Theory Group (Isabelle2005: October 2005)

# Theory Group

Up to index of Isabelle/HOL/HOL-Algebra

theory Group
imports FuncSet Lattice
begin

    (*
      Title:  HOL/Algebra/Group.thy
      Id:     $Id: Group.thy,v 1.25 2005/06/17 14:13:05 haftmann Exp $
      Author: Clemens Ballarin, started 4 February 2003

    Based on work by Florian Kammueller, L C Paulson and Markus Wenzel.
    *)

    header {* Groups *}

    theory Group imports FuncSet Lattice begin

    section {* Monoids and Groups *}

    text {*
      Definitions follow \cite{Jacobson:1985}.
    *}

    subsection {* Definitions *}

    record 'a monoid =  "'a partial_object" +
      mult    :: "['a, 'a] => 'a" (infixl "⊗\<index>" 70)
      one     :: 'a ("\<one>\<index>")

    constdefs (structure G)
      m_inv :: "('a, 'b) monoid_scheme => 'a => 'a" ("inv\<index> _" [81] 80)
      "inv x == (THE y. y ∈ carrier G & x ⊗ y = \<one> & y ⊗ x = \<one>)"

      Units :: "_ => 'a set"
      --{*The set of invertible elements*}
      "Units G == {y. y ∈ carrier G & (∃x ∈ carrier G. x ⊗ y = \<one> & y ⊗ x = \<one>)}"

    consts
      pow :: "[('a, 'm) monoid_scheme, 'a, 'b::number] => 'a" (infixr "'(^')\<index>" 75)

    defs (overloaded)
      nat_pow_def: "pow G a n == nat_rec \<one>G (%u b. b ⊗G a) n"
      int_pow_def: "pow G a z ==
        let p = nat_rec \<one>G (%u b. b ⊗G a)
        in if neg z then invG (p (nat (-z))) else p (nat z)"

    locale monoid = struct G +
      assumes m_closed [intro, simp]:
             "[|x ∈ carrier G; y ∈ carrier G|] ==> x ⊗ y ∈ carrier G"
          and m_assoc:
             "[|x ∈ carrier G; y ∈ carrier G; z ∈ carrier G|]
              ==> (x ⊗ y) ⊗ z = x ⊗ (y ⊗ z)"
          and one_closed [intro, simp]: "\<one> ∈ carrier G"
          and l_one [simp]: "x ∈ carrier G ==> \<one> ⊗ x = x"
          and r_one [simp]: "x ∈ carrier G ==> x ⊗ \<one> = x"

    lemma monoidI:
      includes struct G
      assumes m_closed:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y ∈ carrier G"
        and one_closed: "\<one> ∈ carrier G"
        and m_assoc:
          "!!x y z. [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
          (x ⊗ y) ⊗ z = x ⊗ (y ⊗ z)"
        and l_one: "!!x. x ∈ carrier G ==> \<one> ⊗ x = x"
        and r_one: "!!x. x ∈ carrier G ==> x ⊗ \<one> = x"
      shows "monoid G"
      by (fast intro!: monoid.intro intro: prems)

    lemma (in monoid) Units_closed [dest]:
      "x ∈ Units G ==> x ∈ carrier G"
      by (unfold Units_def) fast

    lemma (in monoid) inv_unique:
      assumes eq: "y ⊗ x = \<one>"  "x ⊗ y' = \<one>"
        and G: "x ∈ carrier G"  "y ∈ carrier G"  "y' ∈ carrier G"
      shows "y = y'"
    proof -
      from G eq have "y = y ⊗ (x ⊗ y')" by simp
      also from G have "... = (y ⊗ x) ⊗ y'" by (simp add: m_assoc)
      also from G eq have "... = y'" by simp
      finally show ?thesis .
    qed

    lemma (in monoid) Units_one_closed [intro, simp]:
      "\<one> ∈ Units G"
      by (unfold Units_def) auto

    lemma (in monoid) Units_inv_closed [intro, simp]:
      "x ∈ Units G ==> inv x ∈ carrier G"
      apply (unfold Units_def m_inv_def, auto)
      apply (rule theI2, fast)
       apply (fast intro: inv_unique, fast)
      done

    lemma (in monoid) Units_l_inv:
      "x ∈ Units G ==> inv x ⊗ x = \<one>"
      apply (unfold Units_def m_inv_def, auto)
      apply (rule theI2, fast)
       apply (fast intro: inv_unique, fast)
      done

    lemma (in monoid) Units_r_inv:
      "x ∈ Units G ==> x ⊗ inv x = \<one>"
      apply (unfold Units_def m_inv_def, auto)
      apply (rule theI2, fast)
       apply (fast intro: inv_unique, fast)
      done

    lemma (in monoid) Units_inv_Units [intro, simp]:
      "x ∈ Units G ==> inv x ∈ Units G"
    proof -
      assume x: "x ∈ Units G"
      show "inv x ∈ Units G"
        by (auto simp add: Units_def
          intro: Units_l_inv Units_r_inv x Units_closed [OF x])
    qed

    lemma (in monoid) Units_l_cancel [simp]:
      "[| x ∈ Units G; y ∈ carrier G; z ∈ carrier G |] ==>
       (x ⊗ y = x ⊗ z) = (y = z)"
    proof
      assume eq: "x ⊗ y = x ⊗ z"
        and G: "x ∈ Units G"  "y ∈ carrier G"  "z ∈ carrier G"
      then have "(inv x ⊗ x) ⊗ y = (inv x ⊗ x) ⊗ z"
        by (simp add: m_assoc Units_closed)
      with G show "y = z" by (simp add: Units_l_inv)
    next
      assume eq: "y = z"
        and G: "x ∈ Units G"  "y ∈ carrier G"  "z ∈ carrier G"
      then show "x ⊗ y = x ⊗ z" by simp
    qed

    lemma (in monoid) Units_inv_inv [simp]:
      "x ∈ Units G ==> inv (inv x) = x"
    proof -
      assume x: "x ∈ Units G"
      then have "inv x ⊗ inv (inv x) = inv x ⊗ x"
        by (simp add: Units_l_inv Units_r_inv)
      with x show ?thesis by (simp add: Units_closed)
    qed

    lemma (in monoid) inv_inj_on_Units:
      "inj_on (m_inv G) (Units G)"
    proof (rule inj_onI)
      fix x y
      assume G: "x ∈ Units G"  "y ∈ Units G" and eq: "inv x = inv y"
      then have "inv (inv x) = inv (inv y)" by simp
      with G show "x = y" by simp
    qed

    lemma (in monoid) Units_inv_comm:
      assumes inv: "x ⊗ y = \<one>"
        and G: "x ∈ Units G"  "y ∈ Units G"
      shows "y ⊗ x = \<one>"
    proof -
      from G have "x ⊗ y ⊗ x = x ⊗ \<one>" by (auto simp add: inv Units_closed)
      with G show ?thesis by (simp del: r_one add: m_assoc Units_closed)
    qed

    text {* Power *}

    lemma (in monoid) nat_pow_closed [intro, simp]:
      "x ∈ carrier G ==> x (^) (n::nat) ∈ carrier G"
      by (induct n) (simp_all add: nat_pow_def)

    lemma (in monoid) nat_pow_0 [simp]:
      "x (^) (0::nat) = \<one>"
      by (simp add: nat_pow_def)

    lemma (in monoid) nat_pow_Suc [simp]:
      "x (^) (Suc n) = x (^) n ⊗ x"
      by (simp add: nat_pow_def)

    lemma (in monoid) nat_pow_one [simp]:
      "\<one> (^) (n::nat) = \<one>"
      by (induct n) simp_all

    lemma (in monoid) nat_pow_mult:
      "x ∈ carrier G ==> x (^) (n::nat) ⊗ x (^) m = x (^) (n + m)"
      by (induct m) (simp_all add: m_assoc [THEN sym])

    lemma (in monoid) nat_pow_pow:
      "x ∈ carrier G ==> (x (^) n) (^) m = x (^) (n * m::nat)"
      by (induct m) (simp, simp add: nat_pow_mult add_commute)

    text {*
      A group is a monoid all of whose elements are invertible.
    *}

    locale group = monoid +
      assumes Units: "carrier G <= Units G"

    lemma (in group) is_group: "group G"
      by (rule group.intro [OF prems])

    theorem groupI:
      includes struct G
      assumes m_closed [simp]:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y ∈ carrier G"
        and one_closed [simp]: "\<one> ∈ carrier G"
        and m_assoc:
          "!!x y z. [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
          (x ⊗ y) ⊗ z = x ⊗ (y ⊗ z)"
        and l_one [simp]: "!!x. x ∈ carrier G ==> \<one> ⊗ x = x"
        and l_inv_ex: "!!x. x ∈ carrier G ==> ∃y ∈ carrier G. y ⊗ x = \<one>"
      shows "group G"
    proof -
      have l_cancel [simp]:
        "!!x y z. [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
        (x ⊗ y = x ⊗ z) = (y = z)"
      proof
        fix x y z
        assume eq: "x ⊗ y = x ⊗ z"
          and G: "x ∈ carrier G"  "y ∈ carrier G"  "z ∈ carrier G"
        with l_inv_ex obtain x_inv where xG: "x_inv ∈ carrier G"
          and l_inv: "x_inv ⊗ x = \<one>" by fast
        from G eq xG have "(x_inv ⊗ x) ⊗ y = (x_inv ⊗ x) ⊗ z"
          by (simp add: m_assoc)
        with G show "y = z" by (simp add: l_inv)
      next
        fix x y z
        assume eq: "y = z"
          and G: "x ∈ carrier G"  "y ∈ carrier G"  "z ∈ carrier G"
        then show "x ⊗ y = x ⊗ z" by simp
      qed
      have r_one:
        "!!x. x ∈ carrier G ==> x ⊗ \<one> = x"
      proof -
        fix x
        assume x: "x ∈ carrier G"
        with l_inv_ex obtain x_inv where xG: "x_inv ∈ carrier G"
          and l_inv: "x_inv ⊗ x = \<one>" by fast
        from x xG have "x_inv ⊗ (x ⊗ \<one>) = x_inv ⊗ x"
          by (simp add: m_assoc [symmetric] l_inv)
        with x xG show "x ⊗ \<one> = x" by simp
      qed
      have inv_ex:
        "!!x. x ∈ carrier G ==> ∃y ∈ carrier G. y ⊗ x = \<one> & x ⊗ y = \<one>"
      proof -
        fix x
        assume x: "x ∈ carrier G"
        with l_inv_ex obtain y where y: "y ∈ carrier G"
          and l_inv: "y ⊗ x = \<one>" by fast
        from x y have "y ⊗ (x ⊗ y) = y ⊗ \<one>"
          by (simp add: m_assoc [symmetric] l_inv r_one)
        with x y have r_inv: "x ⊗ y = \<one>"
          by simp
        from x y show "∃y ∈ carrier G. y ⊗ x = \<one> & x ⊗ y = \<one>"
          by (fast intro: l_inv r_inv)
      qed
      then have carrier_subset_Units: "carrier G <= Units G"
        by (unfold Units_def) fast
      show ?thesis
        by (fast intro!: group.intro monoid.intro group_axioms.intro
          carrier_subset_Units intro: prems r_one)
    qed

    lemma (in monoid) monoid_groupI:
      assumes l_inv_ex:
        "!!x. x ∈ carrier G ==> ∃y ∈ carrier G. y ⊗ x = \<one>"
      shows "group G"
      by (rule groupI) (auto intro: m_assoc l_inv_ex)

    lemma (in group) Units_eq [simp]:
      "Units G = carrier G"
    proof
      show "Units G <= carrier G" by fast
    next
      show "carrier G <= Units G" by (rule Units)
    qed

    lemma (in group) inv_closed [intro, simp]:
      "x ∈ carrier G ==> inv x ∈ carrier G"
      using Units_inv_closed by simp

    lemma (in group) l_inv [simp]:
      "x ∈ carrier G ==> inv x ⊗ x = \<one>"
      using Units_l_inv by simp

    subsection {* Cancellation Laws and Basic Properties *}

    lemma (in group) l_cancel [simp]:
      "[| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
       (x ⊗ y = x ⊗ z) = (y = z)"
      using Units_l_inv by simp

    lemma (in group) r_inv [simp]:
      "x ∈ carrier G ==> x ⊗ inv x = \<one>"
    proof -
      assume x: "x ∈ carrier G"
      then have "inv x ⊗ (x ⊗ inv x) = inv x ⊗ \<one>"
        by (simp add: m_assoc [symmetric] l_inv)
      with x show ?thesis by (simp del: r_one)
    qed

    lemma (in group) r_cancel [simp]:
      "[| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
       (y ⊗ x = z ⊗ x) = (y = z)"
    proof
      assume eq: "y ⊗ x = z ⊗ x"
        and G: "x ∈ carrier G"  "y ∈ carrier G"  "z ∈ carrier G"
      then have "y ⊗ (x ⊗ inv x) = z ⊗ (x ⊗ inv x)"
        by (simp add: m_assoc [symmetric] del: r_inv)
      with G show "y = z" by simp
    next
      assume eq: "y = z"
        and G: "x ∈ carrier G"  "y ∈ carrier G"  "z ∈ carrier G"
      then show "y ⊗ x = z ⊗ x" by simp
    qed

    lemma (in group) inv_one [simp]:
      "inv \<one> = \<one>"
    proof -
      have "inv \<one> = \<one> ⊗ (inv \<one>)" by (simp del: r_inv)
      moreover have "... = \<one>" by simp
      finally show ?thesis .
    qed

    lemma (in group) inv_inv [simp]:
      "x ∈ carrier G ==> inv (inv x) = x"
      using Units_inv_inv by simp

    lemma (in group) inv_inj:
      "inj_on (m_inv G) (carrier G)"
      using inv_inj_on_Units by simp

    lemma (in group) inv_mult_group:
      "[| x ∈ carrier G; y ∈ carrier G |] ==> inv (x ⊗ y) = inv y ⊗ inv x"
    proof -
      assume G: "x ∈ carrier G"  "y ∈ carrier G"
      then have "inv (x ⊗ y) ⊗ (x ⊗ y) = (inv y ⊗ inv x) ⊗ (x ⊗ y)"
        by (simp add: m_assoc l_inv) (simp add: m_assoc [symmetric])
      with G show ?thesis by (simp del: l_inv)
    qed

    lemma (in group) inv_comm:
      "[| x ⊗ y = \<one>; x ∈ carrier G; y ∈ carrier G |] ==> y ⊗ x = \<one>"
      by (rule Units_inv_comm) auto

    lemma (in group) inv_equality:
         "[|y ⊗ x = \<one>; x ∈ carrier G; y ∈ carrier G|] ==> inv x = y"
    apply (simp add: m_inv_def)
    apply (rule the_equality)
     apply (simp add: inv_comm [of y x])
    apply (rule r_cancel [THEN iffD1], auto)
    done

    text {* Power *}

    lemma (in group) int_pow_def2:
      "a (^) (z::int) = (if neg z then inv (a (^) (nat (-z))) else a (^) (nat z))"
      by (simp add: int_pow_def nat_pow_def Let_def)

    lemma (in group) int_pow_0 [simp]:
      "x (^) (0::int) = \<one>"
      by (simp add: int_pow_def2)

    lemma (in group) int_pow_one [simp]:
      "\<one> (^) (z::int) = \<one>"
      by (simp add: int_pow_def2)

    subsection {* Subgroups *}

    locale subgroup = var H + struct G +
      assumes subset: "H ⊆ carrier G"
        and m_closed [intro, simp]: "[|x ∈ H; y ∈ H|] ==> x ⊗ y ∈ H"
        and  one_closed [simp]: "\<one> ∈ H"
        and m_inv_closed [intro,simp]: "x ∈ H ==> inv x ∈ H"

    declare (in subgroup) group.intro [intro]

    lemma (in subgroup) mem_carrier [simp]:
      "x ∈ H ==> x ∈ carrier G"
      using subset by blast

    lemma subgroup_imp_subset:
      "subgroup H G ==> H ⊆ carrier G"
      by (rule subgroup.subset)

    lemma (in subgroup) subgroup_is_group [intro]:
      includes group G
      shows "group (G(|carrier := H|)),)"
      by (rule groupI) (auto intro: m_assoc l_inv mem_carrier)

    text {*
      Since @{term H} is nonempty, it contains some element @{term x}.  Since
      it is closed under inverse, it contains @{text "inv x"}.  Since
      it is closed under product, it contains @{text "x ⊗ inv x = \<one>"}.
    *}

    lemma (in group) one_in_subset:
      "[| H ⊆ carrier G; H ≠ {}; ∀a ∈ H. inv a ∈ H; ∀a∈H. ∀b∈H. a ⊗ b ∈ H |]
       ==> \<one> ∈ H"
    by (force simp add: l_inv)

    text {* A characterization of subgroups: closed, non-empty subset. *}

    lemma (in group) subgroupI:
      assumes subset: "H ⊆ carrier G" and non_empty: "H ≠ {}"
        and inv: "!!a. a ∈ H ==> inv a ∈ H"
        and mult: "!!a b. [|a ∈ H; b ∈ H|] ==> a ⊗ b ∈ H"
      shows "subgroup H G"
    proof (simp add: subgroup_def prems)
      show "\<one> ∈ H" by (rule one_in_subset) (auto simp only: prems)
    qed

    declare monoid.one_closed [iff] group.inv_closed [simp]
      monoid.l_one [simp] monoid.r_one [simp] group.inv_inv [simp]

    lemma subgroup_nonempty:
      "~ subgroup {} G"
      by (blast dest: subgroup.one_closed)

    lemma (in subgroup) finite_imp_card_positive:
      "finite (carrier G) ==> 0 < card H"
    proof (rule classical)
      assume "finite (carrier G)" "~ 0 < card H"
      then have "finite H" by (blast intro: finite_subset [OF subset])
      with prems have "subgroup {} G" by simp
      with subgroup_nonempty show ?thesis by contradiction
    qed

    (*
    lemma (in monoid) Units_subgroup:
      "subgroup (Units G) G"
    *)

    subsection {* Direct Products *}

    constdefs
      DirProd :: "_ => _ => ('a × 'b) monoid"  (infixr "××" 80)
      "G ×× H ≡ (|carrier = carrier G × carrier H,
                    mult = (λ(g, h) (g', h'). (g ⊗G g', h ⊗H h')),
                    one = (\<one>G, \<one>H)|)),"

    lemma DirProd_monoid:
      includes monoid G + monoid H
      shows "monoid (G ×× H)"
    proof -
      from prems
      show ?thesis by (unfold monoid_def DirProd_def, auto)
    qed

    text{*Does not use the previous result because it's easier just to use auto.*}
    lemma DirProd_group:
      includes group G + group H
      shows "group (G ×× H)"
      by (rule groupI)
         (auto intro: G.m_assoc H.m_assoc G.l_inv H.l_inv
               simp add: DirProd_def)

    lemma carrier_DirProd [simp]:
         "carrier (G ×× H) = carrier G × carrier H"
      by (simp add: DirProd_def)

    lemma one_DirProd [simp]:
         "\<one>G ×× H = (\<one>G, \<one>H)"
      by (simp add: DirProd_def)

    lemma mult_DirProd [simp]:
         "(g, h) ⊗(G ×× H) (g', h') = (g ⊗G g', h ⊗H h')"
      by (simp add: DirProd_def)

    lemma inv_DirProd [simp]:
      includes group G + group H
      assumes g: "g ∈ carrier G"
          and h: "h ∈ carrier H"
      shows "m_inv (G ×× H) (g, h) = (invG g, invH h)"
      apply (rule group.inv_equality [OF DirProd_group])
      apply (simp_all add: prems group_def group.l_inv)
      done

    text{*This alternative proof of the previous result demonstrates interpret.
       It uses @{text Prod.inv_equality} (available after @{text interpret})
       instead of @{text "group.inv_equality [OF DirProd_group]"}. *}
    lemma
      includes group G + group H
      assumes g: "g ∈ carrier G"
          and h: "h ∈ carrier H"
      shows "m_inv (G ×× H) (g, h) = (invG g, invH h)"
    proof -
      interpret Prod: group ["G ×× H"]
        by (auto intro: DirProd_group group.intro group.axioms prems)
      show ?thesis by (simp add: Prod.inv_equality g h)
    qed

    subsection {* Homomorphisms and Isomorphisms *}

    constdefs (structure G and H)
      hom :: "_ => _ => ('a => 'b) set"
      "hom G H ==
        {h. h ∈ carrier G -> carrier H &
          (∀x ∈ carrier G. ∀y ∈ carrier G. h (x ⊗G y) = h x ⊗H h y)}"

    lemma hom_mult:
      "[| h ∈ hom G H; x ∈ carrier G; y ∈ carrier G |]
       ==> h (x ⊗G y) = h x ⊗H h y"
      by (simp add: hom_def)

    lemma hom_closed:
      "[| h ∈ hom G H; x ∈ carrier G |] ==> h x ∈ carrier H"
      by (auto simp add: hom_def funcset_mem)

    lemma (in group) hom_compose:
         "[|h ∈ hom G H; i ∈ hom H I|] ==> compose (carrier G) i h ∈ hom G I"
    apply (auto simp add: hom_def funcset_compose)
    apply (simp add: compose_def funcset_mem)
    done

    subsection {* Isomorphisms *}

    constdefs
      iso :: "_ => _ => ('a => 'b) set"  (infixr "≅" 60)
      "G ≅ H == {h. h ∈ hom G H & bij_betw h (carrier G) (carrier H)}"

    lemma iso_refl: "(%x. x) ∈ G ≅ G"
    by (simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def)

    lemma (in group) iso_sym:
         "h ∈ G ≅ H ==> Inv (carrier G) h ∈ H ≅ G"
    apply (simp add: iso_def bij_betw_Inv)
    apply (subgoal_tac "Inv (carrier G) h ∈ carrier H -> carrier G")
     prefer 2 apply (simp add: bij_betw_imp_funcset [OF bij_betw_Inv])
    apply (simp add: hom_def bij_betw_def Inv_f_eq funcset_mem f_Inv_f)
    done

    lemma (in group) iso_trans:
         "[|h ∈ G ≅ H; i ∈ H ≅ I|] ==> (compose (carrier G) i h) ∈ G ≅ I"
    by (auto simp add: iso_def hom_compose bij_betw_compose)

    lemma DirProd_commute_iso:
      shows "(λ(x,y). (y,x)) ∈ (G ×× H) ≅ (H ×× G)"
    by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def)

    lemma DirProd_assoc_iso:
      shows "(λ(x,y,z). (x,(y,z))) ∈ (G ×× H ×× I) ≅ (G ×× (H ×× I))"
    by (auto simp add: iso_def hom_def inj_on_def bij_betw_def Pi_def)

    text{*Basis for homomorphism proofs: we assume two groups @{term G} and
      @{term H}, with a homomorphism @{term h} between them*}
    locale group_hom = group G + group H + var h +
      assumes homh: "h ∈ hom G H"
      notes hom_mult [simp] = hom_mult [OF homh]
        and hom_closed [simp] = hom_closed [OF homh]

    lemma (in group_hom) one_closed [simp]:
      "h \<one> ∈ carrier H"
      by simp

    lemma (in group_hom) hom_one [simp]:
      "h \<one> = \<one>H"
    proof -
      have "h \<one> ⊗H \<one>H = h \<one> ⊗H h \<one>"
        by (simp add: hom_mult [symmetric] del: hom_mult)
      then show ?thesis by (simp del: r_one)
    qed

    lemma (in group_hom) inv_closed [simp]:
      "x ∈ carrier G ==> h (inv x) ∈ carrier H"
      by simp

    lemma (in group_hom) hom_inv [simp]:
      "x ∈ carrier G ==> h (inv x) = invH (h x)"
    proof -
      assume x: "x ∈ carrier G"
      then have "h x ⊗H h (inv x) = \<one>H"
        by (simp add: hom_mult [symmetric] del: hom_mult)
      also from x have "... = h x ⊗H invH (h x)"
        by (simp add: hom_mult [symmetric] del: hom_mult)
      finally have "h x ⊗H h (inv x) = h x ⊗H invH (h x)" .
      with x show ?thesis by (simp del: H.r_inv)
    qed

    subsection {* Commutative Structures *}

    text {*
      Naming convention: multiplicative structures that are commutative
      are called \emph{commutative}, additive structures are called
      \emph{Abelian}.
    *}

    subsection {* Definition *}

    locale comm_monoid = monoid +
      assumes m_comm: "[|x ∈ carrier G; y ∈ carrier G|] ==> x ⊗ y = y ⊗ x"

    lemma (in comm_monoid) m_lcomm:
      "[|x ∈ carrier G; y ∈ carrier G; z ∈ carrier G|] ==>
       x ⊗ (y ⊗ z) = y ⊗ (x ⊗ z)"
    proof -
      assume xyz: "x ∈ carrier G"  "y ∈ carrier G"  "z ∈ carrier G"
      from xyz have "x ⊗ (y ⊗ z) = (x ⊗ y) ⊗ z" by (simp add: m_assoc)
      also from xyz have "... = (y ⊗ x) ⊗ z" by (simp add: m_comm)
      also from xyz have "... = y ⊗ (x ⊗ z)" by (simp add: m_assoc)
      finally show ?thesis .
    qed

    lemmas (in comm_monoid) m_ac = m_assoc m_comm m_lcomm

    lemma comm_monoidI:
      includes struct G
      assumes m_closed:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y ∈ carrier G"
        and one_closed: "\<one> ∈ carrier G"
        and m_assoc:
          "!!x y z. [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
          (x ⊗ y) ⊗ z = x ⊗ (y ⊗ z)"
        and l_one: "!!x. x ∈ carrier G ==> \<one> ⊗ x = x"
        and m_comm:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y = y ⊗ x"
      shows "comm_monoid G"
      using l_one
        by (auto intro!: comm_monoid.intro comm_monoid_axioms.intro monoid.intro
                 intro: prems simp: m_closed one_closed m_comm)

    lemma (in monoid) monoid_comm_monoidI:
      assumes m_comm:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y = y ⊗ x"
      shows "comm_monoid G"
      by (rule comm_monoidI) (auto intro: m_assoc m_comm)

    (*lemma (in comm_monoid) r_one [simp]:
      "x ∈ carrier G ==> x ⊗ \<one> = x"
    proof -
      assume G: "x ∈ carrier G"
      then have "x ⊗ \<one> = \<one> ⊗ x" by (simp add: m_comm)
      also from G have "... = x" by simp
      finally show ?thesis .
    qed*)

    lemma (in comm_monoid) nat_pow_distr:
      "[| x ∈ carrier G; y ∈ carrier G |] ==>
      (x ⊗ y) (^) (n::nat) = x (^) n ⊗ y (^) n"
      by (induct n) (simp, simp add: m_ac)

    locale comm_group = comm_monoid + group

    lemma (in group) group_comm_groupI:
      assumes m_comm: "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==>
          x ⊗ y = y ⊗ x"
      shows "comm_group G"
      by (fast intro: comm_group.intro comm_monoid_axioms.intro
                      is_group prems)

    lemma comm_groupI:
      includes struct G
      assumes m_closed:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y ∈ carrier G"
        and one_closed: "\<one> ∈ carrier G"
        and m_assoc:
          "!!x y z. [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |] ==>
          (x ⊗ y) ⊗ z = x ⊗ (y ⊗ z)"
        and m_comm:
          "!!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗ y = y ⊗ x"
        and l_one: "!!x. x ∈ carrier G ==> \<one> ⊗ x = x"
        and l_inv_ex: "!!x. x ∈ carrier G ==> ∃y ∈ carrier G. y ⊗ x = \<one>"
      shows "comm_group G"
      by (fast intro: group.group_comm_groupI groupI prems)

    lemma (in comm_group) inv_mult:
      "[| x ∈ carrier G; y ∈ carrier G |] ==> inv (x ⊗ y) = inv x ⊗ inv y"
      by (simp add: m_ac inv_mult_group)

    subsection {* Lattice of subgroups of a group *}

    text_raw {* \label{sec:subgroup-lattice} *}

    theorem (in group) subgroups_partial_order:
      "partial_order (| carrier = {H. subgroup H G}, le = op ⊆ |)"
      by (rule partial_order.intro) simp_all

    lemma (in group) subgroup_self:
      "subgroup (carrier G) G"
      by (rule subgroupI) auto

    lemma (in group) subgroup_imp_group:
      "subgroup H G ==> group (G(| carrier := H |))"
      using subgroup.subgroup_is_group [OF _ group.intro] .

    lemma (in group) is_monoid [intro, simp]:
      "monoid G"
      by (auto intro: monoid.intro m_assoc)

    lemma (in group) subgroup_inv_equality:
      "[| subgroup H G; x ∈ H |] ==> m_inv (G (| carrier := H |)) x = inv x"
    apply (rule_tac inv_equality [THEN sym])
      apply (rule group.l_inv [OF subgroup_imp_group, simplified], assumption+)
     apply (rule subsetD [OF subgroup.subset], assumption+)
    apply (rule subsetD [OF subgroup.subset], assumption)
    apply (rule_tac group.inv_closed [OF subgroup_imp_group, simplified], assumption+)
    done

    theorem (in group) subgroups_Inter:
      assumes subgr: "(!!H. H ∈ A ==> subgroup H G)"
        and not_empty: "A ~= {}"
      shows "subgroup (\<Inter>A) G"
    proof (rule subgroupI)
      from subgr [THEN subgroup.subset] and not_empty
      show "\<Inter>A ⊆ carrier G" by blast
    next
      from subgr [THEN subgroup.one_closed]
      show "\<Inter>A ~= {}" by blast
    next
      fix x assume "x ∈ \<Inter>A"
      with subgr [THEN subgroup.m_inv_closed]
      show "inv x ∈ \<Inter>A" by blast
    next
      fix x y assume "x ∈ \<Inter>A" "y ∈ \<Inter>A"
      with subgr [THEN subgroup.m_closed]
      show "x ⊗ y ∈ \<Inter>A" by blast
    qed

    theorem (in group) subgroups_complete_lattice:
      "complete_lattice (| carrier = {H. subgroup H G}, le = op ⊆ |)"
        (is "complete_lattice ?L")
    proof (rule partial_order.complete_lattice_criterion1)
      show "partial_order ?L" by (rule subgroups_partial_order)
    next
      have "greatest ?L (carrier G) (carrier ?L)"
        by (unfold greatest_def) (simp add: subgroup.subset subgroup_self)
      then show "∃G. greatest ?L G (carrier ?L)" ..
    next
      fix A
      assume L: "A ⊆ carrier ?L" and non_empty: "A ~= {}"
      then have Int_subgroup: "subgroup (\<Inter>A) G"
        by (fastsimp intro: subgroups_Inter)
      have "greatest ?L (\<Inter>A) (Lower ?L A)"
        (is "greatest ?L ?Int _")
      proof (rule greatest_LowerI)
        fix H
        assume H: "H ∈ A"
        with L have subgroupH: "subgroup H G" by auto
        from subgroupH have groupH: "group (G (| carrier := H |))" (is "group ?H")
          by (rule subgroup_imp_group)
        from groupH have monoidH: "monoid ?H"
          by (rule group.is_monoid)
        from H have Int_subset: "?Int ⊆ H" by fastsimp
        then show "le ?L ?Int H" by simp
      next
        fix H
        assume H: "H ∈ Lower ?L A"
        with L Int_subgroup show "le ?L H ?Int" by (fastsimp intro: Inter_greatest)
      next
        show "A ⊆ carrier ?L" by (rule L)
      next
        show "?Int ∈ carrier ?L" by simp (rule Int_subgroup)
      qed
      then show "∃I. greatest ?L I (Lower ?L A)" ..
    qed

    end

## Monoids and Groups

### Definitions

lemma monoidI:

      [| !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y ∈ carrier G;
         \<one>G ∈ carrier G;
         !!x y z.
            [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
            ==> x ⊗G y ⊗G z = x ⊗G (y ⊗G z);
         !!x. x ∈ carrier G ==> \<one>G ⊗G x = x;
         !!x. x ∈ carrier G ==> x ⊗G \<one>G = x |]
      ==> monoid G

lemma Units_closed:

      [| monoid G; x ∈ Units G |] ==> x ∈ carrier G

lemma inv_unique:

      [| monoid G; y ⊗G x = \<one>G; x ⊗G y' = \<one>G; x ∈ carrier G; y ∈ carrier G;
         y' ∈ carrier G |]
      ==> y = y'

lemma Units_one_closed:

      monoid G ==> \<one>G ∈ Units G

lemma Units_inv_closed:

      [| monoid G; x ∈ Units G |] ==> invG x ∈ carrier G

lemma Units_l_inv:

      [| monoid G; x ∈ Units G |] ==> invG x ⊗G x = \<one>G

lemma Units_r_inv:

      [| monoid G; x ∈ Units G |] ==> x ⊗G invG x = \<one>G

lemma Units_inv_Units:

      [| monoid G; x ∈ Units G |] ==> invG x ∈ Units G

lemma Units_l_cancel:

      [| monoid G; x ∈ Units G; y ∈ carrier G; z ∈ carrier G |]
      ==> (x ⊗G y = x ⊗G z) = (y = z)

lemma Units_inv_inv:

      [| monoid G; x ∈ Units G |] ==> invG (invG x) = x

lemma inv_inj_on_Units:

      monoid G ==> inj_on (m_inv G) (Units G)

lemma Units_inv_comm:

      [| monoid G; x ⊗G y = \<one>G; x ∈ Units G; y ∈ Units G |] ==> y ⊗G x = \<one>G

lemma nat_pow_closed:

      [| monoid G; x ∈ carrier G |] ==> x (^)G n ∈ carrier G

lemma nat_pow_0:

      monoid G ==> x (^)G 0 = \<one>G

lemma nat_pow_Suc:

      monoid G ==> x (^)G Suc n = x (^)G n ⊗G x

lemma nat_pow_one:

      monoid G ==> \<one>G (^)G n = \<one>G

lemma nat_pow_mult:

      [| monoid G; x ∈ carrier G |] ==> x (^)G n ⊗G x (^)G m = x (^)G (n + m)

lemma nat_pow_pow:

      [| monoid G; x ∈ carrier G |] ==> (x (^)G n) (^)G m = x (^)G (n * m)

lemma is_group:

      group G ==> group G

theorem groupI:

      [| !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y ∈ carrier G;
         \<one>G ∈ carrier G;
         !!x y z.
            [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
            ==> x ⊗G y ⊗G z = x ⊗G (y ⊗G z);
         !!x. x ∈ carrier G ==> \<one>G ⊗G x = x;
         !!x. x ∈ carrier G ==> ∃y∈carrier G. y ⊗G x = \<one>G |]
      ==> group G

lemma monoid_groupI:

      [| monoid G; !!x. x ∈ carrier G ==> ∃y∈carrier G. y ⊗G x = \<one>G |]
      ==> group G

lemma Units_eq:

      group G ==> Units G = carrier G

lemma inv_closed:

      [| group G; x ∈ carrier G |] ==> invG x ∈ carrier G

lemma l_inv:

      [| group G; x ∈ carrier G |] ==> invG x ⊗G x = \<one>G

### Cancellation Laws and Basic Properties

lemma l_cancel:

      [| group G; x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
      ==> (x ⊗G y = x ⊗G z) = (y = z)

lemma r_inv:

      [| group G; x ∈ carrier G |] ==> x ⊗G invG x = \<one>G

lemma r_cancel:

      [| group G; x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
      ==> (y ⊗G x = z ⊗G x) = (y = z)

lemma inv_one:

      group G ==> invG \<one>G = \<one>G

lemma inv_inv:

      [| group G; x ∈ carrier G |] ==> invG (invG x) = x

lemma inv_inj:

      group G ==> inj_on (m_inv G) (carrier G)

lemma inv_mult_group:

      [| group G; x ∈ carrier G; y ∈ carrier G |] ==> invG (x ⊗G y) = invG y ⊗G invG x

lemma inv_comm:

      [| group G; x ⊗G y = \<one>G; x ∈ carrier G; y ∈ carrier G |]
      ==> y ⊗G x = \<one>G

lemma inv_equality:

      [| group G; y ⊗G x = \<one>G; x ∈ carrier G; y ∈ carrier G |] ==> invG x = y

lemma int_pow_def2:

      group G ==> a (^)G z = (if neg z then invG (a (^)G nat (- z)) else a (^)G nat z)

lemma int_pow_0:

      group G ==> x (^)G 0 = \<one>G

lemma int_pow_one:

      group G ==> \<one>G (^)G z = \<one>G

### Subgroups

lemma mem_carrier:

      [| subgroup H G; x ∈ H |] ==> x ∈ carrier G

lemma subgroup_imp_subset:

      subgroup H G ==> H ⊆ carrier G

lemma subgroup_is_group:

      [| subgroup H G; group G |] ==> group (G(| carrier := H |))

lemma one_in_subset:

      [| group G; H ⊆ carrier G; H ≠ {}; ∀a∈H. invG a ∈ H; ∀a∈H. ∀b∈H. a ⊗G b ∈ H |]
      ==> \<one>G ∈ H

lemma subgroupI:

      [| group G; H ⊆ carrier G; H ≠ {}; !!a. a ∈ H ==> invG a ∈ H;
         !!a b. [| a ∈ H; b ∈ H |] ==> a ⊗G b ∈ H |]
      ==> subgroup H G

lemma subgroup_nonempty:

      ¬ subgroup {} G

lemma finite_imp_card_positive:

      [| subgroup H G; finite (carrier G) |] ==> 0 < card H

### Direct Products

lemma DirProd_monoid:

      [| monoid G; monoid H |] ==> monoid (G ×× H)

lemma DirProd_group:

      [| group G; group H |] ==> group (G ×× H)

lemma carrier_DirProd:

      carrier (G ×× H) = carrier G × carrier H

lemma one_DirProd:

      \<one>G ×× H = (\<one>G, \<one>H)

lemma mult_DirProd:

      (g, h) ⊗G ×× H (g', h') = (g ⊗G g', h ⊗H h')

lemma inv_DirProd:

      [| group G; group H; g ∈ carrier G; h ∈ carrier H |]
      ==> invG ×× H (g, h) = (invG g, invH h)

lemma

      [| group G; group H; g ∈ carrier G; h ∈ carrier H |]
      ==> invG ×× H (g, h) = (invG g, invH h)

### Homomorphisms and Isomorphisms

lemma hom_mult:

      [| h ∈ hom G H; x ∈ carrier G; y ∈ carrier G |] ==> h (x ⊗G y) = h x ⊗H h y

lemma hom_closed:

      [| h ∈ hom G H; x ∈ carrier G |] ==> h x ∈ carrier H

lemma hom_compose:

      [| group G; h ∈ hom G H; i ∈ hom H I |] ==> compose (carrier G) i h ∈ hom G I

### Isomorphisms

lemma iso_refl:

      (%x. x) ∈ G ≅ G

lemma iso_sym:

      [| group G; h ∈ G ≅ H |] ==> Inv (carrier G) h ∈ H ≅ G

lemma iso_trans:

      [| group G; h ∈ G ≅ H; i ∈ H ≅ I |] ==> compose (carrier G) i h ∈ G ≅ I

lemma DirProd_commute_iso:

      (%(x, y). (y, x)) ∈ G ×× H ≅ H ×× G

lemma DirProd_assoc_iso:

      (%(x, y, z). (x, y, z)) ∈ G ×× H ×× I ≅ G ×× H ×× I

lemma one_closed:

      group_hom G H h ==> h \<one>G ∈ carrier H

lemma hom_one:

      group_hom G H h ==> h \<one>G = \<one>H

lemma inv_closed:

      [| group_hom G H h; x ∈ carrier G |] ==> h (invG x) ∈ carrier H

lemma hom_inv:

      [| group_hom G H h; x ∈ carrier G |] ==> h (invG x) = invH h x

### Commutative Structures

### Definition

lemma m_lcomm:

      [| comm_monoid G; x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
      ==> x ⊗G (y ⊗G z) = y ⊗G (x ⊗G z)

lemmas m_ac:

      [| comm_monoid G; x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
      ==> x ⊗G y ⊗G z = x ⊗G (y ⊗G z)

      [| comm_monoid G; x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y = y ⊗G x

      [| comm_monoid G; x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
      ==> x ⊗G (y ⊗G z) = y ⊗G (x ⊗G z)

lemma comm_monoidI:

      [| !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y ∈ carrier G;
         \<one>G ∈ carrier G;
         !!x y z.
            [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
            ==> x ⊗G y ⊗G z = x ⊗G (y ⊗G z);
         !!x. x ∈ carrier G ==> \<one>G ⊗G x = x;
         !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y = y ⊗G x |]
      ==> comm_monoid G

lemma monoid_comm_monoidI:

      [| monoid G; !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y = y ⊗G x |]
      ==> comm_monoid G

lemma nat_pow_distr:

      [| comm_monoid G; x ∈ carrier G; y ∈ carrier G |]
      ==> (x ⊗G y) (^)G n = x (^)G n ⊗G y (^)G n

lemma group_comm_groupI:

      [| group G; !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y = y ⊗G x |]
      ==> comm_group G

lemma comm_groupI:

      [| !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y ∈ carrier G;
         \<one>G ∈ carrier G;
         !!x y z.
            [| x ∈ carrier G; y ∈ carrier G; z ∈ carrier G |]
            ==> x ⊗G y ⊗G z = x ⊗G (y ⊗G z);
         !!x y. [| x ∈ carrier G; y ∈ carrier G |] ==> x ⊗G y = y ⊗G x;
         !!x. x ∈ carrier G ==> \<one>G ⊗G x = x;
         !!x. x ∈ carrier G ==> ∃y∈carrier G. y ⊗G x = \<one>G |]
      ==> comm_group G

lemma inv_mult:

      [| comm_group G; x ∈ carrier G; y ∈ carrier G |]
      ==> invG (x ⊗G y) = invG x ⊗G invG y

### Lattice of subgroups of a group

theorem subgroups_partial_order:

      group G ==> Lattice.partial_order (| carrier = {H. subgroup H G}, le = op ⊆ |)

lemma subgroup_self:

      group G ==> subgroup (carrier G) G

lemma subgroup_imp_group:

      [| group G; subgroup H G |] ==> group (G(| carrier := H |))

lemma is_monoid:

      group G ==> monoid G

lemma subgroup_inv_equality:

      [| group G; subgroup H G; x ∈ H |] ==> invG(| carrier := H |) x = invG x

theorem subgroups_Inter:

      [| group G; !!H. H ∈ A ==> subgroup H G; A ≠ {} |] ==> subgroup (Inter A) G

theorem subgroups_complete_lattice:

      group G ==> complete_lattice (| carrier = {H. subgroup H G}, le = op ⊆ |)