src/HOL/Algebra/Group.thy
author wenzelm
Sat May 20 23:36:51 2006 +0200 (2006-05-20)
changeset 19684 6101fbebda1d
parent 16417 9bc16273c2d4
child 19699 1ecda5544e88
permissions -rw-r--r--
pow: unchecked;
     1 (*
     2   Title:  HOL/Algebra/Group.thy
     3   Id:     $Id$
     4   Author: Clemens Ballarin, started 4 February 2003
     5 
     6 Based on work by Florian Kammueller, L C Paulson and Markus Wenzel.
     7 *)
     8 
     9 header {* Groups *}
    10 
    11 theory Group imports FuncSet Lattice begin
    12 
    13 
    14 section {* Monoids and Groups *}
    15 
    16 text {*
    17   Definitions follow \cite{Jacobson:1985}.
    18 *}
    19 
    20 subsection {* Definitions *}
    21 
    22 record 'a monoid =  "'a partial_object" +
    23   mult    :: "['a, 'a] \<Rightarrow> 'a" (infixl "\<otimes>\<index>" 70)
    24   one     :: 'a ("\<one>\<index>")
    25 
    26 constdefs (structure G)
    27   m_inv :: "('a, 'b) monoid_scheme => 'a => 'a" ("inv\<index> _" [81] 80)
    28   "inv x == (THE y. y \<in> carrier G & x \<otimes> y = \<one> & y \<otimes> x = \<one>)"
    29 
    30   Units :: "_ => 'a set"
    31   --{*The set of invertible elements*}
    32   "Units G == {y. y \<in> carrier G & (\<exists>x \<in> carrier G. x \<otimes> y = \<one> & y \<otimes> x = \<one>)}"
    33 
    34 consts
    35   pow :: "[('a, 'm) monoid_scheme, 'a, 'b::number] => 'a" (infixr "'(^')\<index>" 75)
    36 
    37 defs (unchecked overloaded)
    38   nat_pow_def: "pow G a n == nat_rec \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a) n"
    39   int_pow_def: "pow G a z ==
    40     let p = nat_rec \<one>\<^bsub>G\<^esub> (%u b. b \<otimes>\<^bsub>G\<^esub> a)
    41     in if neg z then inv\<^bsub>G\<^esub> (p (nat (-z))) else p (nat z)"
    42 
    43 locale monoid = struct G +
    44   assumes m_closed [intro, simp]:
    45          "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> carrier G"
    46       and m_assoc:
    47          "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> 
    48           \<Longrightarrow> (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
    49       and one_closed [intro, simp]: "\<one> \<in> carrier G"
    50       and l_one [simp]: "x \<in> carrier G \<Longrightarrow> \<one> \<otimes> x = x"
    51       and r_one [simp]: "x \<in> carrier G \<Longrightarrow> x \<otimes> \<one> = x"
    52 
    53 lemma monoidI:
    54   includes struct G
    55   assumes m_closed:
    56       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
    57     and one_closed: "\<one> \<in> carrier G"
    58     and m_assoc:
    59       "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
    60       (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
    61     and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
    62     and r_one: "!!x. x \<in> carrier G ==> x \<otimes> \<one> = x"
    63   shows "monoid G"
    64   by (fast intro!: monoid.intro intro: prems)
    65 
    66 lemma (in monoid) Units_closed [dest]:
    67   "x \<in> Units G ==> x \<in> carrier G"
    68   by (unfold Units_def) fast
    69 
    70 lemma (in monoid) inv_unique:
    71   assumes eq: "y \<otimes> x = \<one>"  "x \<otimes> y' = \<one>"
    72     and G: "x \<in> carrier G"  "y \<in> carrier G"  "y' \<in> carrier G"
    73   shows "y = y'"
    74 proof -
    75   from G eq have "y = y \<otimes> (x \<otimes> y')" by simp
    76   also from G have "... = (y \<otimes> x) \<otimes> y'" by (simp add: m_assoc)
    77   also from G eq have "... = y'" by simp
    78   finally show ?thesis .
    79 qed
    80 
    81 lemma (in monoid) Units_one_closed [intro, simp]:
    82   "\<one> \<in> Units G"
    83   by (unfold Units_def) auto
    84 
    85 lemma (in monoid) Units_inv_closed [intro, simp]:
    86   "x \<in> Units G ==> inv x \<in> carrier G"
    87   apply (unfold Units_def m_inv_def, auto)
    88   apply (rule theI2, fast)
    89    apply (fast intro: inv_unique, fast)
    90   done
    91 
    92 lemma (in monoid) Units_l_inv:
    93   "x \<in> Units G ==> inv x \<otimes> x = \<one>"
    94   apply (unfold Units_def m_inv_def, auto)
    95   apply (rule theI2, fast)
    96    apply (fast intro: inv_unique, fast)
    97   done
    98 
    99 lemma (in monoid) Units_r_inv:
   100   "x \<in> Units G ==> x \<otimes> inv x = \<one>"
   101   apply (unfold Units_def m_inv_def, auto)
   102   apply (rule theI2, fast)
   103    apply (fast intro: inv_unique, fast)
   104   done
   105 
   106 lemma (in monoid) Units_inv_Units [intro, simp]:
   107   "x \<in> Units G ==> inv x \<in> Units G"
   108 proof -
   109   assume x: "x \<in> Units G"
   110   show "inv x \<in> Units G"
   111     by (auto simp add: Units_def
   112       intro: Units_l_inv Units_r_inv x Units_closed [OF x])
   113 qed
   114 
   115 lemma (in monoid) Units_l_cancel [simp]:
   116   "[| x \<in> Units G; y \<in> carrier G; z \<in> carrier G |] ==>
   117    (x \<otimes> y = x \<otimes> z) = (y = z)"
   118 proof
   119   assume eq: "x \<otimes> y = x \<otimes> z"
   120     and G: "x \<in> Units G"  "y \<in> carrier G"  "z \<in> carrier G"
   121   then have "(inv x \<otimes> x) \<otimes> y = (inv x \<otimes> x) \<otimes> z"
   122     by (simp add: m_assoc Units_closed)
   123   with G show "y = z" by (simp add: Units_l_inv)
   124 next
   125   assume eq: "y = z"
   126     and G: "x \<in> Units G"  "y \<in> carrier G"  "z \<in> carrier G"
   127   then show "x \<otimes> y = x \<otimes> z" by simp
   128 qed
   129 
   130 lemma (in monoid) Units_inv_inv [simp]:
   131   "x \<in> Units G ==> inv (inv x) = x"
   132 proof -
   133   assume x: "x \<in> Units G"
   134   then have "inv x \<otimes> inv (inv x) = inv x \<otimes> x"
   135     by (simp add: Units_l_inv Units_r_inv)
   136   with x show ?thesis by (simp add: Units_closed)
   137 qed
   138 
   139 lemma (in monoid) inv_inj_on_Units:
   140   "inj_on (m_inv G) (Units G)"
   141 proof (rule inj_onI)
   142   fix x y
   143   assume G: "x \<in> Units G"  "y \<in> Units G" and eq: "inv x = inv y"
   144   then have "inv (inv x) = inv (inv y)" by simp
   145   with G show "x = y" by simp
   146 qed
   147 
   148 lemma (in monoid) Units_inv_comm:
   149   assumes inv: "x \<otimes> y = \<one>"
   150     and G: "x \<in> Units G"  "y \<in> Units G"
   151   shows "y \<otimes> x = \<one>"
   152 proof -
   153   from G have "x \<otimes> y \<otimes> x = x \<otimes> \<one>" by (auto simp add: inv Units_closed)
   154   with G show ?thesis by (simp del: r_one add: m_assoc Units_closed)
   155 qed
   156 
   157 text {* Power *}
   158 
   159 lemma (in monoid) nat_pow_closed [intro, simp]:
   160   "x \<in> carrier G ==> x (^) (n::nat) \<in> carrier G"
   161   by (induct n) (simp_all add: nat_pow_def)
   162 
   163 lemma (in monoid) nat_pow_0 [simp]:
   164   "x (^) (0::nat) = \<one>"
   165   by (simp add: nat_pow_def)
   166 
   167 lemma (in monoid) nat_pow_Suc [simp]:
   168   "x (^) (Suc n) = x (^) n \<otimes> x"
   169   by (simp add: nat_pow_def)
   170 
   171 lemma (in monoid) nat_pow_one [simp]:
   172   "\<one> (^) (n::nat) = \<one>"
   173   by (induct n) simp_all
   174 
   175 lemma (in monoid) nat_pow_mult:
   176   "x \<in> carrier G ==> x (^) (n::nat) \<otimes> x (^) m = x (^) (n + m)"
   177   by (induct m) (simp_all add: m_assoc [THEN sym])
   178 
   179 lemma (in monoid) nat_pow_pow:
   180   "x \<in> carrier G ==> (x (^) n) (^) m = x (^) (n * m::nat)"
   181   by (induct m) (simp, simp add: nat_pow_mult add_commute)
   182 
   183 text {*
   184   A group is a monoid all of whose elements are invertible.
   185 *}
   186 
   187 locale group = monoid +
   188   assumes Units: "carrier G <= Units G"
   189 
   190 
   191 lemma (in group) is_group: "group G"
   192   by (rule group.intro [OF prems]) 
   193 
   194 theorem groupI:
   195   includes struct G
   196   assumes m_closed [simp]:
   197       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
   198     and one_closed [simp]: "\<one> \<in> carrier G"
   199     and m_assoc:
   200       "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   201       (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
   202     and l_one [simp]: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
   203     and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
   204   shows "group G"
   205 proof -
   206   have l_cancel [simp]:
   207     "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   208     (x \<otimes> y = x \<otimes> z) = (y = z)"
   209   proof
   210     fix x y z
   211     assume eq: "x \<otimes> y = x \<otimes> z"
   212       and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
   213     with l_inv_ex obtain x_inv where xG: "x_inv \<in> carrier G"
   214       and l_inv: "x_inv \<otimes> x = \<one>" by fast
   215     from G eq xG have "(x_inv \<otimes> x) \<otimes> y = (x_inv \<otimes> x) \<otimes> z"
   216       by (simp add: m_assoc)
   217     with G show "y = z" by (simp add: l_inv)
   218   next
   219     fix x y z
   220     assume eq: "y = z"
   221       and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
   222     then show "x \<otimes> y = x \<otimes> z" by simp
   223   qed
   224   have r_one:
   225     "!!x. x \<in> carrier G ==> x \<otimes> \<one> = x"
   226   proof -
   227     fix x
   228     assume x: "x \<in> carrier G"
   229     with l_inv_ex obtain x_inv where xG: "x_inv \<in> carrier G"
   230       and l_inv: "x_inv \<otimes> x = \<one>" by fast
   231     from x xG have "x_inv \<otimes> (x \<otimes> \<one>) = x_inv \<otimes> x"
   232       by (simp add: m_assoc [symmetric] l_inv)
   233     with x xG show "x \<otimes> \<one> = x" by simp
   234   qed
   235   have inv_ex:
   236     "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one> & x \<otimes> y = \<one>"
   237   proof -
   238     fix x
   239     assume x: "x \<in> carrier G"
   240     with l_inv_ex obtain y where y: "y \<in> carrier G"
   241       and l_inv: "y \<otimes> x = \<one>" by fast
   242     from x y have "y \<otimes> (x \<otimes> y) = y \<otimes> \<one>"
   243       by (simp add: m_assoc [symmetric] l_inv r_one)
   244     with x y have r_inv: "x \<otimes> y = \<one>"
   245       by simp
   246     from x y show "\<exists>y \<in> carrier G. y \<otimes> x = \<one> & x \<otimes> y = \<one>"
   247       by (fast intro: l_inv r_inv)
   248   qed
   249   then have carrier_subset_Units: "carrier G <= Units G"
   250     by (unfold Units_def) fast
   251   show ?thesis
   252     by (fast intro!: group.intro monoid.intro group_axioms.intro
   253       carrier_subset_Units intro: prems r_one)
   254 qed
   255 
   256 lemma (in monoid) monoid_groupI:
   257   assumes l_inv_ex:
   258     "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
   259   shows "group G"
   260   by (rule groupI) (auto intro: m_assoc l_inv_ex)
   261 
   262 lemma (in group) Units_eq [simp]:
   263   "Units G = carrier G"
   264 proof
   265   show "Units G <= carrier G" by fast
   266 next
   267   show "carrier G <= Units G" by (rule Units)
   268 qed
   269 
   270 lemma (in group) inv_closed [intro, simp]:
   271   "x \<in> carrier G ==> inv x \<in> carrier G"
   272   using Units_inv_closed by simp
   273 
   274 lemma (in group) l_inv [simp]:
   275   "x \<in> carrier G ==> inv x \<otimes> x = \<one>"
   276   using Units_l_inv by simp
   277 
   278 subsection {* Cancellation Laws and Basic Properties *}
   279 
   280 lemma (in group) l_cancel [simp]:
   281   "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   282    (x \<otimes> y = x \<otimes> z) = (y = z)"
   283   using Units_l_inv by simp
   284 
   285 lemma (in group) r_inv [simp]:
   286   "x \<in> carrier G ==> x \<otimes> inv x = \<one>"
   287 proof -
   288   assume x: "x \<in> carrier G"
   289   then have "inv x \<otimes> (x \<otimes> inv x) = inv x \<otimes> \<one>"
   290     by (simp add: m_assoc [symmetric] l_inv)
   291   with x show ?thesis by (simp del: r_one)
   292 qed
   293 
   294 lemma (in group) r_cancel [simp]:
   295   "[| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   296    (y \<otimes> x = z \<otimes> x) = (y = z)"
   297 proof
   298   assume eq: "y \<otimes> x = z \<otimes> x"
   299     and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
   300   then have "y \<otimes> (x \<otimes> inv x) = z \<otimes> (x \<otimes> inv x)"
   301     by (simp add: m_assoc [symmetric] del: r_inv)
   302   with G show "y = z" by simp
   303 next
   304   assume eq: "y = z"
   305     and G: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
   306   then show "y \<otimes> x = z \<otimes> x" by simp
   307 qed
   308 
   309 lemma (in group) inv_one [simp]:
   310   "inv \<one> = \<one>"
   311 proof -
   312   have "inv \<one> = \<one> \<otimes> (inv \<one>)" by (simp del: r_inv)
   313   moreover have "... = \<one>" by simp
   314   finally show ?thesis .
   315 qed
   316 
   317 lemma (in group) inv_inv [simp]:
   318   "x \<in> carrier G ==> inv (inv x) = x"
   319   using Units_inv_inv by simp
   320 
   321 lemma (in group) inv_inj:
   322   "inj_on (m_inv G) (carrier G)"
   323   using inv_inj_on_Units by simp
   324 
   325 lemma (in group) inv_mult_group:
   326   "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv y \<otimes> inv x"
   327 proof -
   328   assume G: "x \<in> carrier G"  "y \<in> carrier G"
   329   then have "inv (x \<otimes> y) \<otimes> (x \<otimes> y) = (inv y \<otimes> inv x) \<otimes> (x \<otimes> y)"
   330     by (simp add: m_assoc l_inv) (simp add: m_assoc [symmetric])
   331   with G show ?thesis by (simp del: l_inv)
   332 qed
   333 
   334 lemma (in group) inv_comm:
   335   "[| x \<otimes> y = \<one>; x \<in> carrier G; y \<in> carrier G |] ==> y \<otimes> x = \<one>"
   336   by (rule Units_inv_comm) auto
   337 
   338 lemma (in group) inv_equality:
   339      "[|y \<otimes> x = \<one>; x \<in> carrier G; y \<in> carrier G|] ==> inv x = y"
   340 apply (simp add: m_inv_def)
   341 apply (rule the_equality)
   342  apply (simp add: inv_comm [of y x])
   343 apply (rule r_cancel [THEN iffD1], auto)
   344 done
   345 
   346 text {* Power *}
   347 
   348 lemma (in group) int_pow_def2:
   349   "a (^) (z::int) = (if neg z then inv (a (^) (nat (-z))) else a (^) (nat z))"
   350   by (simp add: int_pow_def nat_pow_def Let_def)
   351 
   352 lemma (in group) int_pow_0 [simp]:
   353   "x (^) (0::int) = \<one>"
   354   by (simp add: int_pow_def2)
   355 
   356 lemma (in group) int_pow_one [simp]:
   357   "\<one> (^) (z::int) = \<one>"
   358   by (simp add: int_pow_def2)
   359 
   360 subsection {* Subgroups *}
   361 
   362 locale subgroup = var H + struct G + 
   363   assumes subset: "H \<subseteq> carrier G"
   364     and m_closed [intro, simp]: "\<lbrakk>x \<in> H; y \<in> H\<rbrakk> \<Longrightarrow> x \<otimes> y \<in> H"
   365     and  one_closed [simp]: "\<one> \<in> H"
   366     and m_inv_closed [intro,simp]: "x \<in> H \<Longrightarrow> inv x \<in> H"
   367 
   368 declare (in subgroup) group.intro [intro]
   369 
   370 lemma (in subgroup) mem_carrier [simp]:
   371   "x \<in> H \<Longrightarrow> x \<in> carrier G"
   372   using subset by blast
   373 
   374 lemma subgroup_imp_subset:
   375   "subgroup H G \<Longrightarrow> H \<subseteq> carrier G"
   376   by (rule subgroup.subset)
   377 
   378 lemma (in subgroup) subgroup_is_group [intro]:
   379   includes group G
   380   shows "group (G\<lparr>carrier := H\<rparr>)" 
   381   by (rule groupI) (auto intro: m_assoc l_inv mem_carrier)
   382 
   383 text {*
   384   Since @{term H} is nonempty, it contains some element @{term x}.  Since
   385   it is closed under inverse, it contains @{text "inv x"}.  Since
   386   it is closed under product, it contains @{text "x \<otimes> inv x = \<one>"}.
   387 *}
   388 
   389 lemma (in group) one_in_subset:
   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 |]
   391    ==> \<one> \<in> H"
   392 by (force simp add: l_inv)
   393 
   394 text {* A characterization of subgroups: closed, non-empty subset. *}
   395 
   396 lemma (in group) subgroupI:
   397   assumes subset: "H \<subseteq> carrier G" and non_empty: "H \<noteq> {}"
   398     and inv: "!!a. a \<in> H \<Longrightarrow> inv a \<in> H"
   399     and mult: "!!a b. \<lbrakk>a \<in> H; b \<in> H\<rbrakk> \<Longrightarrow> a \<otimes> b \<in> H"
   400   shows "subgroup H G"
   401 proof (simp add: subgroup_def prems)
   402   show "\<one> \<in> H" by (rule one_in_subset) (auto simp only: prems)
   403 qed
   404 
   405 declare monoid.one_closed [iff] group.inv_closed [simp]
   406   monoid.l_one [simp] monoid.r_one [simp] group.inv_inv [simp]
   407 
   408 lemma subgroup_nonempty:
   409   "~ subgroup {} G"
   410   by (blast dest: subgroup.one_closed)
   411 
   412 lemma (in subgroup) finite_imp_card_positive:
   413   "finite (carrier G) ==> 0 < card H"
   414 proof (rule classical)
   415   assume "finite (carrier G)" "~ 0 < card H"
   416   then have "finite H" by (blast intro: finite_subset [OF subset])
   417   with prems have "subgroup {} G" by simp
   418   with subgroup_nonempty show ?thesis by contradiction
   419 qed
   420 
   421 (*
   422 lemma (in monoid) Units_subgroup:
   423   "subgroup (Units G) G"
   424 *)
   425 
   426 subsection {* Direct Products *}
   427 
   428 constdefs
   429   DirProd :: "_ \<Rightarrow> _ \<Rightarrow> ('a \<times> 'b) monoid"  (infixr "\<times>\<times>" 80)
   430   "G \<times>\<times> H \<equiv> \<lparr>carrier = carrier G \<times> carrier H,
   431                 mult = (\<lambda>(g, h) (g', h'). (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')),
   432                 one = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)\<rparr>"
   433 
   434 lemma DirProd_monoid:
   435   includes monoid G + monoid H
   436   shows "monoid (G \<times>\<times> H)"
   437 proof -
   438   from prems
   439   show ?thesis by (unfold monoid_def DirProd_def, auto) 
   440 qed
   441 
   442 
   443 text{*Does not use the previous result because it's easier just to use auto.*}
   444 lemma DirProd_group:
   445   includes group G + group H
   446   shows "group (G \<times>\<times> H)"
   447   by (rule groupI)
   448      (auto intro: G.m_assoc H.m_assoc G.l_inv H.l_inv
   449            simp add: DirProd_def)
   450 
   451 lemma carrier_DirProd [simp]:
   452      "carrier (G \<times>\<times> H) = carrier G \<times> carrier H"
   453   by (simp add: DirProd_def)
   454 
   455 lemma one_DirProd [simp]:
   456      "\<one>\<^bsub>G \<times>\<times> H\<^esub> = (\<one>\<^bsub>G\<^esub>, \<one>\<^bsub>H\<^esub>)"
   457   by (simp add: DirProd_def)
   458 
   459 lemma mult_DirProd [simp]:
   460      "(g, h) \<otimes>\<^bsub>(G \<times>\<times> H)\<^esub> (g', h') = (g \<otimes>\<^bsub>G\<^esub> g', h \<otimes>\<^bsub>H\<^esub> h')"
   461   by (simp add: DirProd_def)
   462 
   463 lemma inv_DirProd [simp]:
   464   includes group G + group H
   465   assumes g: "g \<in> carrier G"
   466       and h: "h \<in> carrier H"
   467   shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
   468   apply (rule group.inv_equality [OF DirProd_group])
   469   apply (simp_all add: prems group_def group.l_inv)
   470   done
   471 
   472 text{*This alternative proof of the previous result demonstrates interpret.
   473    It uses @{text Prod.inv_equality} (available after @{text interpret})
   474    instead of @{text "group.inv_equality [OF DirProd_group]"}. *}
   475 lemma
   476   includes group G + group H
   477   assumes g: "g \<in> carrier G"
   478       and h: "h \<in> carrier H"
   479   shows "m_inv (G \<times>\<times> H) (g, h) = (inv\<^bsub>G\<^esub> g, inv\<^bsub>H\<^esub> h)"
   480 proof -
   481   interpret Prod: group ["G \<times>\<times> H"]
   482     by (auto intro: DirProd_group group.intro group.axioms prems)
   483   show ?thesis by (simp add: Prod.inv_equality g h)
   484 qed
   485   
   486 
   487 subsection {* Homomorphisms and Isomorphisms *}
   488 
   489 constdefs (structure G and H)
   490   hom :: "_ => _ => ('a => 'b) set"
   491   "hom G H ==
   492     {h. h \<in> carrier G -> carrier H &
   493       (\<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)}"
   494 
   495 lemma hom_mult:
   496   "[| h \<in> hom G H; x \<in> carrier G; y \<in> carrier G |]
   497    ==> h (x \<otimes>\<^bsub>G\<^esub> y) = h x \<otimes>\<^bsub>H\<^esub> h y"
   498   by (simp add: hom_def)
   499 
   500 lemma hom_closed:
   501   "[| h \<in> hom G H; x \<in> carrier G |] ==> h x \<in> carrier H"
   502   by (auto simp add: hom_def funcset_mem)
   503 
   504 lemma (in group) hom_compose:
   505      "[|h \<in> hom G H; i \<in> hom H I|] ==> compose (carrier G) i h \<in> hom G I"
   506 apply (auto simp add: hom_def funcset_compose) 
   507 apply (simp add: compose_def funcset_mem)
   508 done
   509 
   510 
   511 subsection {* Isomorphisms *}
   512 
   513 constdefs
   514   iso :: "_ => _ => ('a => 'b) set"  (infixr "\<cong>" 60)
   515   "G \<cong> H == {h. h \<in> hom G H & bij_betw h (carrier G) (carrier H)}"
   516 
   517 lemma iso_refl: "(%x. x) \<in> G \<cong> G"
   518 by (simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
   519 
   520 lemma (in group) iso_sym:
   521      "h \<in> G \<cong> H \<Longrightarrow> Inv (carrier G) h \<in> H \<cong> G"
   522 apply (simp add: iso_def bij_betw_Inv) 
   523 apply (subgoal_tac "Inv (carrier G) h \<in> carrier H \<rightarrow> carrier G") 
   524  prefer 2 apply (simp add: bij_betw_imp_funcset [OF bij_betw_Inv]) 
   525 apply (simp add: hom_def bij_betw_def Inv_f_eq funcset_mem f_Inv_f) 
   526 done
   527 
   528 lemma (in group) iso_trans: 
   529      "[|h \<in> G \<cong> H; i \<in> H \<cong> I|] ==> (compose (carrier G) i h) \<in> G \<cong> I"
   530 by (auto simp add: iso_def hom_compose bij_betw_compose)
   531 
   532 lemma DirProd_commute_iso:
   533   shows "(\<lambda>(x,y). (y,x)) \<in> (G \<times>\<times> H) \<cong> (H \<times>\<times> G)"
   534 by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
   535 
   536 lemma DirProd_assoc_iso:
   537   shows "(\<lambda>(x,y,z). (x,(y,z))) \<in> (G \<times>\<times> H \<times>\<times> I) \<cong> (G \<times>\<times> (H \<times>\<times> I))"
   538 by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def) 
   539 
   540 
   541 text{*Basis for homomorphism proofs: we assume two groups @{term G} and
   542   @{term H}, with a homomorphism @{term h} between them*}
   543 locale group_hom = group G + group H + var h +
   544   assumes homh: "h \<in> hom G H"
   545   notes hom_mult [simp] = hom_mult [OF homh]
   546     and hom_closed [simp] = hom_closed [OF homh]
   547 
   548 lemma (in group_hom) one_closed [simp]:
   549   "h \<one> \<in> carrier H"
   550   by simp
   551 
   552 lemma (in group_hom) hom_one [simp]:
   553   "h \<one> = \<one>\<^bsub>H\<^esub>"
   554 proof -
   555   have "h \<one> \<otimes>\<^bsub>H\<^esub> \<one>\<^bsub>H\<^esub> = h \<one> \<otimes>\<^bsub>H\<^esub> h \<one>"
   556     by (simp add: hom_mult [symmetric] del: hom_mult)
   557   then show ?thesis by (simp del: r_one)
   558 qed
   559 
   560 lemma (in group_hom) inv_closed [simp]:
   561   "x \<in> carrier G ==> h (inv x) \<in> carrier H"
   562   by simp
   563 
   564 lemma (in group_hom) hom_inv [simp]:
   565   "x \<in> carrier G ==> h (inv x) = inv\<^bsub>H\<^esub> (h x)"
   566 proof -
   567   assume x: "x \<in> carrier G"
   568   then have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = \<one>\<^bsub>H\<^esub>"
   569     by (simp add: hom_mult [symmetric] del: hom_mult)
   570   also from x have "... = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)"
   571     by (simp add: hom_mult [symmetric] del: hom_mult)
   572   finally have "h x \<otimes>\<^bsub>H\<^esub> h (inv x) = h x \<otimes>\<^bsub>H\<^esub> inv\<^bsub>H\<^esub> (h x)" .
   573   with x show ?thesis by (simp del: H.r_inv)
   574 qed
   575 
   576 subsection {* Commutative Structures *}
   577 
   578 text {*
   579   Naming convention: multiplicative structures that are commutative
   580   are called \emph{commutative}, additive structures are called
   581   \emph{Abelian}.
   582 *}
   583 
   584 subsection {* Definition *}
   585 
   586 locale comm_monoid = monoid +
   587   assumes m_comm: "\<lbrakk>x \<in> carrier G; y \<in> carrier G\<rbrakk> \<Longrightarrow> x \<otimes> y = y \<otimes> x"
   588 
   589 lemma (in comm_monoid) m_lcomm:
   590   "\<lbrakk>x \<in> carrier G; y \<in> carrier G; z \<in> carrier G\<rbrakk> \<Longrightarrow>
   591    x \<otimes> (y \<otimes> z) = y \<otimes> (x \<otimes> z)"
   592 proof -
   593   assume xyz: "x \<in> carrier G"  "y \<in> carrier G"  "z \<in> carrier G"
   594   from xyz have "x \<otimes> (y \<otimes> z) = (x \<otimes> y) \<otimes> z" by (simp add: m_assoc)
   595   also from xyz have "... = (y \<otimes> x) \<otimes> z" by (simp add: m_comm)
   596   also from xyz have "... = y \<otimes> (x \<otimes> z)" by (simp add: m_assoc)
   597   finally show ?thesis .
   598 qed
   599 
   600 lemmas (in comm_monoid) m_ac = m_assoc m_comm m_lcomm
   601 
   602 lemma comm_monoidI:
   603   includes struct G
   604   assumes m_closed:
   605       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
   606     and one_closed: "\<one> \<in> carrier G"
   607     and m_assoc:
   608       "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   609       (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
   610     and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
   611     and m_comm:
   612       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
   613   shows "comm_monoid G"
   614   using l_one
   615     by (auto intro!: comm_monoid.intro comm_monoid_axioms.intro monoid.intro 
   616              intro: prems simp: m_closed one_closed m_comm)
   617 
   618 lemma (in monoid) monoid_comm_monoidI:
   619   assumes m_comm:
   620       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
   621   shows "comm_monoid G"
   622   by (rule comm_monoidI) (auto intro: m_assoc m_comm)
   623 
   624 (*lemma (in comm_monoid) r_one [simp]:
   625   "x \<in> carrier G ==> x \<otimes> \<one> = x"
   626 proof -
   627   assume G: "x \<in> carrier G"
   628   then have "x \<otimes> \<one> = \<one> \<otimes> x" by (simp add: m_comm)
   629   also from G have "... = x" by simp
   630   finally show ?thesis .
   631 qed*)
   632 
   633 lemma (in comm_monoid) nat_pow_distr:
   634   "[| x \<in> carrier G; y \<in> carrier G |] ==>
   635   (x \<otimes> y) (^) (n::nat) = x (^) n \<otimes> y (^) n"
   636   by (induct n) (simp, simp add: m_ac)
   637 
   638 locale comm_group = comm_monoid + group
   639 
   640 lemma (in group) group_comm_groupI:
   641   assumes m_comm: "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==>
   642       x \<otimes> y = y \<otimes> x"
   643   shows "comm_group G"
   644   by (fast intro: comm_group.intro comm_monoid_axioms.intro
   645                   is_group prems)
   646 
   647 lemma comm_groupI:
   648   includes struct G
   649   assumes m_closed:
   650       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y \<in> carrier G"
   651     and one_closed: "\<one> \<in> carrier G"
   652     and m_assoc:
   653       "!!x y z. [| x \<in> carrier G; y \<in> carrier G; z \<in> carrier G |] ==>
   654       (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
   655     and m_comm:
   656       "!!x y. [| x \<in> carrier G; y \<in> carrier G |] ==> x \<otimes> y = y \<otimes> x"
   657     and l_one: "!!x. x \<in> carrier G ==> \<one> \<otimes> x = x"
   658     and l_inv_ex: "!!x. x \<in> carrier G ==> \<exists>y \<in> carrier G. y \<otimes> x = \<one>"
   659   shows "comm_group G"
   660   by (fast intro: group.group_comm_groupI groupI prems)
   661 
   662 lemma (in comm_group) inv_mult:
   663   "[| x \<in> carrier G; y \<in> carrier G |] ==> inv (x \<otimes> y) = inv x \<otimes> inv y"
   664   by (simp add: m_ac inv_mult_group)
   665 
   666 subsection {* Lattice of subgroups of a group *}
   667 
   668 text_raw {* \label{sec:subgroup-lattice} *}
   669 
   670 theorem (in group) subgroups_partial_order:
   671   "partial_order (| carrier = {H. subgroup H G}, le = op \<subseteq> |)"
   672   by (rule partial_order.intro) simp_all
   673 
   674 lemma (in group) subgroup_self:
   675   "subgroup (carrier G) G"
   676   by (rule subgroupI) auto
   677 
   678 lemma (in group) subgroup_imp_group:
   679   "subgroup H G ==> group (G(| carrier := H |))"
   680   using subgroup.subgroup_is_group [OF _ group.intro] .
   681 
   682 lemma (in group) is_monoid [intro, simp]:
   683   "monoid G"
   684   by (auto intro: monoid.intro m_assoc) 
   685 
   686 lemma (in group) subgroup_inv_equality:
   687   "[| subgroup H G; x \<in> H |] ==> m_inv (G (| carrier := H |)) x = inv x"
   688 apply (rule_tac inv_equality [THEN sym])
   689   apply (rule group.l_inv [OF subgroup_imp_group, simplified], assumption+)
   690  apply (rule subsetD [OF subgroup.subset], assumption+)
   691 apply (rule subsetD [OF subgroup.subset], assumption)
   692 apply (rule_tac group.inv_closed [OF subgroup_imp_group, simplified], assumption+)
   693 done
   694 
   695 theorem (in group) subgroups_Inter:
   696   assumes subgr: "(!!H. H \<in> A ==> subgroup H G)"
   697     and not_empty: "A ~= {}"
   698   shows "subgroup (\<Inter>A) G"
   699 proof (rule subgroupI)
   700   from subgr [THEN subgroup.subset] and not_empty
   701   show "\<Inter>A \<subseteq> carrier G" by blast
   702 next
   703   from subgr [THEN subgroup.one_closed]
   704   show "\<Inter>A ~= {}" by blast
   705 next
   706   fix x assume "x \<in> \<Inter>A"
   707   with subgr [THEN subgroup.m_inv_closed]
   708   show "inv x \<in> \<Inter>A" by blast
   709 next
   710   fix x y assume "x \<in> \<Inter>A" "y \<in> \<Inter>A"
   711   with subgr [THEN subgroup.m_closed]
   712   show "x \<otimes> y \<in> \<Inter>A" by blast
   713 qed
   714 
   715 theorem (in group) subgroups_complete_lattice:
   716   "complete_lattice (| carrier = {H. subgroup H G}, le = op \<subseteq> |)"
   717     (is "complete_lattice ?L")
   718 proof (rule partial_order.complete_lattice_criterion1)
   719   show "partial_order ?L" by (rule subgroups_partial_order)
   720 next
   721   have "greatest ?L (carrier G) (carrier ?L)"
   722     by (unfold greatest_def) (simp add: subgroup.subset subgroup_self)
   723   then show "\<exists>G. greatest ?L G (carrier ?L)" ..
   724 next
   725   fix A
   726   assume L: "A \<subseteq> carrier ?L" and non_empty: "A ~= {}"
   727   then have Int_subgroup: "subgroup (\<Inter>A) G"
   728     by (fastsimp intro: subgroups_Inter)
   729   have "greatest ?L (\<Inter>A) (Lower ?L A)"
   730     (is "greatest ?L ?Int _")
   731   proof (rule greatest_LowerI)
   732     fix H
   733     assume H: "H \<in> A"
   734     with L have subgroupH: "subgroup H G" by auto
   735     from subgroupH have groupH: "group (G (| carrier := H |))" (is "group ?H")
   736       by (rule subgroup_imp_group)
   737     from groupH have monoidH: "monoid ?H"
   738       by (rule group.is_monoid)
   739     from H have Int_subset: "?Int \<subseteq> H" by fastsimp
   740     then show "le ?L ?Int H" by simp
   741   next
   742     fix H
   743     assume H: "H \<in> Lower ?L A"
   744     with L Int_subgroup show "le ?L H ?Int" by (fastsimp intro: Inter_greatest)
   745   next
   746     show "A \<subseteq> carrier ?L" by (rule L)
   747   next
   748     show "?Int \<in> carrier ?L" by simp (rule Int_subgroup)
   749   qed
   750   then show "\<exists>I. greatest ?L I (Lower ?L A)" ..
   751 qed
   752 
   753 end