src/HOL/Algebra/Group.thy
author paulson
Thu Jun 17 17:18:30 2004 +0200 (2004-06-17)
changeset 14963 d584e32f7d46
parent 14852 fffab59e0050
child 15076 4b3d280ef06a
permissions -rw-r--r--
removal of magmas and semigroups
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(*
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  Title:  HOL/Algebra/Group.thy
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  Id:     $Id$
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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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header {* Groups *}
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theory Group = FuncSet + Lattice:
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section {* Monoids and Groups *}
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text {*
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  Definitions follow \cite{Jacobson:1985}.
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*}
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subsection {* Definitions *}
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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 = struct G +
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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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  includes struct G
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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: prems)
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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_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:
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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:
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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)
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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"
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    by (simp add: Units_l_inv Units_r_inv)
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  with x show ?thesis by (simp add: Units_closed)
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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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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"
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  by (rule group.intro [OF prems]) 
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theorem groupI:
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  includes struct G
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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 (fast intro!: group.intro monoid.intro group_axioms.intro
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      carrier_subset_Units intro: prems r_one)
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qed
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lemma (in monoid) monoid_groupI:
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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 [simp]:
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  "x \<in> carrier G ==> inv x \<otimes> x = \<one>"
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  using Units_l_inv by simp
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subsection {* Cancellation Laws and Basic Properties *}
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lemma (in group) l_cancel [simp]:
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  "[| 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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  using Units_l_inv by simp
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lemma (in group) r_inv [simp]:
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  "x \<in> carrier G ==> x \<otimes> inv x = \<one>"
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proof -
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  assume x: "x \<in> carrier G"
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  then have "inv x \<otimes> (x \<otimes> inv x) = inv x \<otimes> \<one>"
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    by (simp add: m_assoc [symmetric] l_inv)
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  with x show ?thesis by (simp del: r_one)
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qed
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lemma (in group) r_cancel [simp]:
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  "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
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   (y \<otimes> x = z \<otimes> x) = (y = z)"
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proof
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  assume eq: "y \<otimes> x = z \<otimes> x"
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    and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
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  then have "y \<otimes> (x \<otimes> inv x) = z \<otimes> (x \<otimes> inv x)"
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    by (simp add: m_assoc [symmetric] del: r_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> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
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  then show "y \<otimes> x = z \<otimes> x" by simp
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qed
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lemma (in group) inv_one [simp]:
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  "inv \<one> = \<one>"
ballarin@13854
   311
proof -
paulson@14963
   312
  have "inv \<one> = \<one> \<otimes> (inv \<one>)" by (simp del: r_inv)
paulson@14963
   313
  moreover have "... = \<one>" by simp
ballarin@13854
   314
  finally show ?thesis .
ballarin@13854
   315
qed
ballarin@13854
   316
ballarin@13813
   317
lemma (in group) inv_inv [simp]:
ballarin@13813
   318
  "x \<in> carrier G ==> inv (inv x) = x"
ballarin@13936
   319
  using Units_inv_inv by simp
ballarin@13936
   320
ballarin@13936
   321
lemma (in group) inv_inj:
ballarin@13936
   322
  "inj_on (m_inv G) (carrier G)"
ballarin@13936
   323
  using inv_inj_on_Units by simp
ballarin@13813
   324
ballarin@13854
   325
lemma (in group) inv_mult_group:
ballarin@13813
   326
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv y \<otimes> inv x"
ballarin@13813
   327
proof -
wenzelm@14693
   328
  assume G: "x \<in> carrier G"  "y \<in> carrier G"
ballarin@13813
   329
  then have "inv (x \<otimes> y) \<otimes> (x \<otimes> y) = (inv y \<otimes> inv x) \<otimes> (x \<otimes> y)"
paulson@14963
   330
    by (simp add: m_assoc l_inv) (simp add: m_assoc [symmetric])
paulson@14963
   331
  with G show ?thesis by (simp del: l_inv)
ballarin@13813
   332
qed
ballarin@13813
   333
ballarin@13940
   334
lemma (in group) inv_comm:
ballarin@13940
   335
  "[| x \<otimes> y = \<one>; x \<in> carrier G; y \<in> carrier G |] ==> y \<otimes> x = \<one>"
wenzelm@14693
   336
  by (rule Units_inv_comm) auto
ballarin@13940
   337
paulson@13944
   338
lemma (in group) inv_equality:
paulson@13943
   339
     "[|y \<otimes> x = \<one>; x \<in> carrier G; y \<in> carrier G|] ==> inv x = y"
paulson@13943
   340
apply (simp add: m_inv_def)
paulson@13943
   341
apply (rule the_equality)
wenzelm@14693
   342
 apply (simp add: inv_comm [of y x])
wenzelm@14693
   343
apply (rule r_cancel [THEN iffD1], auto)
paulson@13943
   344
done
paulson@13943
   345
ballarin@13936
   346
text {* Power *}
ballarin@13936
   347
ballarin@13936
   348
lemma (in group) int_pow_def2:
ballarin@13936
   349
  "a (^) (z::int) = (if neg z then inv (a (^) (nat (-z))) else a (^) (nat z))"
ballarin@13936
   350
  by (simp add: int_pow_def nat_pow_def Let_def)
ballarin@13936
   351
ballarin@13936
   352
lemma (in group) int_pow_0 [simp]:
ballarin@13936
   353
  "x (^) (0::int) = \<one>"
ballarin@13936
   354
  by (simp add: int_pow_def2)
ballarin@13936
   355
ballarin@13936
   356
lemma (in group) int_pow_one [simp]:
ballarin@13936
   357
  "\<one> (^) (z::int) = \<one>"
ballarin@13936
   358
  by (simp add: int_pow_def2)
ballarin@13936
   359
paulson@14963
   360
subsection {* Subgroups *}
ballarin@13813
   361
paulson@14963
   362
locale subgroup = var H + struct G + 
paulson@14963
   363
  assumes subset: "H \<subseteq> carrier G"
paulson@14963
   364
    and m_closed [intro, simp]: "\<lbrakk>x \<in> H; y \<in> H\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> H"
paulson@14963
   365
    and  one_closed [simp]: "\<one> \<in> H"
paulson@14963
   366
    and m_inv_closed [intro,simp]: "x \<in> H \<Longrightarrow> inv x \<in> H"
ballarin@13813
   367
ballarin@13813
   368
declare (in subgroup) group.intro [intro]
ballarin@13949
   369
paulson@14963
   370
lemma (in subgroup) mem_carrier [simp]:
paulson@14963
   371
  "x \<in> H \<Longrightarrow> x \<in> carrier G"
paulson@14963
   372
  using subset by blast
ballarin@13813
   373
paulson@14963
   374
lemma subgroup_imp_subset:
paulson@14963
   375
  "subgroup H G \<Longrightarrow> H \<subseteq> carrier G"
paulson@14963
   376
  by (rule subgroup.subset)
paulson@14963
   377
paulson@14963
   378
lemma (in subgroup) subgroup_is_group [intro]:
ballarin@13813
   379
  includes group G
paulson@14963
   380
  shows "group (G\<lparr>carrier := H\<rparr>)" 
paulson@14963
   381
  by (rule groupI) (auto intro: m_assoc l_inv mem_carrier)
ballarin@13813
   382
ballarin@13813
   383
text {*
ballarin@13813
   384
  Since @{term H} is nonempty, it contains some element @{term x}.  Since
ballarin@13813
   385
  it is closed under inverse, it contains @{text "inv x"}.  Since
ballarin@13813
   386
  it is closed under product, it contains @{text "x \<otimes> inv x = \<one>"}.
ballarin@13813
   387
*}
ballarin@13813
   388
ballarin@13813
   389
lemma (in group) one_in_subset:
ballarin@13813
   390
  "[| 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
   391
   ==> \<one> \<in> H"
ballarin@13813
   392
by (force simp add: l_inv)
ballarin@13813
   393
ballarin@13813
   394
text {* A characterization of subgroups: closed, non-empty subset. *}
ballarin@13813
   395
ballarin@13813
   396
lemma (in group) subgroupI:
ballarin@13813
   397
  assumes subset: "H \<subseteq> carrier G" and non_empty: "H \<noteq> {}"
paulson@14963
   398
    and inv: "!!a. a \<in> H \<Longrightarrow> inv a \<in> H"
paulson@14963
   399
    and mult: "!!a b. \<lbrakk>a \<in> H; b \<in> H\<rbrakk> \<Longrightarrow> a \<otimes> b \<in> H"
ballarin@13813
   400
  shows "subgroup H G"
paulson@14963
   401
proof (simp add: subgroup_def prems)
paulson@14963
   402
  show "\<one> \<in> H" by (rule one_in_subset) (auto simp only: prems)
ballarin@13813
   403
qed
ballarin@13813
   404
ballarin@13936
   405
declare monoid.one_closed [iff] group.inv_closed [simp]
ballarin@13936
   406
  monoid.l_one [simp] monoid.r_one [simp] group.inv_inv [simp]
ballarin@13813
   407
ballarin@13813
   408
lemma subgroup_nonempty:
ballarin@13813
   409
  "~ subgroup {} G"
ballarin@13813
   410
  by (blast dest: subgroup.one_closed)
ballarin@13813
   411
ballarin@13813
   412
lemma (in subgroup) finite_imp_card_positive:
ballarin@13813
   413
  "finite (carrier G) ==> 0 < card H"
ballarin@13813
   414
proof (rule classical)
paulson@14963
   415
  assume "finite (carrier G)" "~ 0 < card H"
paulson@14963
   416
  then have "finite H" by (blast intro: finite_subset [OF subset])
paulson@14963
   417
  with prems have "subgroup {} G" by simp
ballarin@13813
   418
  with subgroup_nonempty show ?thesis by contradiction
ballarin@13813
   419
qed
ballarin@13813
   420
ballarin@13936
   421
(*
ballarin@13936
   422
lemma (in monoid) Units_subgroup:
ballarin@13936
   423
  "subgroup (Units G) G"
ballarin@13936
   424
*)
ballarin@13936
   425
ballarin@13813
   426
subsection {* Direct Products *}
ballarin@13813
   427
paulson@14963
   428
constdefs
paulson@14963
   429
  DirProd :: "_ \<Rightarrow> _ \<Rightarrow> ('a \<times> 'b) monoid"  (infixr "\<times>\<times>" 80)
paulson@14963
   430
  "G \<times>\<times> H \<equiv> \<lparr>carrier = carrier G \<times> carrier H,
paulson@14963
   431
                mult = (\<lambda>(g, h) (g', h'). (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')),
paulson@14963
   432
                one = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)\<rparr>"
ballarin@13813
   433
paulson@14963
   434
lemma DirProd_monoid:
paulson@14963
   435
  includes monoid G + monoid H
paulson@14963
   436
  shows "monoid (G \<times>\<times> H)"
paulson@14963
   437
proof -
paulson@14963
   438
  from prems
paulson@14963
   439
  show ?thesis by (unfold monoid_def DirProd_def, auto) 
paulson@14963
   440
qed
ballarin@13813
   441
ballarin@13813
   442
paulson@14963
   443
text{*Does not use the previous result because it's easier just to use auto.*}
paulson@14963
   444
lemma DirProd_group:
ballarin@13813
   445
  includes group G + group H
paulson@14963
   446
  shows "group (G \<times>\<times> H)"
ballarin@13936
   447
  by (rule groupI)
paulson@14963
   448
     (auto intro: G.m_assoc H.m_assoc G.l_inv H.l_inv
paulson@14963
   449
           simp add: DirProd_def)
ballarin@13813
   450
paulson@14963
   451
lemma carrier_DirProd [simp]:
paulson@14963
   452
     "carrier (G \<times>\<times> H) = carrier G \<times> carrier H"
paulson@14963
   453
  by (simp add: DirProd_def)
paulson@13944
   454
paulson@14963
   455
lemma one_DirProd [simp]:
paulson@14963
   456
     "\<one>\<^bsub>G \<times>\<times> H\<^esub> = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)"
paulson@14963
   457
  by (simp add: DirProd_def)
paulson@13944
   458
paulson@14963
   459
lemma mult_DirProd [simp]:
paulson@14963
   460
     "(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
   461
  by (simp add: DirProd_def)
paulson@13944
   462
paulson@14963
   463
lemma inv_DirProd [simp]:
paulson@13944
   464
  includes group G + group H
paulson@13944
   465
  assumes g: "g \<in> carrier G"
paulson@13944
   466
      and h: "h \<in> carrier H"
paulson@14963
   467
  shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
paulson@14963
   468
  apply (rule group.inv_equality [OF DirProd_group])
paulson@13944
   469
  apply (simp_all add: prems group_def group.l_inv)
paulson@13944
   470
  done
paulson@13944
   471
paulson@14963
   472
text{*This alternative proof of the previous result demonstrates instantiate.
paulson@14963
   473
   It uses @{text Prod.inv_equality} (available after instantiation) instead of
paulson@14963
   474
   @{text "group.inv_equality [OF DirProd_group]"}. *}
paulson@14963
   475
lemma
paulson@14963
   476
  includes group G + group H
paulson@14963
   477
  assumes g: "g \<in> carrier G"
paulson@14963
   478
      and h: "h \<in> carrier H"
paulson@14963
   479
  shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
paulson@14963
   480
proof -
paulson@14963
   481
  have "group (G \<times>\<times> H)"
paulson@14963
   482
    by (blast intro: DirProd_group group.intro prems)
paulson@14963
   483
  then instantiate Prod: group
paulson@14963
   484
  show ?thesis by (simp add: Prod.inv_equality g h)
paulson@14963
   485
qed
paulson@14963
   486
  
paulson@14963
   487
paulson@14963
   488
subsection {* Homomorphisms and Isomorphisms *}
ballarin@13813
   489
wenzelm@14651
   490
constdefs (structure G and H)
wenzelm@14651
   491
  hom :: "_ => _ => ('a => 'b) set"
ballarin@13813
   492
  "hom G H ==
ballarin@13813
   493
    {h. h \<in> carrier G -> carrier H &
wenzelm@14693
   494
      (\<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
   495
ballarin@13813
   496
lemma hom_mult:
wenzelm@14693
   497
  "[| h \<in> hom G H; x \<in> carrier G; y \<in> carrier G |]
wenzelm@14693
   498
   ==> h (x \<otimes>\<^bsub>G\<^esub> y) = h x \<otimes>\<^bsub>H\<^esub> h y"
wenzelm@14693
   499
  by (simp add: hom_def)
ballarin@13813
   500
ballarin@13813
   501
lemma hom_closed:
ballarin@13813
   502
  "[| h \<in> hom G H; x \<in> carrier G |] ==> h x \<in> carrier H"
ballarin@13813
   503
  by (auto simp add: hom_def funcset_mem)
ballarin@13813
   504
paulson@14761
   505
lemma (in group) hom_compose:
paulson@14761
   506
     "[|h \<in> hom G H; i \<in> hom H I|] ==> compose (carrier G) i h \<in> hom G I"
paulson@14761
   507
apply (auto simp add: hom_def funcset_compose) 
paulson@14761
   508
apply (simp add: compose_def funcset_mem)
paulson@13943
   509
done
paulson@13943
   510
paulson@14761
   511
paulson@14761
   512
subsection {* Isomorphisms *}
paulson@14761
   513
paulson@14803
   514
constdefs
paulson@14803
   515
  iso :: "_ => _ => ('a => 'b) set"  (infixr "\<cong>" 60)
paulson@14803
   516
  "G \<cong> H == {h. h \<in> hom G H & bij_betw h (carrier G) (carrier H)}"
paulson@14761
   517
paulson@14803
   518
lemma iso_refl: "(%x. x) \<in> G \<cong> G"
paulson@14761
   519
by (simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
paulson@14761
   520
paulson@14761
   521
lemma (in group) iso_sym:
paulson@14803
   522
     "h \<in> G \<cong> H \<Longrightarrow> Inv (carrier G) h \<in> H \<cong> G"
paulson@14761
   523
apply (simp add: iso_def bij_betw_Inv) 
paulson@14761
   524
apply (subgoal_tac "Inv (carrier G) h \<in> carrier H \<rightarrow> carrier G") 
paulson@14761
   525
 prefer 2 apply (simp add: bij_betw_imp_funcset [OF bij_betw_Inv]) 
paulson@14761
   526
apply (simp add: hom_def bij_betw_def Inv_f_eq funcset_mem f_Inv_f) 
paulson@14761
   527
done
paulson@14761
   528
paulson@14761
   529
lemma (in group) iso_trans: 
paulson@14803
   530
     "[|h \<in> G \<cong> H; i \<in> H \<cong> I|] ==> (compose (carrier G) i h) \<in> G \<cong> I"
paulson@14761
   531
by (auto simp add: iso_def hom_compose bij_betw_compose)
paulson@14761
   532
paulson@14963
   533
lemma DirProd_commute_iso:
paulson@14963
   534
  shows "(\<lambda>(x,y). (y,x)) \<in> (G \<times>\<times> H) \<cong> (H \<times>\<times> G)"
paulson@14761
   535
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
paulson@14761
   536
paulson@14963
   537
lemma DirProd_assoc_iso:
paulson@14963
   538
  shows "(\<lambda>(x,y,z). (x,(y,z))) \<in> (G \<times>\<times> H \<times>\<times> I) \<cong> (G \<times>\<times> (H \<times>\<times> I))"
paulson@14761
   539
by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
paulson@14761
   540
paulson@14761
   541
paulson@14963
   542
text{*Basis for homomorphism proofs: we assume two groups @{term G} and
paulson@14963
   543
  @term{H}, with a homomorphism @{term h} between them*}
ballarin@13813
   544
locale group_hom = group G + group H + var h +
ballarin@13813
   545
  assumes homh: "h \<in> hom G H"
ballarin@13813
   546
  notes hom_mult [simp] = hom_mult [OF homh]
ballarin@13813
   547
    and hom_closed [simp] = hom_closed [OF homh]
ballarin@13813
   548
ballarin@13813
   549
lemma (in group_hom) one_closed [simp]:
ballarin@13813
   550
  "h \<one> \<in> carrier H"
ballarin@13813
   551
  by simp
ballarin@13813
   552
ballarin@13813
   553
lemma (in group_hom) hom_one [simp]:
wenzelm@14693
   554
  "h \<one> = \<one>\<^bsub>H\<^esub>"
ballarin@13813
   555
proof -
wenzelm@14693
   556
  have "h \<one> \<otimes>\<^bsub>H\<^esub> \<one>\<^bsub>H\<^esub> = h \<one> \<otimes>\<^sub>2 h \<one>"
ballarin@13813
   557
    by (simp add: hom_mult [symmetric] del: hom_mult)
ballarin@13813
   558
  then show ?thesis by (simp del: r_one)
ballarin@13813
   559
qed
ballarin@13813
   560
ballarin@13813
   561
lemma (in group_hom) inv_closed [simp]:
ballarin@13813
   562
  "x \<in> carrier G ==> h (inv x) \<in> carrier H"
ballarin@13813
   563
  by simp
ballarin@13813
   564
ballarin@13813
   565
lemma (in group_hom) hom_inv [simp]:
wenzelm@14693
   566
  "x \<in> carrier G ==> h (inv x) = inv\<^bsub>H\<^esub> (h x)"
ballarin@13813
   567
proof -
ballarin@13813
   568
  assume x: "x \<in> carrier G"
wenzelm@14693
   569
  then have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = \<one>\<^bsub>H\<^esub>"
paulson@14963
   570
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   571
  also from x have "... = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)"
paulson@14963
   572
    by (simp add: hom_mult [symmetric] del: hom_mult)
wenzelm@14693
   573
  finally have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)" .
paulson@14963
   574
  with x show ?thesis by (simp del: H.r_inv)
ballarin@13813
   575
qed
ballarin@13813
   576
ballarin@13949
   577
subsection {* Commutative Structures *}
ballarin@13936
   578
ballarin@13936
   579
text {*
ballarin@13936
   580
  Naming convention: multiplicative structures that are commutative
ballarin@13936
   581
  are called \emph{commutative}, additive structures are called
ballarin@13936
   582
  \emph{Abelian}.
ballarin@13936
   583
*}
ballarin@13813
   584
ballarin@13813
   585
subsection {* Definition *}
ballarin@13813
   586
paulson@14963
   587
locale comm_monoid = monoid +
paulson@14963
   588
  assumes m_comm: "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y = y \<otimes> x"
ballarin@13813
   589
paulson@14963
   590
lemma (in comm_monoid) m_lcomm:
paulson@14963
   591
  "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> \<Longrightarrow>
ballarin@13813
   592
   x \<otimes> (y \<otimes> z) = y \<otimes> (x \<otimes> z)"
ballarin@13813
   593
proof -
wenzelm@14693
   594
  assume xyz: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
ballarin@13813
   595
  from xyz have "x \<otimes> (y \<otimes> z) = (x \<otimes> y) \<otimes> z" by (simp add: m_assoc)
ballarin@13813
   596
  also from xyz have "... = (y \<otimes> x) \<otimes> z" by (simp add: m_comm)
ballarin@13813
   597
  also from xyz have "... = y \<otimes> (x \<otimes> z)" by (simp add: m_assoc)
ballarin@13813
   598
  finally show ?thesis .
ballarin@13813
   599
qed
ballarin@13813
   600
paulson@14963
   601
lemmas (in comm_monoid) m_ac = m_assoc m_comm m_lcomm
ballarin@13813
   602
ballarin@13936
   603
lemma comm_monoidI:
wenzelm@14693
   604
  includes struct G
ballarin@13936
   605
  assumes m_closed:
wenzelm@14693
   606
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   607
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   608
    and m_assoc:
ballarin@13936
   609
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   610
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
wenzelm@14693
   611
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
ballarin@13936
   612
    and m_comm:
wenzelm@14693
   613
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   614
  shows "comm_monoid G"
ballarin@13936
   615
  using l_one
paulson@14963
   616
    by (auto intro!: comm_monoid.intro comm_monoid_axioms.intro monoid.intro 
paulson@14963
   617
             intro: prems simp: m_closed one_closed m_comm)
ballarin@13817
   618
ballarin@13936
   619
lemma (in monoid) monoid_comm_monoidI:
ballarin@13936
   620
  assumes m_comm:
wenzelm@14693
   621
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
ballarin@13936
   622
  shows "comm_monoid G"
ballarin@13936
   623
  by (rule comm_monoidI) (auto intro: m_assoc m_comm)
paulson@14963
   624
wenzelm@14693
   625
(*lemma (in comm_monoid) r_one [simp]:
ballarin@13817
   626
  "x \<in> carrier G ==> x \<otimes> \<one> = x"
ballarin@13817
   627
proof -
ballarin@13817
   628
  assume G: "x \<in> carrier G"
ballarin@13817
   629
  then have "x \<otimes> \<one> = \<one> \<otimes> x" by (simp add: m_comm)
ballarin@13817
   630
  also from G have "... = x" by simp
ballarin@13817
   631
  finally show ?thesis .
wenzelm@14693
   632
qed*)
paulson@14963
   633
ballarin@13936
   634
lemma (in comm_monoid) nat_pow_distr:
ballarin@13936
   635
  "[| x \<in> carrier G; y \<in> carrier G |] ==>
ballarin@13936
   636
  (x \<otimes> y) (^) (n::nat) = x (^) n \<otimes> y (^) n"
ballarin@13936
   637
  by (induct n) (simp, simp add: m_ac)
ballarin@13936
   638
ballarin@13936
   639
locale comm_group = comm_monoid + group
ballarin@13936
   640
ballarin@13936
   641
lemma (in group) group_comm_groupI:
ballarin@13936
   642
  assumes m_comm: "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==>
wenzelm@14693
   643
      x \<otimes> y = y \<otimes> x"
ballarin@13936
   644
  shows "comm_group G"
paulson@14963
   645
  by (fast intro: comm_group.intro comm_monoid_axioms.intro
paulson@14761
   646
                  is_group prems)
ballarin@13817
   647
ballarin@13936
   648
lemma comm_groupI:
wenzelm@14693
   649
  includes struct G
ballarin@13936
   650
  assumes m_closed:
wenzelm@14693
   651
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
wenzelm@14693
   652
    and one_closed: "\<one> \<in> carrier G"
ballarin@13936
   653
    and m_assoc:
ballarin@13936
   654
      "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
wenzelm@14693
   655
      (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
ballarin@13936
   656
    and m_comm:
wenzelm@14693
   657
      "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
wenzelm@14693
   658
    and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
paulson@14963
   659
    and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
ballarin@13936
   660
  shows "comm_group G"
ballarin@13936
   661
  by (fast intro: group.group_comm_groupI groupI prems)
ballarin@13936
   662
ballarin@13936
   663
lemma (in comm_group) inv_mult:
ballarin@13854
   664
  "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv x \<otimes> inv y"
ballarin@13936
   665
  by (simp add: m_ac inv_mult_group)
ballarin@13854
   666
ballarin@14751
   667
subsection {* Lattice of subgroups of a group *}
ballarin@14751
   668
ballarin@14751
   669
text_raw {* \label{sec:subgroup-lattice} *}
ballarin@14751
   670
ballarin@14751
   671
theorem (in group) subgroups_partial_order:
ballarin@14751
   672
  "partial_order (| carrier = {H. subgroup H G}, le = op \<subseteq> |)"
ballarin@14751
   673
  by (rule partial_order.intro) simp_all
ballarin@14751
   674
ballarin@14751
   675
lemma (in group) subgroup_self:
ballarin@14751
   676
  "subgroup (carrier G) G"
ballarin@14751
   677
  by (rule subgroupI) auto
ballarin@14751
   678
ballarin@14751
   679
lemma (in group) subgroup_imp_group:
ballarin@14751
   680
  "subgroup H G ==> group (G(| carrier := H |))"
paulson@14963
   681
  using subgroup.subgroup_is_group [OF _ group.intro] .
ballarin@14751
   682
ballarin@14751
   683
lemma (in group) is_monoid [intro, simp]:
ballarin@14751
   684
  "monoid G"
paulson@14963
   685
  by (auto intro: monoid.intro m_assoc) 
ballarin@14751
   686
ballarin@14751
   687
lemma (in group) subgroup_inv_equality:
ballarin@14751
   688
  "[| subgroup H G; x \<in> H |] ==> m_inv (G (| carrier := H |)) x = inv x"
ballarin@14751
   689
apply (rule_tac inv_equality [THEN sym])
paulson@14761
   690
  apply (rule group.l_inv [OF subgroup_imp_group, simplified], assumption+)
paulson@14761
   691
 apply (rule subsetD [OF subgroup.subset], assumption+)
paulson@14761
   692
apply (rule subsetD [OF subgroup.subset], assumption)
paulson@14761
   693
apply (rule_tac group.inv_closed [OF subgroup_imp_group, simplified], assumption+)
ballarin@14751
   694
done
ballarin@14751
   695
ballarin@14751
   696
theorem (in group) subgroups_Inter:
ballarin@14751
   697
  assumes subgr: "(!!H. H \<in> A ==> subgroup H G)"
ballarin@14751
   698
    and not_empty: "A ~= {}"
ballarin@14751
   699
  shows "subgroup (\<Inter>A) G"
ballarin@14751
   700
proof (rule subgroupI)
ballarin@14751
   701
  from subgr [THEN subgroup.subset] and not_empty
ballarin@14751
   702
  show "\<Inter>A \<subseteq> carrier G" by blast
ballarin@14751
   703
next
ballarin@14751
   704
  from subgr [THEN subgroup.one_closed]
ballarin@14751
   705
  show "\<Inter>A ~= {}" by blast
ballarin@14751
   706
next
ballarin@14751
   707
  fix x assume "x \<in> \<Inter>A"
ballarin@14751
   708
  with subgr [THEN subgroup.m_inv_closed]
ballarin@14751
   709
  show "inv x \<in> \<Inter>A" by blast
ballarin@14751
   710
next
ballarin@14751
   711
  fix x y assume "x \<in> \<Inter>A" "y \<in> \<Inter>A"
ballarin@14751
   712
  with subgr [THEN subgroup.m_closed]
ballarin@14751
   713
  show "x \<otimes> y \<in> \<Inter>A" by blast
ballarin@14751
   714
qed
ballarin@14751
   715
ballarin@14751
   716
theorem (in group) subgroups_complete_lattice:
ballarin@14751
   717
  "complete_lattice (| carrier = {H. subgroup H G}, le = op \<subseteq> |)"
ballarin@14751
   718
    (is "complete_lattice ?L")
ballarin@14751
   719
proof (rule partial_order.complete_lattice_criterion1)
ballarin@14751
   720
  show "partial_order ?L" by (rule subgroups_partial_order)
ballarin@14751
   721
next
ballarin@14751
   722
  have "greatest ?L (carrier G) (carrier ?L)"
ballarin@14751
   723
    by (unfold greatest_def) (simp add: subgroup.subset subgroup_self)
paulson@14963
   724
  then show "\<exists>G. greatest ?L G (carrier ?L)" ..
ballarin@14751
   725
next
ballarin@14751
   726
  fix A
ballarin@14751
   727
  assume L: "A \<subseteq> carrier ?L" and non_empty: "A ~= {}"
ballarin@14751
   728
  then have Int_subgroup: "subgroup (\<Inter>A) G"
ballarin@14751
   729
    by (fastsimp intro: subgroups_Inter)
ballarin@14751
   730
  have "greatest ?L (\<Inter>A) (Lower ?L A)"
ballarin@14751
   731
    (is "greatest ?L ?Int _")
ballarin@14751
   732
  proof (rule greatest_LowerI)
ballarin@14751
   733
    fix H
ballarin@14751
   734
    assume H: "H \<in> A"
ballarin@14751
   735
    with L have subgroupH: "subgroup H G" by auto
ballarin@14751
   736
    from subgroupH have groupH: "group (G (| carrier := H |))" (is "group ?H")
ballarin@14751
   737
      by (rule subgroup_imp_group)
ballarin@14751
   738
    from groupH have monoidH: "monoid ?H"
ballarin@14751
   739
      by (rule group.is_monoid)
ballarin@14751
   740
    from H have Int_subset: "?Int \<subseteq> H" by fastsimp
ballarin@14751
   741
    then show "le ?L ?Int H" by simp
ballarin@14751
   742
  next
ballarin@14751
   743
    fix H
ballarin@14751
   744
    assume H: "H \<in> Lower ?L A"
ballarin@14751
   745
    with L Int_subgroup show "le ?L H ?Int" by (fastsimp intro: Inter_greatest)
ballarin@14751
   746
  next
ballarin@14751
   747
    show "A \<subseteq> carrier ?L" by (rule L)
ballarin@14751
   748
  next
ballarin@14751
   749
    show "?Int \<in> carrier ?L" by simp (rule Int_subgroup)
ballarin@14751
   750
  qed
paulson@14963
   751
  then show "\<exists>I. greatest ?L I (Lower ?L A)" ..
ballarin@14751
   752
qed
ballarin@14751
   753
ballarin@13813
   754
end