bsub>G\<^esub> (p (nat (-z))) else p (nat z))"
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end
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locale monoid =
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  fixes G (structure)
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  assumes m_closed [intro, simp]:
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         "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> carrier G"
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      and m_assoc:
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         "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> 
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          \<Longrightarrow> (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
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      and one_closed [intro, simp]: "\<one> \<in> carrier G"
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      and l_one [simp]: "x \<in> carrier G \<Longrightarrow> \<one> \<otimes> x = x"
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      and r_one [simp]: "x \<in> carrier G \<Longrightarrow> x \<otimes> \<one> = x"
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lemma monoidI:
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  fixes G (structure)
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  assumes m_closed:
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      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
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    and one_closed: "\<one> \<in> carrier G"
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    and m_assoc:
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      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
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      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
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    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
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    and r_one: "!!x. x \<in> carrier G ==> x \<otimes> \<one> = x"
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  shows "monoid G"
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  by (fast intro!: monoid.intro intro: assms)
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lemma (in monoid) Units_closed [dest]:
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  "x \<in> Units G ==> x \<in> carrier G"
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  by (unfold Units_def) fast
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lemma (in monoid) inv_unique:
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  assumes eq: "y \<otimes> x = \<one>"  "x \<otimes> y' = \<one>"
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    and G: "x \<in> carrier G"  "y \<in> carrier G"  "y' \<in> carrier G"
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  shows "y = y'"
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proof -
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  from G eq have "y = y \<otimes> (x \<otimes> y')" by simp
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  also from G have "... = (y \<otimes> x) \<otimes> y'" by (simp add: m_assoc)
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  also from G eq have "... = y'" by simp
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  finally show ?thesis .
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qed
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lemma (in monoid) Units_m_closed [intro, simp]:
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  assumes x: "x \<in> Units G" and y: "y \<in> Units G"
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  shows "x \<otimes> y \<in> Units G"
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proof -
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  from x obtain x' where x: "x \<in> carrier G" "x' \<in> carrier G" and xinv: "x \<otimes> x' = \<one>" "x' \<otimes> x = \<one>"
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    unfolding Units_def by fast
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  from y obtain y' where y: "y \<in> carrier G" "y' \<in> carrier G" and yinv: "y \<otimes> y' = \<one>" "y' \<otimes> y = \<one>"
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    unfolding Units_def by fast
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  from x y xinv yinv have "y' \<otimes> (x' \<otimes> x) \<otimes> y = \<one>" by simp
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  moreover from x y xinv yinv have "x \<otimes> (y \<otimes> y') \<otimes> x' = \<one>" by simp
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  moreover note x y
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  ultimately show ?thesis unfolding Units_def
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    -- "Must avoid premature use of @{text hyp_subst_tac}."
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    apply (rule_tac CollectI)
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    apply (rule)
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    apply (fast)
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    apply (rule bexI [where x = "y' \<otimes> x'"])
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    apply (auto simp: m_assoc)
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    done
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qed
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lemma (in monoid) Units_one_closed [intro, simp]:
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  "\<one> \<in> Units G"
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  by (unfold Units_def) auto
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lemma (in monoid) Units_inv_closed [intro, simp]:
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  "x \<in> Units G ==> inv x \<in> carrier G"
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  apply (unfold Units_def m_inv_def, auto)
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  apply (rule theI2, fast)
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   apply (fast intro: inv_unique, fast)
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  done
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lemma (in monoid) Units_l_inv_ex:
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  "x \<in> Units G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
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  by (unfold Units_def) auto
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lemma (in monoid) Units_r_inv_ex:
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  "x \<in> Units G ==> \<exists>y \<in> carrier G. x \<otimes> y = \<one>"
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  by (unfold Units_def) auto
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lemma (in monoid) Units_l_inv [simp]:
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  "x \<in> Units G ==> inv x \<otimes> x = \<one>"
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  apply (unfold Units_def m_inv_def, auto)
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  apply (rule theI2, fast)
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   apply (fast intro: inv_unique, fast)
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  done
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lemma (in monoid) Units_r_inv [simp]:
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  "x \<in> Units G ==> x \<otimes> inv x = \<one>"
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  apply (unfold Units_def m_inv_def, auto)
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  apply (rule theI2, fast)
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   apply (fast intro: inv_unique, fast)
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  done
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lemma (in monoid) Units_inv_Units [intro, simp]:
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  "x \<in> Units G ==> inv x \<in> Units G"
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proof -
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  assume x: "x \<in> Units G"
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  show "inv x \<in> Units G"
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    by (auto simp add: Units_def
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      intro: Units_l_inv Units_r_inv x Units_closed [OF x])
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qed
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lemma (in monoid) Units_l_cancel [simp]:
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  "[| x \<in> Units G; y \<in> carrier G; z \<in> carrier G |] ==>
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   (x \<otimes> y = x \<otimes> z) = (y = z)"
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proof
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  assume eq: "x \<otimes> y = x \<otimes> z"
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    and G: "x \<in> Units G"  "y \<in> carrier G"  "z \<in> carrier G"
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  then have "(inv x \<otimes> x) \<otimes> y = (inv x \<otimes> x) \<otimes> z"
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    by (simp add: m_assoc Units_closed del: Units_l_inv)
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  with G show "y = z" by simp
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next
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  assume eq: "y = z"
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    and G: "x \<in> Units G"  "y \<in> carrier G"  "z \<in> carrier G"
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  then show "x \<otimes> y = x \<otimes> z" by simp
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qed
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lemma (in monoid) Units_inv_inv [simp]:
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  "x \<in> Units G ==> inv (inv x) = x"
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proof -
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  assume x: "x \<in> Units G"
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  then have "inv x \<otimes> inv (inv x) = inv x \<otimes> x" by simp
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  with x show ?thesis by (simp add: Units_closed del: Units_l_inv Units_r_inv)
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qed
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lemma (in monoid) inv_inj_on_Units:
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  "inj_on (m_inv G) (Units G)"
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proof (rule inj_onI)
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  fix x y
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  assume G: "x \<in> Units G"  "y \<in> Units G" and eq: "inv x = inv y"
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  then have "inv (inv x) = inv (inv y)" by simp
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  with G show "x = y" by simp
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qed
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lemma (in monoid) Units_inv_comm:
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  assumes inv: "x \<otimes> y = \<one>"
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    and G: "x \<in> Units G"  "y \<in> Units G"
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  shows "y \<otimes> x = \<one>"
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proof -
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  from G have "x \<otimes> y \<otimes> x = x \<otimes> \<one>" by (auto simp add: inv Units_closed)
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  with G show ?thesis by (simp del: r_one add: m_assoc Units_closed)
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qed
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text {* Power *}
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lemma (in monoid) nat_pow_closed [intro, simp]:
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  "x \<in> carrier G ==> x (^) (n::nat) \<in> carrier G"
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  by (induct n) (simp_all add: nat_pow_def)
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lemma (in monoid) nat_pow_0 [simp]:
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  "x (^) (0::nat) = \<one>"
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  by (simp add: nat_pow_def)
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lemma (in monoid) nat_pow_Suc [simp]:
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  "x (^) (Suc n) = x (^) n \<otimes> x"
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  by (simp add: nat_pow_def)
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lemma (in monoid) nat_pow_one [simp]:
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  "\<one> (^) (n::nat) = \<one>"
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  by (induct n) simp_all
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lemma (in monoid) nat_pow_mult:
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  "x \<in> carrier G ==> x (^) (n::nat) \<otimes> x (^) m = x (^) (n + m)"
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  by (induct m) (simp_all add: m_assoc [THEN sym])
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lemma (in monoid) nat_pow_pow:
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  "x \<in> carrier G ==> (x (^) n) (^) m = x (^) (n * m::nat)"
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  by (induct m) (simp, simp add: nat_pow_mult add.commute)
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(* Jacobson defines submonoid here. *)
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(* Jacobson defines the order of a monoid here. *)
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subsection {* Groups *}
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text {*
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  A group is a monoid all of whose elements are invertible.
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*}
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locale group = monoid +
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  assumes Units: "carrier G <= Units G"
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lemma (in group) is_group: "group G" by (rule group_axioms)
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theorem groupI:
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  fixes G (structure)
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  assumes m_closed [simp]:
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      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
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    and one_closed [simp]: "\<one> \<in> carrier G"
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    and m_assoc:
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      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
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      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
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    and l_one [simp]: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
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    and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
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  shows "group G"
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proof -
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  have l_cancel [simp]:
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    "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
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    (x \<otimes> y = x \<otimes> z) = (y = z)"
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  proof
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    fix x y z
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    assume eq: "x \<otimes> y = x \<otimes> z"
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      and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
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    with l_inv_ex obtain x_inv where xG: "x_inv \<in> carrier G"
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      and l_inv: "x_inv \<otimes> x = \<one>" by fast
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    from G eq xG have "(x_inv \<otimes> x) \<otimes> y = (x_inv \<otimes> x) \<otimes> z"
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      by (simp add: m_assoc)
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    with G show "y = z" by (simp add: l_inv)
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  next
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    fix x y z
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    assume eq: "y = z"
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      and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
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    then show "x \<otimes> y = x \<otimes> z" by simp
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  qed
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  have r_one:
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    "!!x. x \<in> carrier G ==> x \<otimes> \<one> = x"
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  proof -
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    fix x
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    assume x: "x \<in> carrier G"
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    with l_inv_ex obtain x_inv where xG: "x_inv \<in> carrier G"
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      and l_inv: "x_inv \<otimes> x = \<one>" by fast
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    from x xG have "x_inv \<otimes> (x \<otimes> \<one>) = x_inv \<otimes> x"
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      by (simp add: m_assoc [symmetric] l_inv)
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    with x xG show "x \<otimes> \<one> = x" by simp
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  qed
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  have inv_ex:
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    "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one> & x \<otimes> y = \<one>"
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  proof -
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    fix x
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    assume x: "x \<in> carrier G"
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    with l_inv_ex obtain y where y: "y \<in> carrier G"
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      and l_inv: "y \<otimes> x = \<one>" by fast
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    from x y have "y \<otimes> (x \<otimes> y) = y \<otimes> \<one>"
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      by (simp add: m_assoc [symmetric] l_inv r_one)
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    with x y have r_inv: "x \<otimes> y = \<one>"
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      by simp
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    from x y show "\<exists>y \<in> carrier G. y \<otimes> x = \<one> & x \<otimes> y = \<one>"
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      by (fast intro: l_inv r_inv)
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  qed
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  then have carrier_subset_Units: "carrier G <= Units G"
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    by (unfold Units_def) fast
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  show ?thesis
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    by standard (auto simp: r_one m_assoc carrier_subset_Units)
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qed
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lemma (in monoid) group_l_invI:
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  assumes l_inv_ex:
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    "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
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  shows "group G"
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  by (rule groupI) (auto intro: m_assoc l_inv_ex)
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lemma (in group) Units_eq [simp]:
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  "Units G = carrier G"
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proof
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  show "Units G <= carrier G" by fast
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next
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  show "carrier G <= Units G" by (rule Units)
ballarin@13936
   305
qed
ballarin@13936
   306
ballarin@13936
   307
lemma (in group) inv_closed [intro, simp]:
ballarin@13936
   308
  "x \<in> carrier G ==> inv x \<in> carrier G"
ballarin@13936
   309
  using Units_inv_closed by simp
ballarin@13936
   310
ballarin@19981
   311
lemma (in group) l_inv_ex [simp]:
ballarin@19981
   312
  "x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
ballarin@19981
   313
  using Units_l_inv_ex by simp
ballarin@19981
   314
ballarin@19981
   315
lemma (in group) r_inv_ex [simp]:
ballarin@19981
   316
  "x \<in> carrier G ==> \<exists>y \<in> carrier G. x \<otimes> y = \<one>"
ballarin@19981
   317
  using Units_r_inv_ex by simp
ballarin@19981
   318
paulson@14963
   319
lemma (in group) l_inv [simp]:
ballarin@13936
   320
  "x \<in> carrier G ==> inv x \<otimes> x = \<one>"
ballarin@13936
   321
  using Units_l_inv by simp
ballarin@13813
   322
ballarin@20318
   323
ballarin@13813
   324
subsection {* Cancellation Laws and Basic Properties *}
ballarin@13813
   325
ballarin@13813
   326
lemma (in group) l_cancel [simp]:
ballarin@13813
   327
  "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
ballarin@13813
   328
   (x \<otimes> y = x \<otimes> z) = (y = z)"
ballarin@13936
   329
  using Units_l_inv by simp
ballarin@13940
   330
paulson@14963
   331
lemma (in group) r_inv [simp]:
ballarin@13813
   332
  "x \<in> carrier G ==> x \<otimes> inv x = \<one>"
ballarin@13813
   333
proof -
ballarin@13813
   334
  assume x: "x \<in> carrier G"
ballarin@13813
   335
  then have "inv x \<otimes> (x \<otimes> inv x) = inv x \<otimes> \<one>"
wenzelm@44472
   336
    by (simp add: m_assoc [symmetric])
ballarin@13813
   337
  with x show ?thesis by (simp del: r_one)
ballarin@13813
   338
qed
ballarin@13813
   339
ballarin@13813
   340
lemma (in group) r_cancel [simp]:
ballarin@13813
   341
  "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
ballarin@13813
   342
   (y \<otimes> x = z \<otimes> x) = (y = z)"
ballarin@13813
   343
proof
ballarin@13813
   344
  assume eq: "y \<otimes> x = z \<otimes> x"
wenzelm@14693
   345
    and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   346
  then have "y \<otimes> (x \<otimes> inv x) = z \<otimes> (x \<otimes> inv x)"
ballarin@27698
   347
    by (simp add: m_assoc [symmetric] del: r_inv Units_r_inv)
paulson@14963
   348
  with G show "y = z" by simp
ballarin@13813
   349
next
ballarin@13813
   350
  assume eq: "y = z"
wenzelm@14693
   351
    and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   352
  then show "y \<otimes> x = z \<otimes> x" by simp
ballarin@13813
   353
qed
ballarin@13813
   354
ballarin@13854
   355
lemma (in group) inv_one [simp]:
ballarin@13854
   356
  "inv \<one> = \<one>"
ballarin@13854
   357
proof -
ballarin@27698
   358
  have "inv \<one> = \<one> \<otimes> (inv \<one>)" by (simp del: r_inv Units_r_inv)
paulson@14963
   359
  moreover have "... = \<one>" by simp
ballarin@13854
   360
  finally show ?thesis .
ballarin@13854
   361
qed
ballarin@13854
   362
ballarin@13813
   363
lemma (in group) inv_inv [simp]:
ballarin@13813
   364
  "x \<in> carrier G ==> inv (inv x) = x"
ballarin@13936
   365
  using Units_inv_inv by simp
ballarin@13936
   366
ballarin@13936
   367
lemma (in group) inv_inj:
ballarin@13936
   368
  "inj_on (m_inv G) (carrier G)"
ballarin@13936
   369
  using inv_inj_on_Units by simp
ballarin@13813
   370
ballarin@13854
   371
lemma (in group) inv_mult_group:
ballarin@13813
   372
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv y \<otimes> inv x"
ballarin@13813
   373
proof -
wenzelm@14693
   374
  assume G: "x \<in> carrier G"  "y \<in> carrier G"
ballarin@13813
   375
  then have "inv (x \<otimes> y) \<otimes> (x \<otimes> y) = (inv y \<otimes> inv x) \<otimes> (x \<otimes> y)"
wenzelm@44472
   376
    by (simp add: m_assoc) (simp add: m_assoc [symmetric])
ballarin@27698
   377
  with G show ?thesis by (simp del: l_inv Units_l_inv)
ballarin@13813
   378
qed
ballarin@13813
   379
ballarin@13940
   380
lemma (in group) inv_comm:
ballarin@13940
   381
  "[| x \<otimes> y = \<one>; x \<in> carrier G; y \<in> carrier G |] ==> y \<otimes> x = \<one>"
wenzelm@14693
   382
  by (rule Units_inv_comm) auto
ballarin@13940
   383
paulson@13944
   384
lemma (in group) inv_equality:
paulson@13943
   385
     "[|y \<otimes> x = \<one>; x \<in> carrier G; y \<in> carrier G|] ==> inv x = y"
paulson@13943
   386
apply (simp add: m_inv_def)
paulson@13943
   387
apply (rule the_equality)
wenzelm@14693
   388
 apply (simp add: inv_comm [of y x])
wenzelm@14693
   389
apply (rule r_cancel [THEN iffD1], auto)
paulson@13943
   390
done
paulson@13943
   391
ballarin@57271
   392
(* Contributed by Joachim Breitner *)
ballarin@57271
   393
lemma (in group) inv_solve_left:
ballarin@57271
   394
  "\<lbrakk> a \<in> carrier G; b \<in> carrier G; c \<in> carrier G \<rbrakk> \<Longrightarrow> a = inv b \<otimes> c \<longleftrightarrow> c = b \<otimes> a"
ballarin@57271
   395
  by (metis inv_equality l_inv_ex l_one m_assoc r_inv)
ballarin@57271
   396
lemma (in group) inv_solve_right:
ballarin@57271
   397
  "\<lbrakk> a \<in> carrier G; b \<in> carrier G; c \<in> carrier G \<rbrakk> \<Longrightarrow> a = b \<otimes> inv c \<longleftrightarrow> b = a \<otimes> c"
ballarin@57271
   398
  by (metis inv_equality l_inv_ex l_one m_assoc r_inv)
ballarin@57271
   399
ballarin@13936
   400
text {* Power *}
ballarin@13936
   401
ballarin@13936
   402
lemma (in group) int_pow_def2:
huffman@46559
   403
  "a (^) (z::int) = (if z < 0 then inv (a (^) (nat (-z))) else a (^) (nat z))"
ballarin@13936
   404
  by (simp add: int_pow_def nat_pow_def Let_def)
ballarin@13936
   405
ballarin@13936
   406
lemma (in group) int_pow_0 [simp]:
ballarin@13936
   407
  "x (^) (0::int) = \<one>"
ballarin@13936
   408
  by (simp add: int_pow_def2)
ballarin@13936
   409
ballarin@13936
   410
lemma (in group) int_pow_one [simp]:
ballarin@13936
   411
  "\<one> (^) (z::int) = \<one>"
ballarin@13936
   412
  by (simp add: int_pow_def2)
ballarin@13936
   413
ballarin@57271
   414
(* The following are contributed by Joachim Breitner *)
ballarin@20318
   415
ballarin@57271
   416
lemma (in group) int_pow_closed [intro, simp]:
ballarin@57271
   417
  "x \<in> carrier G ==> x (^) (i::int) \<in> carrier G"
ballarin@57271
   418
  by (simp add: int_pow_def2)
ballarin@57271
   419
ballarin@57271
   420
lemma (in group) int_pow_1 [simp]:
ballarin@57271
   421
  "x \<in> carrier G \<Longrightarrow> x (^) (1::int) = x"
ballarin@57271
   422
  by (simp add: int_pow_def2)
ballarin@57271
   423
ballarin@57271
   424
lemma (in group) int_pow_neg:
ballarin@57271
   425
  "x \<in> carrier G \<Longrightarrow> x (^) (-i::int) = inv (x (^) i)"
ballarin@57271
   426
  by (simp add: int_pow_def2)
ballarin@57271
   427
ballarin@57271
   428
lemma (in group) int_pow_mult:
ballarin@57271
   429
  "x \<in> carrier G \<Longrightarrow> x (^) (i + j::int) = x (^) i \<otimes> x (^) j"
ballarin@57271
   430
proof -
ballarin@57271
   431
  have [simp]: "-i - j = -j - i" by simp
ballarin@57271
   432
  assume "x : carrier G" then
ballarin@57271
   433
  show ?thesis
ballarin@57271
   434
    by (auto simp add: int_pow_def2 inv_solve_left inv_solve_right nat_add_distrib [symmetric] nat_pow_mult )
ballarin@57271
   435
qed
ballarin@57271
   436
ballarin@57271
   437
 
paulson@14963
   438
subsection {* Subgroups *}
ballarin@13813
   439
ballarin@19783
   440
locale subgroup =
ballarin@19783
   441
  fixes H and G (structure)
paulson@14963
   442
  assumes subset: "H \<subseteq> carrier G"
paulson@14963
   443
    and m_closed [intro, simp]: "\<lbrakk>x \<in> H; y \<in> H\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> H"
ballarin@20318
   444
    and one_closed [simp]: "\<one> \<in> H"
paulson@14963
   445
    and m_inv_closed [intro,simp]: "x \<in> H \<Longrightarrow> inv x \<in> H"
ballarin@13813
   446
ballarin@20318
   447
lemma (in subgroup) is_subgroup:
wenzelm@26199
   448
  "subgroup H G" by (rule subgroup_axioms)
ballarin@20318
   449
ballarin@13813
   450
declare (in subgroup) group.intro [intro]
ballarin@13949
   451
paulson@14963
   452
lemma (in subgroup) mem_carrier [simp]:
paulson@14963
   453
  "x \<in> H \<Longrightarrow> x \<in> carrier G"
paulson@14963
   454
  using subset by blast
ballarin@13813
   455
paulson@14963
   456
lemma subgroup_imp_subset:
paulson@14963
   457
  "subgroup H G \<Longrightarrow> H \<subseteq> carrier G"
paulson@14963
   458
  by (rule subgroup.subset)
paulson@14963
   459
paulson@14963
   460
lemma (in subgroup) subgroup_is_group [intro]:
ballarin@27611
   461
  assumes "group G"
ballarin@27611
   462
  shows "group (G\<lparr>carrier := H\<rparr>)"
ballarin@27611
   463
proof -
ballarin@29237
   464
  interpret group G by fact
ballarin@27611
   465
  show ?thesis
ballarin@27698
   466
    apply (rule monoid.group_l_invI)
ballarin@27698
   467
    apply (unfold_locales) [1]
ballarin@27698
   468
    apply (auto intro: m_assoc l_inv mem_carrier)
ballarin@27698
   469
    done
ballarin@27611
   470
qed
ballarin@13813
   471
ballarin@13813
   472
text {*
ballarin@13813
   473
  Since @{term H} is nonempty, it contains some element @{term x}.  Since
ballarin@13813
   474
  it is closed under inverse, it contains @{text "inv x"}.  Since
ballarin@13813
   475
  it is closed under product, it contains @{text "x \<otimes> inv x = \<one>"}.
ballarin@13813
   476
*}
ballarin@13813
   477
ballarin@13813
   478
lemma (in group) one_in_subset:
ballarin@13813
   479
  "[| H \<subseteq> carrier G; H \<noteq> {}; \<forall>a \<in> H. inv a \<in> H; \<forall>a\<in>H. \<forall>b\<in>H. a \<otimes> b \<in> H |]
ballarin@13813
   480
   ==> \<one> \<in> H"
wenzelm@44472
   481
by force
ballarin@13813
   482
ballarin@13813
   483
text {* A characterization of subgroups: closed, non-empty subset. *}
ballarin@13813
   484
ballarin@13813
   485
lemma (in group) subgroupI:
ballarin@13813
   486
  assumes subset: "H \<subseteq> carrier G" and non_empty: "H \<noteq> {}"
paulson@14963
   487
    and inv: "!!a. a \<in> H \<Longrightarrow> inv a \<in> H"
paulson@14963
   488
    and mult: "!!a b. \<lbrakk>a \<in> H; b \<in> H\<rbrakk> \<Longrightarrow> a \<otimes> b \<in> H"
ballarin@13813
   489
  shows "subgroup H G"
ballarin@27714
   490
proof (simp add: subgroup_def assms)
ballarin@27714
   491
  show "\<one> \<in> H" by (rule one_in_subset) (auto simp only: assms)
ballarin@13813
   492
qed
ballarin@13813
   493
ballarin@13936
   494
declare monoid.one_closed [iff] group.inv_closed [simp]
ballarin@13936
   495
  monoid.l_one [simp] monoid.r_one [simp] group.inv_inv [simp]
ballarin@13813
   496
ballarin@13813
   497
lemma subgroup_nonempty:
ballarin@13813
   498
  "~ subgroup {} G"
ballarin@13813
   499
  by (blast dest: subgroup.one_closed)
ballarin@13813
   500
ballarin@13813
   501
lemma (in subgroup) finite_imp_card_positive:
ballarin@13813
   502
  "finite (carrier G) ==> 0 < card H"
ballarin@13813
   503
proof (rule classical)
wenzelm@41528
   504
  assume "finite (carrier G)" and a: "~ 0 < card H"
paulson@14963
   505
  then have "finite H" by (blast intro: finite_subset [OF subset])
wenzelm@41528
   506
  with is_subgroup a have "subgroup {} G" by simp
ballarin@13813
   507
  with subgroup_nonempty show ?thesis by contradiction
ballarin@13813
   508
qed
ballarin@13813
   509
ballarin@13936
   510
(*
ballarin@13936
   511
lemma (in monoid) Units_subgroup:
ballarin@13936
   512
  "subgroup (Units G) G"
ballarin@13936
   513
*)
ballarin@13936
   514
ballarin@20318
   515
ballarin@13813
   516
subsection {* Direct Products *}
ballarin@13813
   517
wenzelm@35848
   518
definition
wenzelm@35848
   519
  DirProd :: "_ \<Rightarrow> _ \<Rightarrow> ('a \<times> 'b) monoid" (infixr "\<times>\<times>" 80) where
wenzelm@35848
   520
  "G \<times>\<times> H =
wenzelm@35848
   521
    \<lparr>carrier = carrier G \<times> carrier H,
wenzelm@35848
   522
     mult = (\<lambda>(g, h) (g', h'). (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')),
wenzelm@35848
   523
     one = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)\<rparr>"
ballarin@13813
   524
paulson@14963
   525
lemma DirProd_monoid:
ballarin@27611
   526
  assumes "monoid G" and "monoid H"
paulson@14963
   527
  shows "monoid (G \<times>\<times> H)"
paulson@14963
   528
proof -
wenzelm@30729
   529
  interpret G: monoid G by fact
wenzelm@30729
   530
  interpret H: monoid H by fact
ballarin@27714
   531
  from assms
paulson@14963
   532
  show ?thesis by (unfold monoid_def DirProd_def, auto) 
paulson@14963
   533
qed
ballarin@13813
   534
ballarin@13813
   535
paulson@14963
   536
text{*Does not use the previous result because it's easier just to use auto.*}
paulson@14963
   537
lemma DirProd_group:
ballarin@27611
   538
  assumes "group G" and "group H"
paulson@14963
   539
  shows "group (G \<times>\<times> H)"
ballarin@27611
   540
proof -
wenzelm@30729
   541
  interpret G: group G by fact
wenzelm@30729
   542
  interpret H: group H by fact
ballarin@27611
   543
  show ?thesis by (rule groupI)
paulson@14963
   544
     (auto intro: G.m_assoc H.m_assoc G.l_inv H.l_inv
paulson@14963
   545
           simp add: DirProd_def)
ballarin@27611
   546
qed
ballarin@13813
   547
paulson@14963
   548
lemma carrier_DirProd [simp]:
paulson@14963
   549
     "carrier (G \<times>\<times> H) = carrier G \<times> carrier H"
paulson@14963
   550
  by (simp add: DirProd_def)
paulson@13944
   551
paulson@14963
   552
lemma one_DirProd [simp]:
paulson@14963
   553
     "\<one>\<^bsub>G \<times>\<times> H\<^esub> = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)"
paulson@14963
   554
  by (simp add: DirProd_def)
paulson@13944
   555
paulson@14963
   556
lemma mult_DirProd [simp]:
paulson@14963
   557
     "(g, h) \<otimes>\<^bsub>(G \<times>\<times> H)\<^esub> (g', h') = (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')"
paulson@14963
   558
  by (simp add: DirProd_def)
paulson@13944
   559
paulson@14963
   560
lemma inv_DirProd [simp]:
ballarin@27611
   561
  assumes "group G" and "group H"
paulson@13944
   562
  assumes g: "g \<in> carrier G"
paulson@13944
   563
      and h: "h \<in> carrier H"
paulson@14963
   564
  shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
ballarin@27611
   565
proof -
wenzelm@30729
   566
  interpret G: group G by fact
wenzelm@30729
   567
  interpret H: group H by fact
wenzelm@30729
   568
  interpret Prod: group "G \<times>\<times> H"
ballarin@27714
   569
    by (auto intro: DirProd_group group.intro group.axioms assms)
paulson@14963
   570
  show ?thesis by (simp add: Prod.inv_equality g h)
paulson@14963
   571
qed
ballarin@27698
   572
paulson@14963
   573
paulson@14963
   574
subsection {* Homomorphisms and Isomorphisms *}
ballarin@13813
   575
wenzelm@35847
   576
definition
wenzelm@35847
   577
  hom :: "_ => _ => ('a => 'b) set" where
wenzelm@35848
   578
  "hom G H =
ballarin@13813
   579
    {h. h \<in> carrier G -> carrier H &
wenzelm@14693
   580
      (\<forall>x \<in> carrier G. \<forall>y \<in> carrier G. h (x \<otimes>\<^bsub>G\<^esub> y) = h x \<otimes>\<^bsub>H\<^esub> h y)}"
ballarin@13813
   581
paulson@14761
   582
lemma (in group) hom_compose:
nipkow@31754
   583
  "[|h \<in> hom G H; i \<in> hom H I|] ==> compose (carrier G) i h \<in> hom G I"
nipkow@44890
   584
by (fastforce simp add: hom_def compose_def)
paulson@13943
   585
wenzelm@35848
   586
definition
wenzelm@35848
   587
  iso :: "_ => _ => ('a => 'b) set" (infixr "\<cong>" 60)
wenzelm@35848
   588
  where "G \<cong> H = {h. h \<in> hom G H & bij_betw h (carrier G) (carrier H)}"
paulson@14761
   589
paulson@14803
   590
lemma iso_refl: "(%x. x) \<in> G \<cong> G"
nipkow@31727
   591
by (simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def)
paulson@14761
   592
paulson@14761
   593
lemma (in group) iso_sym:
nipkow@33057
   594
     "h \<in> G \<cong> H \<Longrightarrow> inv_into (carrier G) h \<in> H \<cong> G"
nipkow@33057
   595
apply (simp add: iso_def bij_betw_inv_into) 
nipkow@33057
   596
apply (subgoal_tac "inv_into (carrier G) h \<in> carrier H \<rightarrow> carrier G") 
nipkow@33057
   597
 prefer 2 apply (simp add: bij_betw_imp_funcset [OF bij_betw_inv_into]) 
nipkow@33057
   598
apply (simp add: hom_def bij_betw_def inv_into_f_eq f_inv_into_f Pi_def)
paulson@14761
   599
done
paulson@14761
   600
paulson@14761
   601
lemma (in group) iso_trans: 
paulson@14803
   602
     "[|h \<in> G \<cong> H; i \<in> H \<cong> I|] ==> (compose (carrier G) i h) \<in> G \<cong> I"
paulson@14761
   603
by (auto simp add: iso_def hom_compose bij_betw_compose)
paulson@14761
   604
paulson@14963
   605
lemma DirProd_commute_iso:
paulson@14963
   606
  shows "(\<lambda>(x,y). (y,x)) \<in> (G \<times>\<times> H) \<cong> (H \<times>\<times> G)"
nipkow@31754
   607
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def)
paulson@14761
   608
paulson@14963
   609
lemma DirProd_assoc_iso:
paulson@14963
   610
  shows "(\<lambda>(x,y,z). (x,(y,z))) \<in> (G \<times>\<times> H \<times>\<times> I) \<cong> (G \<times>\<times> (H \<times>\<times> I))"
nipkow@31727
   611
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def)
paulson@14761
   612
paulson@14761
   613
paulson@14963
   614
text{*Basis for homomorphism proofs: we assume two groups @{term G} and
ballarin@15076
   615
  @{term H}, with a homomorphism @{term h} between them*}
ballarin@29237
   616
locale group_hom = G: group G + H: group H for G (structure) and H (structure) +
ballarin@29237
   617
  fixes h
ballarin@13813
   618
  assumes homh: "h \<in> hom G H"
ballarin@29240
   619
ballarin@29240
   620
lemma (in group_hom) hom_mult [simp]:
ballarin@29240
   621
  "[| x \<in> carrier G; y \<in> carrier G |] ==> h (x \<otimes>\<^bsub>G\<^esub> y) = h x \<otimes>\<^bsub>H\<^esub> h y"
ballarin@29240
   622
proof -
ballarin@29240
   623
  assume "x \<in> carrier G" "y \<in> carrier G"
ballarin@29240
   624
  with homh [unfolded hom_def] show ?thesis by simp
ballarin@29240
   625
qed
ballarin@29240
   626
ballarin@29240
   627
lemma (in group_hom) hom_closed [simp]:
ballarin@29240
   628
  "x \<in> carrier G ==> h x \<in> carrier H"
ballarin@29240
   629
proof -
ballarin@29240
   630
  assume "x \<in> carrier G"
nipkow@31754
   631
  with homh [unfolded hom_def] show ?thesis by auto
ballarin@29240
   632
qed
ballarin@13813
   633
ballarin@13813
   634
lemma (in group_hom) one_closed [simp]:
ballarin@13813
   635
  "h \<one> \<in> carrier H"
ballarin@13813
   636
  by simp
ballarin@13813
   637
ballarin@13813
   638
lemma (in group_hom) hom_one [simp]:
wenzelm@14693
   639
  "h \<one> = \<one>\<^bsub>H\<^esub>"
ballarin@13813
   640
proof -
ballarin@15076
   641
  have "h \<one> \<otimes>\<^bsub>H\<^esub> \<one>\<^bsub>H\<^esub> = h \<one> \<otimes>\<^bsub>H\<^esub> h \<one>"
ballarin@13813
   642
    by (simp add: hom_mult [symmetric] del: hom_mult)
ballarin@13813
   643
  then show ?thesis by (simp del: r_one)
ballarin@13813
   644
qed
ballarin@13813
   645
ballarin@13813
   646
lemma (in group_hom) inv_closed [simp]:
ballarin@13813
   647
  "x \<in> carrier G ==> h (inv x) \<in> carrier H"
ballarin@13813
   648
  by simp
ballarin@13813
   649
ballarin@13813
   650
lemma (in group_hom) hom_inv [simp]:
wenzelm@14693
   651
  "x \<in> carrier G ==> h (inv x) = inv\<^bsub>H\<^esub> (h x)"
ballarin@13813
   652
proof -
ballarin@13813
   653
  assume x: "x \<in> carrier G"
wenzelm@14693
   654
  then have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = \<one>\<^bsub>H\<^esub>"
paulson@14963
   655
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   656
  also from x have "... = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)"
paulson@14963
   657
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   658
  finally have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)" .
ballarin@27698
   659
  with x show ?thesis by (simp del: H.r_inv H.Units_r_inv)
ballarin@13813
   660
qed
ballarin@13813
   661
ballarin@57271
   662
(* Contributed by Joachim Breitner *)
ballarin@57271
   663
lemma (in group) int_pow_is_hom:
ballarin@57271
   664
  "x \<in> carrier G \<Longrightarrow> (op(^) x) \<in> hom \<lparr> carrier = UNIV, mult = op +, one = 0::int \<rparr> G "
ballarin@57271
   665
  unfolding hom_def by (simp add: int_pow_mult)
ballarin@57271
   666
ballarin@20318
   667
ballarin@13949
   668
subsection {* Commutative Structures *}
ballarin@13936
   669
ballarin@13936
   670
text {*
ballarin@13936
   671
  Naming convention: multiplicative structures that are commutative
ballarin@13936
   672
  are called \emph{commutative}, additive structures are called
ballarin@13936
   673
  \emph{Abelian}.
ballarin@13936
   674
*}
ballarin@13813
   675
paulson@14963
   676
locale comm_monoid = monoid +
paulson@14963
   677
  assumes m_comm: "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y = y \<otimes> x"
ballarin@13813
   678
paulson@14963
   679
lemma (in comm_monoid) m_lcomm:
paulson@14963
   680
  "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> \<Longrightarrow>
ballarin@13813
   681
   x \<otimes> (y \<otimes> z) = y \<otimes> (x \<otimes> z)"
ballarin@13813
   682
proof -
wenzelm@14693
   683
  assume xyz: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   684
  from xyz have "x \<otimes> (y \<otimes> z) = (x \<otimes> y) \<otimes> z" by (simp add: m_assoc)
ballarin@13813
   685
  also from xyz have "... = (y \<otimes> x) \<otimes> z" by (simp add: m_comm)
ballarin@13813
   686
  also from xyz have "... = y \<otimes> (x \<otimes> z)" by (simp add: m_assoc)
ballarin@13813
   687
  finally show ?thesis .
ballarin@13813
   688
qed
ballarin@13813
   689
paulson@14963
   690
lemmas (in comm_monoid) m_ac = m_assoc m_comm m_lcomm
ballarin@13813
   691
ballarin@13936
   692
lemma comm_monoidI:
ballarin@19783
   693
  fixes G (structure)
ballarin@13936
   694
  assumes m_closed:
wenzelm@14693
   695
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   696
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   697
    and m_assoc:
ballarin@13936
   698
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   699
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
wenzelm@14693
   700
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
ballarin@13936
   701
    and m_comm:
wenzelm@14693
   702
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   703
  shows "comm_monoid G"
ballarin@13936
   704
  using l_one
paulson@14963
   705
    by (auto intro!: comm_monoid.intro comm_monoid_axioms.intro monoid.intro 
ballarin@27714
   706
             intro: assms simp: m_closed one_closed m_comm)
ballarin@13817
   707
ballarin@13936
   708
lemma (in monoid) monoid_comm_monoidI:
ballarin@13936
   709
  assumes m_comm:
wenzelm@14693
   710
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   711
  shows "comm_monoid G"
ballarin@13936
   712
  by (rule comm_monoidI) (auto intro: m_assoc m_comm)
paulson@14963
   713
wenzelm@14693
   714
(*lemma (in comm_monoid) r_one [simp]:
ballarin@13817
   715
  "x \<in> carrier G ==> x \<otimes> \<one> = x"
ballarin@13817
   716
proof -
ballarin@13817
   717
  assume G: "x \<in> carrier G"
ballarin@13817
   718
  then have "x \<otimes> \<one> = \<one> \<otimes> x" by (simp add: m_comm)
ballarin@13817
   719
  also from G have "... = x" by simp
ballarin@13817
   720
  finally show ?thesis .
wenzelm@14693
   721
qed*)
paulson@14963
   722
ballarin@13936
   723
lemma (in comm_monoid) nat_pow_distr:
ballarin@13936
   724
  "[| x \<in> carrier G; y \<in> carrier G |] ==>
ballarin@13936
   725
  (x \<otimes> y) (^) (n::nat) = x (^) n \<otimes> y (^) n"
ballarin@13936
   726
  by (induct n) (simp, simp add: m_ac)
ballarin@13936
   727
ballarin@13936
   728
locale comm_group = comm_monoid + group
ballarin@13936
   729
ballarin@13936
   730
lemma (in group) group_comm_groupI:
ballarin@13936
   731
  assumes m_comm: "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==>
wenzelm@14693
   732
      x \<otimes> y = y \<otimes> x"
ballarin@13936
   733
  shows "comm_group G"
wenzelm@61169
   734
  by standard (simp_all add: m_comm)
ballarin@13817
   735
ballarin@13936
   736
lemma comm_groupI:
ballarin@19783
   737
  fixes G (structure)
ballarin@13936
   738
  assumes m_closed:
wenzelm@14693
   739
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   740
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   741
    and m_assoc:
ballarin@13936
   742
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   743
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
ballarin@13936
   744
    and m_comm:
wenzelm@14693
   745
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
wenzelm@14693
   746
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
paulson@14963
   747
    and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
ballarin@13936
   748
  shows "comm_group G"
ballarin@27714
   749
  by (fast intro: group.group_comm_groupI groupI assms)
ballarin@13936
   750
ballarin@13936
   751
lemma (in comm_group) inv_mult:
ballarin@13854
   752
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv x \<otimes> inv y"
ballarin@13936
   753
  by (simp add: m_ac inv_mult_group)
ballarin@13854
   754
ballarin@20318
   755
ballarin@20318
   756
subsection {* The Lattice of Subgroups of a Group *}
ballarin@14751
   757
ballarin@14751
   758
text_raw {* \label{sec:subgroup-lattice} *}
ballarin@14751
   759
ballarin@14751
   760
theorem (in group) subgroups_partial_order:
wenzelm@55926
   761
  "partial_order \<lparr>carrier = {H. subgroup H G}, eq = op =, le = op \<subseteq>\<rparr>"
wenzelm@61169
   762
  by standard simp_all
ballarin@14751
   763
ballarin@14751
   764
lemma (in group) subgroup_self:
ballarin@14751
   765
  "subgroup (carrier G) G"
ballarin@14751
   766
  by (rule subgroupI) auto
ballarin@14751
   767
ballarin@14751
   768
lemma (in group) subgroup_imp_group:
wenzelm@55926
   769
  "subgroup H G ==> group (G\<lparr>carrier := H\<rparr>)"
wenzelm@26199
   770
  by (erule subgroup.subgroup_is_group) (rule group_axioms)
ballarin@14751
   771
ballarin@14751
   772
lemma (in group) is_monoid [intro, simp]:
ballarin@14751
   773
  "monoid G"
paulson@14963
   774
  by (auto intro: monoid.intro m_assoc) 
ballarin@14751
   775
ballarin@14751
   776
lemma (in group) subgroup_inv_equality:
wenzelm@55926
   777
  "[| subgroup H G; x \<in> H |] ==> m_inv (G \<lparr>carrier := H\<rparr>) x = inv x"
ballarin@14751
   778
apply (rule_tac inv_equality [THEN sym])
paulson@14761
   779
  apply (rule group.l_inv [OF subgroup_imp_group, simplified], assumption+)
paulson@14761
   780
 apply (rule subsetD [OF subgroup.subset], assumption+)
paulson@14761
   781
apply (rule subsetD [OF subgroup.subset], assumption)
paulson@14761
   782
apply (rule_tac group.inv_closed [OF subgroup_imp_group, simplified], assumption+)
ballarin@14751
   783
done
ballarin@14751
   784
ballarin@14751
   785
theorem (in group) subgroups_Inter:
ballarin@14751
   786
  assumes subgr: "(!!H. H \<in> A ==> subgroup H G)"
ballarin@14751
   787
    and not_empty: "A ~= {}"
ballarin@14751
   788
  shows "subgroup (\<Inter>A) G"
ballarin@14751
   789
proof (rule subgroupI)
ballarin@14751
   790
  from subgr [THEN subgroup.subset] and not_empty
ballarin@14751
   791
  show "\<Inter>A \<subseteq> carrier G" by blast
ballarin@14751
   792
next
ballarin@14751
   793
  from subgr [THEN subgroup.one_closed]
ballarin@14751
   794
  show "\<Inter>A ~= {}" by blast
ballarin@14751
   795
next
ballarin@14751
   796
  fix x assume "x \<in> \<Inter>A"
ballarin@14751
   797
  with subgr [THEN subgroup.m_inv_closed]
ballarin@14751
   798
  show "inv x \<in> \<Inter>A" by blast
ballarin@14751
   799
next
ballarin@14751
   800
  fix x y assume "x \<in> \<Inter>A" "y \<in> \<Inter>A"
ballarin@14751
   801
  with subgr [THEN subgroup.m_closed]
ballarin@14751
   802
  show "x \<otimes> y \<in> \<Inter>A" by blast
ballarin@14751
   803
qed
ballarin@14751
   804
ballarin@14751
   805
theorem (in group) subgroups_complete_lattice:
wenzelm@55926
   806
  "complete_lattice \<lparr>carrier = {H. subgroup H G}, eq = op =, le = op \<subseteq>\<rparr>"
ballarin@22063
   807
    (is "complete_lattice ?L")
ballarin@14751
   808
proof (rule partial_order.complete_lattice_criterion1)
ballarin@22063
   809
  show "partial_order ?L" by (rule subgroups_partial_order)
ballarin@14751
   810
next
wenzelm@46008
   811
  have "greatest ?L (carrier G) (carrier ?L)"
wenzelm@46008
   812
    by (unfold greatest_def) (simp add: subgroup.subset subgroup_self)
wenzelm@46008
   813
  then show "\<exists>G. greatest ?L G (carrier ?L)" ..
ballarin@14751
   814
next
ballarin@14751
   815
  fix A
ballarin@22063
   816
  assume L: "A \<subseteq> carrier ?L" and non_empty: "A ~= {}"
ballarin@14751
   817
  then have Int_subgroup: "subgroup (\<Inter>A) G"
nipkow@44890
   818
    by (fastforce intro: subgroups_Inter)
wenzelm@46008
   819
  have "greatest ?L (\<Inter>A) (Lower ?L A)" (is "greatest _ ?Int _")
wenzelm@46008
   820
  proof (rule greatest_LowerI)
wenzelm@46008
   821
    fix H
wenzelm@46008
   822
    assume H: "H \<in> A"
wenzelm@46008
   823
    with L have subgroupH: "subgroup H G" by auto
wenzelm@55926
   824
    from subgroupH have groupH: "group (G \<lparr>carrier := H\<rparr>)" (is "group ?H")
wenzelm@46008
   825
      by (rule subgroup_imp_group)
wenzelm@46008
   826
    from groupH have monoidH: "monoid ?H"
wenzelm@46008
   827
      by (rule group.is_monoid)
wenzelm@46008
   828
    from H have Int_subset: "?Int \<subseteq> H" by fastforce
wenzelm@46008
   829
    then show "le ?L ?Int H" by simp
wenzelm@46008
   830
  next
wenzelm@46008
   831
    fix H
wenzelm@46008
   832
    assume H: "H \<in> Lower ?L A"
wenzelm@46008
   833
    with L Int_subgroup show "le ?L H ?Int"
wenzelm@46008
   834
      by (fastforce simp: Lower_def intro: Inter_greatest)
wenzelm@46008
   835
  next
wenzelm@46008
   836
    show "A \<subseteq> carrier ?L" by (rule L)
wenzelm@46008
   837
  next
wenzelm@46008
   838
    show "?Int \<in> carrier ?L" by simp (rule Int_subgroup)
ballarin@14751
   839
  qed
wenzelm@46008
   840
  then show "\<exists>I. greatest ?L I (Lower ?L A)" ..
ballarin@14751
   841
qed
ballarin@14751
   842
ballarin@13813
   843
end