src/HOL/Algebra/Group.thy
author nipkow
Thu Oct 22 09:27:48 2009 +0200 (2009-10-22)
changeset 33057 764547b68538
parent 32989 c28279b29ff1
child 35416 d8d7d1b785af
permissions -rw-r--r--
inv_onto -> inv_into
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(*
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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 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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constdefs (structure G)
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  m_inv :: "('a, 'b) monoid_scheme => 'a => 'a" ("inv\<index> _" [81] 80)
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  "inv x == (THE y. y \<in> carrier G & x \<otimes> y = \<one> & y \<otimes> x = \<one>)"
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  Units :: "_ => 'a set"
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  --{*The set of invertible elements*}
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  "Units G == {y. y \<in> carrier G & (\<exists>x \<in> carrier G. x \<otimes> y = \<one> & y \<otimes> x = \<one>)}"
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consts
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  pow :: "[('a, 'm) monoid_scheme, 'a, 'b::number] => 'a" (infixr "'(^')\<index>" 75)
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defs (overloaded)
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  nat_pow_def: "pow G a n == nat_rec \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a) n"
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  int_pow_def: "pow G a z ==
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    let p = nat_rec \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a)
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    in if neg z then inv\<^bsub>G\<^esub> (p (nat (-z))) else p (nat z)"
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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 add: Units_l_inv)
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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 proof qed (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)
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qed
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lemma (in group) inv_closed [intro, simp]:
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  "x \<in> carrier G ==> inv x \<in> carrier G"
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  using Units_inv_closed by simp
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lemma (in group) l_inv_ex [simp]:
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  "x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
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  using Units_l_inv_ex by simp
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ballarin@19981
   309
lemma (in group) r_inv_ex [simp]:
ballarin@19981
   310
  "x \<in> carrier G ==> \<exists>y \<in> carrier G. x \<otimes> y = \<one>"
ballarin@19981
   311
  using Units_r_inv_ex by simp
ballarin@19981
   312
paulson@14963
   313
lemma (in group) l_inv [simp]:
ballarin@13936
   314
  "x \<in> carrier G ==> inv x \<otimes> x = \<one>"
ballarin@13936
   315
  using Units_l_inv by simp
ballarin@13813
   316
ballarin@20318
   317
ballarin@13813
   318
subsection {* Cancellation Laws and Basic Properties *}
ballarin@13813
   319
ballarin@13813
   320
lemma (in group) l_cancel [simp]:
ballarin@13813
   321
  "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
ballarin@13813
   322
   (x \<otimes> y = x \<otimes> z) = (y = z)"
ballarin@13936
   323
  using Units_l_inv by simp
ballarin@13940
   324
paulson@14963
   325
lemma (in group) r_inv [simp]:
ballarin@13813
   326
  "x \<in> carrier G ==> x \<otimes> inv x = \<one>"
ballarin@13813
   327
proof -
ballarin@13813
   328
  assume x: "x \<in> carrier G"
ballarin@13813
   329
  then have "inv x \<otimes> (x \<otimes> inv x) = inv x \<otimes> \<one>"
ballarin@13813
   330
    by (simp add: m_assoc [symmetric] l_inv)
ballarin@13813
   331
  with x show ?thesis by (simp del: r_one)
ballarin@13813
   332
qed
ballarin@13813
   333
ballarin@13813
   334
lemma (in group) r_cancel [simp]:
ballarin@13813
   335
  "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
ballarin@13813
   336
   (y \<otimes> x = z \<otimes> x) = (y = z)"
ballarin@13813
   337
proof
ballarin@13813
   338
  assume eq: "y \<otimes> x = z \<otimes> x"
wenzelm@14693
   339
    and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   340
  then have "y \<otimes> (x \<otimes> inv x) = z \<otimes> (x \<otimes> inv x)"
ballarin@27698
   341
    by (simp add: m_assoc [symmetric] del: r_inv Units_r_inv)
paulson@14963
   342
  with G show "y = z" by simp
ballarin@13813
   343
next
ballarin@13813
   344
  assume eq: "y = z"
wenzelm@14693
   345
    and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   346
  then show "y \<otimes> x = z \<otimes> x" by simp
ballarin@13813
   347
qed
ballarin@13813
   348
ballarin@13854
   349
lemma (in group) inv_one [simp]:
ballarin@13854
   350
  "inv \<one> = \<one>"
ballarin@13854
   351
proof -
ballarin@27698
   352
  have "inv \<one> = \<one> \<otimes> (inv \<one>)" by (simp del: r_inv Units_r_inv)
paulson@14963
   353
  moreover have "... = \<one>" by simp
ballarin@13854
   354
  finally show ?thesis .
ballarin@13854
   355
qed
ballarin@13854
   356
ballarin@13813
   357
lemma (in group) inv_inv [simp]:
ballarin@13813
   358
  "x \<in> carrier G ==> inv (inv x) = x"
ballarin@13936
   359
  using Units_inv_inv by simp
ballarin@13936
   360
ballarin@13936
   361
lemma (in group) inv_inj:
ballarin@13936
   362
  "inj_on (m_inv G) (carrier G)"
ballarin@13936
   363
  using inv_inj_on_Units by simp
ballarin@13813
   364
ballarin@13854
   365
lemma (in group) inv_mult_group:
ballarin@13813
   366
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv y \<otimes> inv x"
ballarin@13813
   367
proof -
wenzelm@14693
   368
  assume G: "x \<in> carrier G"  "y \<in> carrier G"
ballarin@13813
   369
  then have "inv (x \<otimes> y) \<otimes> (x \<otimes> y) = (inv y \<otimes> inv x) \<otimes> (x \<otimes> y)"
paulson@14963
   370
    by (simp add: m_assoc l_inv) (simp add: m_assoc [symmetric])
ballarin@27698
   371
  with G show ?thesis by (simp del: l_inv Units_l_inv)
ballarin@13813
   372
qed
ballarin@13813
   373
ballarin@13940
   374
lemma (in group) inv_comm:
ballarin@13940
   375
  "[| x \<otimes> y = \<one>; x \<in> carrier G; y \<in> carrier G |] ==> y \<otimes> x = \<one>"
wenzelm@14693
   376
  by (rule Units_inv_comm) auto
ballarin@13940
   377
paulson@13944
   378
lemma (in group) inv_equality:
paulson@13943
   379
     "[|y \<otimes> x = \<one>; x \<in> carrier G; y \<in> carrier G|] ==> inv x = y"
paulson@13943
   380
apply (simp add: m_inv_def)
paulson@13943
   381
apply (rule the_equality)
wenzelm@14693
   382
 apply (simp add: inv_comm [of y x])
wenzelm@14693
   383
apply (rule r_cancel [THEN iffD1], auto)
paulson@13943
   384
done
paulson@13943
   385
ballarin@13936
   386
text {* Power *}
ballarin@13936
   387
ballarin@13936
   388
lemma (in group) int_pow_def2:
ballarin@13936
   389
  "a (^) (z::int) = (if neg z then inv (a (^) (nat (-z))) else a (^) (nat z))"
ballarin@13936
   390
  by (simp add: int_pow_def nat_pow_def Let_def)
ballarin@13936
   391
ballarin@13936
   392
lemma (in group) int_pow_0 [simp]:
ballarin@13936
   393
  "x (^) (0::int) = \<one>"
ballarin@13936
   394
  by (simp add: int_pow_def2)
ballarin@13936
   395
ballarin@13936
   396
lemma (in group) int_pow_one [simp]:
ballarin@13936
   397
  "\<one> (^) (z::int) = \<one>"
ballarin@13936
   398
  by (simp add: int_pow_def2)
ballarin@13936
   399
ballarin@20318
   400
paulson@14963
   401
subsection {* Subgroups *}
ballarin@13813
   402
ballarin@19783
   403
locale subgroup =
ballarin@19783
   404
  fixes H and G (structure)
paulson@14963
   405
  assumes subset: "H \<subseteq> carrier G"
paulson@14963
   406
    and m_closed [intro, simp]: "\<lbrakk>x \<in> H; y \<in> H\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> H"
ballarin@20318
   407
    and one_closed [simp]: "\<one> \<in> H"
paulson@14963
   408
    and m_inv_closed [intro,simp]: "x \<in> H \<Longrightarrow> inv x \<in> H"
ballarin@13813
   409
ballarin@20318
   410
lemma (in subgroup) is_subgroup:
wenzelm@26199
   411
  "subgroup H G" by (rule subgroup_axioms)
ballarin@20318
   412
ballarin@13813
   413
declare (in subgroup) group.intro [intro]
ballarin@13949
   414
paulson@14963
   415
lemma (in subgroup) mem_carrier [simp]:
paulson@14963
   416
  "x \<in> H \<Longrightarrow> x \<in> carrier G"
paulson@14963
   417
  using subset by blast
ballarin@13813
   418
paulson@14963
   419
lemma subgroup_imp_subset:
paulson@14963
   420
  "subgroup H G \<Longrightarrow> H \<subseteq> carrier G"
paulson@14963
   421
  by (rule subgroup.subset)
paulson@14963
   422
paulson@14963
   423
lemma (in subgroup) subgroup_is_group [intro]:
ballarin@27611
   424
  assumes "group G"
ballarin@27611
   425
  shows "group (G\<lparr>carrier := H\<rparr>)"
ballarin@27611
   426
proof -
ballarin@29237
   427
  interpret group G by fact
ballarin@27611
   428
  show ?thesis
ballarin@27698
   429
    apply (rule monoid.group_l_invI)
ballarin@27698
   430
    apply (unfold_locales) [1]
ballarin@27698
   431
    apply (auto intro: m_assoc l_inv mem_carrier)
ballarin@27698
   432
    done
ballarin@27611
   433
qed
ballarin@13813
   434
ballarin@13813
   435
text {*
ballarin@13813
   436
  Since @{term H} is nonempty, it contains some element @{term x}.  Since
ballarin@13813
   437
  it is closed under inverse, it contains @{text "inv x"}.  Since
ballarin@13813
   438
  it is closed under product, it contains @{text "x \<otimes> inv x = \<one>"}.
ballarin@13813
   439
*}
ballarin@13813
   440
ballarin@13813
   441
lemma (in group) one_in_subset:
ballarin@13813
   442
  "[| 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
   443
   ==> \<one> \<in> H"
ballarin@13813
   444
by (force simp add: l_inv)
ballarin@13813
   445
ballarin@13813
   446
text {* A characterization of subgroups: closed, non-empty subset. *}
ballarin@13813
   447
ballarin@13813
   448
lemma (in group) subgroupI:
ballarin@13813
   449
  assumes subset: "H \<subseteq> carrier G" and non_empty: "H \<noteq> {}"
paulson@14963
   450
    and inv: "!!a. a \<in> H \<Longrightarrow> inv a \<in> H"
paulson@14963
   451
    and mult: "!!a b. \<lbrakk>a \<in> H; b \<in> H\<rbrakk> \<Longrightarrow> a \<otimes> b \<in> H"
ballarin@13813
   452
  shows "subgroup H G"
ballarin@27714
   453
proof (simp add: subgroup_def assms)
ballarin@27714
   454
  show "\<one> \<in> H" by (rule one_in_subset) (auto simp only: assms)
ballarin@13813
   455
qed
ballarin@13813
   456
ballarin@13936
   457
declare monoid.one_closed [iff] group.inv_closed [simp]
ballarin@13936
   458
  monoid.l_one [simp] monoid.r_one [simp] group.inv_inv [simp]
ballarin@13813
   459
ballarin@13813
   460
lemma subgroup_nonempty:
ballarin@13813
   461
  "~ subgroup {} G"
ballarin@13813
   462
  by (blast dest: subgroup.one_closed)
ballarin@13813
   463
ballarin@13813
   464
lemma (in subgroup) finite_imp_card_positive:
ballarin@13813
   465
  "finite (carrier G) ==> 0 < card H"
ballarin@13813
   466
proof (rule classical)
paulson@14963
   467
  assume "finite (carrier G)" "~ 0 < card H"
paulson@14963
   468
  then have "finite H" by (blast intro: finite_subset [OF subset])
paulson@14963
   469
  with prems have "subgroup {} G" by simp
ballarin@13813
   470
  with subgroup_nonempty show ?thesis by contradiction
ballarin@13813
   471
qed
ballarin@13813
   472
ballarin@13936
   473
(*
ballarin@13936
   474
lemma (in monoid) Units_subgroup:
ballarin@13936
   475
  "subgroup (Units G) G"
ballarin@13936
   476
*)
ballarin@13936
   477
ballarin@20318
   478
ballarin@13813
   479
subsection {* Direct Products *}
ballarin@13813
   480
paulson@14963
   481
constdefs
paulson@14963
   482
  DirProd :: "_ \<Rightarrow> _ \<Rightarrow> ('a \<times> 'b) monoid"  (infixr "\<times>\<times>" 80)
paulson@14963
   483
  "G \<times>\<times> H \<equiv> \<lparr>carrier = carrier G \<times> carrier H,
paulson@14963
   484
                mult = (\<lambda>(g, h) (g', h'). (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')),
paulson@14963
   485
                one = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)\<rparr>"
ballarin@13813
   486
paulson@14963
   487
lemma DirProd_monoid:
ballarin@27611
   488
  assumes "monoid G" and "monoid H"
paulson@14963
   489
  shows "monoid (G \<times>\<times> H)"
paulson@14963
   490
proof -
wenzelm@30729
   491
  interpret G: monoid G by fact
wenzelm@30729
   492
  interpret H: monoid H by fact
ballarin@27714
   493
  from assms
paulson@14963
   494
  show ?thesis by (unfold monoid_def DirProd_def, auto) 
paulson@14963
   495
qed
ballarin@13813
   496
ballarin@13813
   497
paulson@14963
   498
text{*Does not use the previous result because it's easier just to use auto.*}
paulson@14963
   499
lemma DirProd_group:
ballarin@27611
   500
  assumes "group G" and "group H"
paulson@14963
   501
  shows "group (G \<times>\<times> H)"
ballarin@27611
   502
proof -
wenzelm@30729
   503
  interpret G: group G by fact
wenzelm@30729
   504
  interpret H: group H by fact
ballarin@27611
   505
  show ?thesis by (rule groupI)
paulson@14963
   506
     (auto intro: G.m_assoc H.m_assoc G.l_inv H.l_inv
paulson@14963
   507
           simp add: DirProd_def)
ballarin@27611
   508
qed
ballarin@13813
   509
paulson@14963
   510
lemma carrier_DirProd [simp]:
paulson@14963
   511
     "carrier (G \<times>\<times> H) = carrier G \<times> carrier H"
paulson@14963
   512
  by (simp add: DirProd_def)
paulson@13944
   513
paulson@14963
   514
lemma one_DirProd [simp]:
paulson@14963
   515
     "\<one>\<^bsub>G \<times>\<times> H\<^esub> = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)"
paulson@14963
   516
  by (simp add: DirProd_def)
paulson@13944
   517
paulson@14963
   518
lemma mult_DirProd [simp]:
paulson@14963
   519
     "(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
   520
  by (simp add: DirProd_def)
paulson@13944
   521
paulson@14963
   522
lemma inv_DirProd [simp]:
ballarin@27611
   523
  assumes "group G" and "group H"
paulson@13944
   524
  assumes g: "g \<in> carrier G"
paulson@13944
   525
      and h: "h \<in> carrier H"
paulson@14963
   526
  shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
ballarin@27611
   527
proof -
wenzelm@30729
   528
  interpret G: group G by fact
wenzelm@30729
   529
  interpret H: group H by fact
wenzelm@30729
   530
  interpret Prod: group "G \<times>\<times> H"
ballarin@27714
   531
    by (auto intro: DirProd_group group.intro group.axioms assms)
paulson@14963
   532
  show ?thesis by (simp add: Prod.inv_equality g h)
paulson@14963
   533
qed
ballarin@27698
   534
paulson@14963
   535
paulson@14963
   536
subsection {* Homomorphisms and Isomorphisms *}
ballarin@13813
   537
wenzelm@14651
   538
constdefs (structure G and H)
wenzelm@14651
   539
  hom :: "_ => _ => ('a => 'b) set"
ballarin@13813
   540
  "hom G H ==
ballarin@13813
   541
    {h. h \<in> carrier G -> carrier H &
wenzelm@14693
   542
      (\<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
   543
paulson@14761
   544
lemma (in group) hom_compose:
nipkow@31754
   545
  "[|h \<in> hom G H; i \<in> hom H I|] ==> compose (carrier G) i h \<in> hom G I"
nipkow@31754
   546
by (fastsimp simp add: hom_def compose_def)
paulson@13943
   547
paulson@14803
   548
constdefs
paulson@14803
   549
  iso :: "_ => _ => ('a => 'b) set"  (infixr "\<cong>" 60)
paulson@14803
   550
  "G \<cong> H == {h. h \<in> hom G H & bij_betw h (carrier G) (carrier H)}"
paulson@14761
   551
paulson@14803
   552
lemma iso_refl: "(%x. x) \<in> G \<cong> G"
nipkow@31727
   553
by (simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def)
paulson@14761
   554
paulson@14761
   555
lemma (in group) iso_sym:
nipkow@33057
   556
     "h \<in> G \<cong> H \<Longrightarrow> inv_into (carrier G) h \<in> H \<cong> G"
nipkow@33057
   557
apply (simp add: iso_def bij_betw_inv_into) 
nipkow@33057
   558
apply (subgoal_tac "inv_into (carrier G) h \<in> carrier H \<rightarrow> carrier G") 
nipkow@33057
   559
 prefer 2 apply (simp add: bij_betw_imp_funcset [OF bij_betw_inv_into]) 
nipkow@33057
   560
apply (simp add: hom_def bij_betw_def inv_into_f_eq f_inv_into_f Pi_def)
paulson@14761
   561
done
paulson@14761
   562
paulson@14761
   563
lemma (in group) iso_trans: 
paulson@14803
   564
     "[|h \<in> G \<cong> H; i \<in> H \<cong> I|] ==> (compose (carrier G) i h) \<in> G \<cong> I"
paulson@14761
   565
by (auto simp add: iso_def hom_compose bij_betw_compose)
paulson@14761
   566
paulson@14963
   567
lemma DirProd_commute_iso:
paulson@14963
   568
  shows "(\<lambda>(x,y). (y,x)) \<in> (G \<times>\<times> H) \<cong> (H \<times>\<times> G)"
nipkow@31754
   569
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def)
paulson@14761
   570
paulson@14963
   571
lemma DirProd_assoc_iso:
paulson@14963
   572
  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
   573
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def)
paulson@14761
   574
paulson@14761
   575
paulson@14963
   576
text{*Basis for homomorphism proofs: we assume two groups @{term G} and
ballarin@15076
   577
  @{term H}, with a homomorphism @{term h} between them*}
ballarin@29237
   578
locale group_hom = G: group G + H: group H for G (structure) and H (structure) +
ballarin@29237
   579
  fixes h
ballarin@13813
   580
  assumes homh: "h \<in> hom G H"
ballarin@29240
   581
ballarin@29240
   582
lemma (in group_hom) hom_mult [simp]:
ballarin@29240
   583
  "[| 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
   584
proof -
ballarin@29240
   585
  assume "x \<in> carrier G" "y \<in> carrier G"
ballarin@29240
   586
  with homh [unfolded hom_def] show ?thesis by simp
ballarin@29240
   587
qed
ballarin@29240
   588
ballarin@29240
   589
lemma (in group_hom) hom_closed [simp]:
ballarin@29240
   590
  "x \<in> carrier G ==> h x \<in> carrier H"
ballarin@29240
   591
proof -
ballarin@29240
   592
  assume "x \<in> carrier G"
nipkow@31754
   593
  with homh [unfolded hom_def] show ?thesis by auto
ballarin@29240
   594
qed
ballarin@13813
   595
ballarin@13813
   596
lemma (in group_hom) one_closed [simp]:
ballarin@13813
   597
  "h \<one> \<in> carrier H"
ballarin@13813
   598
  by simp
ballarin@13813
   599
ballarin@13813
   600
lemma (in group_hom) hom_one [simp]:
wenzelm@14693
   601
  "h \<one> = \<one>\<^bsub>H\<^esub>"
ballarin@13813
   602
proof -
ballarin@15076
   603
  have "h \<one> \<otimes>\<^bsub>H\<^esub> \<one>\<^bsub>H\<^esub> = h \<one> \<otimes>\<^bsub>H\<^esub> h \<one>"
ballarin@13813
   604
    by (simp add: hom_mult [symmetric] del: hom_mult)
ballarin@13813
   605
  then show ?thesis by (simp del: r_one)
ballarin@13813
   606
qed
ballarin@13813
   607
ballarin@13813
   608
lemma (in group_hom) inv_closed [simp]:
ballarin@13813
   609
  "x \<in> carrier G ==> h (inv x) \<in> carrier H"
ballarin@13813
   610
  by simp
ballarin@13813
   611
ballarin@13813
   612
lemma (in group_hom) hom_inv [simp]:
wenzelm@14693
   613
  "x \<in> carrier G ==> h (inv x) = inv\<^bsub>H\<^esub> (h x)"
ballarin@13813
   614
proof -
ballarin@13813
   615
  assume x: "x \<in> carrier G"
wenzelm@14693
   616
  then have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = \<one>\<^bsub>H\<^esub>"
paulson@14963
   617
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   618
  also from x have "... = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)"
paulson@14963
   619
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   620
  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
   621
  with x show ?thesis by (simp del: H.r_inv H.Units_r_inv)
ballarin@13813
   622
qed
ballarin@13813
   623
ballarin@20318
   624
ballarin@13949
   625
subsection {* Commutative Structures *}
ballarin@13936
   626
ballarin@13936
   627
text {*
ballarin@13936
   628
  Naming convention: multiplicative structures that are commutative
ballarin@13936
   629
  are called \emph{commutative}, additive structures are called
ballarin@13936
   630
  \emph{Abelian}.
ballarin@13936
   631
*}
ballarin@13813
   632
paulson@14963
   633
locale comm_monoid = monoid +
paulson@14963
   634
  assumes m_comm: "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y = y \<otimes> x"
ballarin@13813
   635
paulson@14963
   636
lemma (in comm_monoid) m_lcomm:
paulson@14963
   637
  "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> \<Longrightarrow>
ballarin@13813
   638
   x \<otimes> (y \<otimes> z) = y \<otimes> (x \<otimes> z)"
ballarin@13813
   639
proof -
wenzelm@14693
   640
  assume xyz: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   641
  from xyz have "x \<otimes> (y \<otimes> z) = (x \<otimes> y) \<otimes> z" by (simp add: m_assoc)
ballarin@13813
   642
  also from xyz have "... = (y \<otimes> x) \<otimes> z" by (simp add: m_comm)
ballarin@13813
   643
  also from xyz have "... = y \<otimes> (x \<otimes> z)" by (simp add: m_assoc)
ballarin@13813
   644
  finally show ?thesis .
ballarin@13813
   645
qed
ballarin@13813
   646
paulson@14963
   647
lemmas (in comm_monoid) m_ac = m_assoc m_comm m_lcomm
ballarin@13813
   648
ballarin@13936
   649
lemma comm_monoidI:
ballarin@19783
   650
  fixes G (structure)
ballarin@13936
   651
  assumes m_closed:
wenzelm@14693
   652
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   653
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   654
    and m_assoc:
ballarin@13936
   655
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   656
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
wenzelm@14693
   657
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
ballarin@13936
   658
    and m_comm:
wenzelm@14693
   659
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   660
  shows "comm_monoid G"
ballarin@13936
   661
  using l_one
paulson@14963
   662
    by (auto intro!: comm_monoid.intro comm_monoid_axioms.intro monoid.intro 
ballarin@27714
   663
             intro: assms simp: m_closed one_closed m_comm)
ballarin@13817
   664
ballarin@13936
   665
lemma (in monoid) monoid_comm_monoidI:
ballarin@13936
   666
  assumes m_comm:
wenzelm@14693
   667
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   668
  shows "comm_monoid G"
ballarin@13936
   669
  by (rule comm_monoidI) (auto intro: m_assoc m_comm)
paulson@14963
   670
wenzelm@14693
   671
(*lemma (in comm_monoid) r_one [simp]:
ballarin@13817
   672
  "x \<in> carrier G ==> x \<otimes> \<one> = x"
ballarin@13817
   673
proof -
ballarin@13817
   674
  assume G: "x \<in> carrier G"
ballarin@13817
   675
  then have "x \<otimes> \<one> = \<one> \<otimes> x" by (simp add: m_comm)
ballarin@13817
   676
  also from G have "... = x" by simp
ballarin@13817
   677
  finally show ?thesis .
wenzelm@14693
   678
qed*)
paulson@14963
   679
ballarin@13936
   680
lemma (in comm_monoid) nat_pow_distr:
ballarin@13936
   681
  "[| x \<in> carrier G; y \<in> carrier G |] ==>
ballarin@13936
   682
  (x \<otimes> y) (^) (n::nat) = x (^) n \<otimes> y (^) n"
ballarin@13936
   683
  by (induct n) (simp, simp add: m_ac)
ballarin@13936
   684
ballarin@13936
   685
locale comm_group = comm_monoid + group
ballarin@13936
   686
ballarin@13936
   687
lemma (in group) group_comm_groupI:
ballarin@13936
   688
  assumes m_comm: "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==>
wenzelm@14693
   689
      x \<otimes> y = y \<otimes> x"
ballarin@13936
   690
  shows "comm_group G"
haftmann@28823
   691
  proof qed (simp_all add: m_comm)
ballarin@13817
   692
ballarin@13936
   693
lemma comm_groupI:
ballarin@19783
   694
  fixes G (structure)
ballarin@13936
   695
  assumes m_closed:
wenzelm@14693
   696
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   697
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   698
    and m_assoc:
ballarin@13936
   699
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   700
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
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"
wenzelm@14693
   703
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
paulson@14963
   704
    and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
ballarin@13936
   705
  shows "comm_group G"
ballarin@27714
   706
  by (fast intro: group.group_comm_groupI groupI assms)
ballarin@13936
   707
ballarin@13936
   708
lemma (in comm_group) inv_mult:
ballarin@13854
   709
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv x \<otimes> inv y"
ballarin@13936
   710
  by (simp add: m_ac inv_mult_group)
ballarin@13854
   711
ballarin@20318
   712
ballarin@20318
   713
subsection {* The Lattice of Subgroups of a Group *}
ballarin@14751
   714
ballarin@14751
   715
text_raw {* \label{sec:subgroup-lattice} *}
ballarin@14751
   716
ballarin@14751
   717
theorem (in group) subgroups_partial_order:
ballarin@27713
   718
  "partial_order (| carrier = {H. subgroup H G}, eq = op =, le = op \<subseteq> |)"
haftmann@28823
   719
  proof qed simp_all
ballarin@14751
   720
ballarin@14751
   721
lemma (in group) subgroup_self:
ballarin@14751
   722
  "subgroup (carrier G) G"
ballarin@14751
   723
  by (rule subgroupI) auto
ballarin@14751
   724
ballarin@14751
   725
lemma (in group) subgroup_imp_group:
ballarin@14751
   726
  "subgroup H G ==> group (G(| carrier := H |))"
wenzelm@26199
   727
  by (erule subgroup.subgroup_is_group) (rule group_axioms)
ballarin@14751
   728
ballarin@14751
   729
lemma (in group) is_monoid [intro, simp]:
ballarin@14751
   730
  "monoid G"
paulson@14963
   731
  by (auto intro: monoid.intro m_assoc) 
ballarin@14751
   732
ballarin@14751
   733
lemma (in group) subgroup_inv_equality:
ballarin@14751
   734
  "[| subgroup H G; x \<in> H |] ==> m_inv (G (| carrier := H |)) x = inv x"
ballarin@14751
   735
apply (rule_tac inv_equality [THEN sym])
paulson@14761
   736
  apply (rule group.l_inv [OF subgroup_imp_group, simplified], assumption+)
paulson@14761
   737
 apply (rule subsetD [OF subgroup.subset], assumption+)
paulson@14761
   738
apply (rule subsetD [OF subgroup.subset], assumption)
paulson@14761
   739
apply (rule_tac group.inv_closed [OF subgroup_imp_group, simplified], assumption+)
ballarin@14751
   740
done
ballarin@14751
   741
ballarin@14751
   742
theorem (in group) subgroups_Inter:
ballarin@14751
   743
  assumes subgr: "(!!H. H \<in> A ==> subgroup H G)"
ballarin@14751
   744
    and not_empty: "A ~= {}"
ballarin@14751
   745
  shows "subgroup (\<Inter>A) G"
ballarin@14751
   746
proof (rule subgroupI)
ballarin@14751
   747
  from subgr [THEN subgroup.subset] and not_empty
ballarin@14751
   748
  show "\<Inter>A \<subseteq> carrier G" by blast
ballarin@14751
   749
next
ballarin@14751
   750
  from subgr [THEN subgroup.one_closed]
ballarin@14751
   751
  show "\<Inter>A ~= {}" by blast
ballarin@14751
   752
next
ballarin@14751
   753
  fix x assume "x \<in> \<Inter>A"
ballarin@14751
   754
  with subgr [THEN subgroup.m_inv_closed]
ballarin@14751
   755
  show "inv x \<in> \<Inter>A" by blast
ballarin@14751
   756
next
ballarin@14751
   757
  fix x y assume "x \<in> \<Inter>A" "y \<in> \<Inter>A"
ballarin@14751
   758
  with subgr [THEN subgroup.m_closed]
ballarin@14751
   759
  show "x \<otimes> y \<in> \<Inter>A" by blast
ballarin@14751
   760
qed
ballarin@14751
   761
ballarin@14751
   762
theorem (in group) subgroups_complete_lattice:
ballarin@27713
   763
  "complete_lattice (| carrier = {H. subgroup H G}, eq = op =, le = op \<subseteq> |)"
ballarin@22063
   764
    (is "complete_lattice ?L")
ballarin@14751
   765
proof (rule partial_order.complete_lattice_criterion1)
ballarin@22063
   766
  show "partial_order ?L" by (rule subgroups_partial_order)
ballarin@14751
   767
next
berghofe@26805
   768
  show "\<exists>G. greatest ?L G (carrier ?L)"
berghofe@26805
   769
  proof
berghofe@26805
   770
    show "greatest ?L (carrier G) (carrier ?L)"
berghofe@26805
   771
      by (unfold greatest_def)
berghofe@26805
   772
        (simp add: subgroup.subset subgroup_self)
berghofe@26805
   773
  qed
ballarin@14751
   774
next
ballarin@14751
   775
  fix A
ballarin@22063
   776
  assume L: "A \<subseteq> carrier ?L" and non_empty: "A ~= {}"
ballarin@14751
   777
  then have Int_subgroup: "subgroup (\<Inter>A) G"
ballarin@14751
   778
    by (fastsimp intro: subgroups_Inter)
berghofe@26805
   779
  show "\<exists>I. greatest ?L I (Lower ?L A)"
berghofe@26805
   780
  proof
berghofe@26805
   781
    show "greatest ?L (\<Inter>A) (Lower ?L A)"
berghofe@26805
   782
      (is "greatest _ ?Int _")
berghofe@26805
   783
    proof (rule greatest_LowerI)
berghofe@26805
   784
      fix H
berghofe@26805
   785
      assume H: "H \<in> A"
berghofe@26805
   786
      with L have subgroupH: "subgroup H G" by auto
berghofe@26805
   787
      from subgroupH have groupH: "group (G (| carrier := H |))" (is "group ?H")
wenzelm@32960
   788
        by (rule subgroup_imp_group)
berghofe@26805
   789
      from groupH have monoidH: "monoid ?H"
wenzelm@32960
   790
        by (rule group.is_monoid)
berghofe@26805
   791
      from H have Int_subset: "?Int \<subseteq> H" by fastsimp
berghofe@26805
   792
      then show "le ?L ?Int H" by simp
berghofe@26805
   793
    next
berghofe@26805
   794
      fix H
berghofe@26805
   795
      assume H: "H \<in> Lower ?L A"
berghofe@26805
   796
      with L Int_subgroup show "le ?L H ?Int"
wenzelm@32960
   797
        by (fastsimp simp: Lower_def intro: Inter_greatest)
berghofe@26805
   798
    next
berghofe@26805
   799
      show "A \<subseteq> carrier ?L" by (rule L)
berghofe@26805
   800
    next
berghofe@26805
   801
      show "?Int \<in> carrier ?L" by simp (rule Int_subgroup)
berghofe@26805
   802
    qed
ballarin@14751
   803
  qed
ballarin@14751
   804
qed
ballarin@14751
   805
ballarin@13813
   806
end