formal/coq/math-comp/fingroup/automorphism.v

(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp Require Import ssreflect ssrbool ssrfun eqtype ssrnat fintype.
From mathcomp Require Import finset fingroup perm morphism.

(******************************************************************************)
(* Group automorphisms and characteristic subgroups.                          *)
(* Unlike morphisms on a group G, which are functions of type gT -> rT, with  *)
(* a canonical structure of dependent type {morphim G >-> rT}, automorphisms  *)
(* are permutations of type {perm gT} contained in Aut G : {set {perm gT}}.   *)
(* This lets us use the finGroupType of {perm gT}. Note also that while       *)
(* morphisms on G are undefined outside G, automorphisms have their support   *)
(* in G, i.e., they are the identity outside G.                               *)
(* Definitions:                                                               *)
(*    Aut G (or [Aut G]) == the automorphism group of G.                      *)
(*             [Aut G]%G == the group structure for Aut G.                    *)
(*            autm AutGa == the morphism on G induced by a, given             *)
(*                          AutGa : a \in Aut G.                              *)
(*       perm_in injf fA == the permutation with support B in induced by f,   *)
(*                          given injf : {in A &, injective f} and            *)
(*                          fA : f @: A \subset A.                            *)
(*           aut injf fG == the automorphism of G induced by the morphism f,  *)
(*                          given injf : 'injm f and fG : f @* G \subset G.   *)
(*    Aut_isom injf sDom == the injective homomorphism that maps Aut G to     *)
(*                          Aut (f @* G), with f : {morphism D >-> rT} and    *)
(*                          given injf: 'injm f and sDom : G \subset D.       *)
(*              conjgm G == the conjugation automorphism on G.                *)
(*             H \char G == H is a characteristic subgroup of G.              *)
(******************************************************************************)

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

Import GroupScope.

(***********************************************************************)
(* A group automorphism, defined as a permutation on a subset of a     *)
(* finGroupType that respects the morphism law.                        *)
(* Here perm_on is used as a closure rule for the set A.               *)
(***********************************************************************)

Section Automorphism.

Variable gT : finGroupType.
Implicit Type A : {set gT}.
Implicit Types a b : {perm gT}.

Definition Aut A := [set a | perm_on A a & morphic A a].

Lemma Aut_morphic A a : a \in Aut A -> morphic A a.
Proof. by case/setIdP. Qed.

Lemma out_Aut A a x : a \in Aut A -> x \notin A -> a x = x.
Proof. by case/setIdP=> Aa _; apply: out_perm. Qed.

Lemma eq_Aut A : {in Aut A &, forall a b, {in A, a =1 b} -> a = b}.
Proof.
move=> a g Aa Ag /= eqag; apply/permP=> x.
by have [/eqag // | /out_Aut out] := boolP (x \in A); rewrite !out.
Qed.

(* The morphism that is represented by a given element of Aut A. *)

Definition autm A a (AutAa : a \in Aut A) := morphm (Aut_morphic AutAa).
Lemma autmE A a (AutAa : a \in Aut A) : autm AutAa = a.
Proof. by []. Qed.

Canonical autm_morphism A a aM := Eval hnf in [morphism of @autm A a aM].

Section AutGroup.

Variable G : {group gT}.

Lemma Aut_group_set : group_set (Aut G).
Proof.
apply/group_setP; split=> [|a b].
  by rewrite inE perm_on1; apply/morphicP=> ? *; rewrite !permE.
rewrite !inE => /andP[Ga aM] /andP[Gb bM]; rewrite perm_onM //=.
apply/morphicP=> x y Gx Gy; rewrite !permM (morphicP aM) //.
by rewrite (morphicP bM) ?perm_closed.
Qed.

Canonical Aut_group := group Aut_group_set.

Variable (a : {perm gT}) (AutGa : a \in Aut G).
Notation f := (autm AutGa).
Notation fE := (autmE AutGa).

Lemma injm_autm : 'injm f.
Proof. by apply/injmP; apply: in2W; apply: perm_inj. Qed.

Lemma ker_autm : 'ker f = 1. Proof. by move/trivgP: injm_autm. Qed.

Lemma im_autm : f @* G = G.
Proof.
apply/setP=> x; rewrite morphimEdom (can_imset_pre _ (permK a)) inE.
by have /[1!inE] /andP[/perm_closed <-] := AutGa; rewrite permKV.
Qed.

Lemma Aut_closed x : x \in G -> a x \in G.
Proof. by move=> Gx; rewrite -im_autm; apply: mem_morphim. Qed.

End AutGroup.

Lemma Aut1 : Aut 1 = 1.
Proof.
apply/trivgP/subsetP=> a /= AutGa; apply/set1P.
apply: eq_Aut (AutGa) (group1 _) _ => _ /set1P->.
by rewrite -(autmE AutGa) morph1 perm1.
Qed.

End Automorphism.

Arguments Aut _ _%g.
Notation "[ 'Aut' G ]" := (Aut_group G)
  (at level 0, format "[ 'Aut'  G ]") : Group_scope.
Notation "[ 'Aut' G ]" := (Aut G)
  (at level 0, only parsing) : group_scope.

Prenex Implicits Aut autm.

(* The permutation function (total on the underlying groupType) that is the *)
(* representant of a given morphism f with domain A in (Aut A).             *)

Section PermIn.

Variables (T : finType) (A : {set T}) (f : T -> T).

Hypotheses (injf : {in A &, injective f}) (sBf : f @: A \subset A).

Lemma perm_in_inj : injective (fun x => if x \in A then f x else x).
Proof.
move=> x y /=; wlog Ay: x y / y \in A.
  by move=> IH eqfxy; case: ifP (eqfxy); [symmetry | case: ifP => //]; auto.
rewrite Ay; case: ifP => [Ax | nAx def_x]; first exact: injf.
by case/negP: nAx; rewrite def_x (subsetP sBf) ?imset_f.
Qed.

Definition perm_in := perm perm_in_inj.

Lemma perm_in_on : perm_on A perm_in.
Proof.
by apply/subsetP=> x; rewrite inE /= permE; case: ifP => // _; case/eqP.
Qed.

Lemma perm_inE : {in A, perm_in =1 f}.
Proof. by move=> x Ax; rewrite /= permE Ax. Qed.

End PermIn.

(* properties of injective endomorphisms *)

Section MakeAut.

Variables (gT : finGroupType) (G : {group gT}) (f : {morphism G >-> gT}).
Implicit Type A : {set gT}.

Hypothesis injf : 'injm f.

Lemma morphim_fixP A : A \subset G -> reflect (f @* A = A) (f @* A \subset A).
Proof.
rewrite /morphim => sAG; have:= eqEcard (f @: A) A.
rewrite (setIidPr sAG) card_in_imset ?leqnn ?andbT  => [<-|]; first exact: eqP.
by move/injmP: injf; apply: sub_in2; apply/subsetP.
Qed.

Hypothesis Gf : f @* G = G.

Lemma aut_closed : f @: G \subset G.
Proof. by rewrite -morphimEdom; apply/morphim_fixP. Qed.

Definition aut := perm_in (injmP injf) aut_closed.

Lemma autE : {in G, aut =1 f}.
Proof. exact: perm_inE. Qed.

Lemma morphic_aut : morphic G aut.
Proof. by apply/morphicP=> x y Gx Gy /=; rewrite !autE ?groupM // morphM. Qed.

Lemma Aut_aut : aut \in Aut G.
Proof. by rewrite inE morphic_aut perm_in_on. Qed.

Lemma imset_autE A : A \subset G -> aut @: A = f @* A.
Proof.
move=> sAG; rewrite /morphim (setIidPr sAG).
by apply: eq_in_imset; apply: sub_in1 autE; apply/subsetP.
Qed.

Lemma preim_autE A : A \subset G -> aut @^-1: A = f @*^-1 A.
Proof.
move=> sAG; apply/setP=> x; rewrite !inE permE /=.
by case Gx: (x \in G) => //; apply/negP=> Ax; rewrite (subsetP sAG) in Gx.
Qed.

End MakeAut.

Arguments morphim_fixP {gT G f}.
Prenex Implicits aut.

Section AutIsom.

Variables (gT rT : finGroupType) (G D : {group gT}) (f : {morphism D >-> rT}).

Hypotheses (injf : 'injm f) (sGD : G \subset D).
Let domG := subsetP sGD.

Lemma Aut_isom_subproof a :
  {a' | a' \in Aut (f @* G) & a \in Aut G -> {in G, a' \o f =1 f \o a}}.
Proof.
set Aut_a := autm (subgP (subg [Aut G] a)).
have aDom: 'dom (f \o Aut_a \o invm injf) = f @* G.
  rewrite /dom /= morphpre_invm -morphpreIim; congr (f @* _).
  by rewrite [_ :&: D](setIidPl _) ?injmK ?injm_autm ?im_autm.
have [af [def_af ker_af _ im_af]] := domP _ aDom.
have inj_a': 'injm af by rewrite ker_af !injm_comp ?injm_autm ?injm_invm.
have im_a': af @* (f @* G) = f @* G.
  by rewrite im_af !morphim_comp morphim_invm // im_autm.
pose a' := aut inj_a' im_a'; exists a' => [|AutGa x Gx]; first exact: Aut_aut.
have Dx := domG Gx; rewrite /= [a' _]autE ?mem_morphim //.
by rewrite def_af /= invmE // autmE subgK.
Qed.

Definition Aut_isom a := s2val (Aut_isom_subproof a).

Lemma Aut_Aut_isom a : Aut_isom a \in Aut (f @* G).
Proof. by rewrite /Aut_isom; case: (Aut_isom_subproof a). Qed.

Lemma Aut_isomE a : a \in Aut G -> {in G, forall x, Aut_isom a (f x) = f (a x)}.
Proof. by rewrite /Aut_isom; case: (Aut_isom_subproof a). Qed.

Lemma Aut_isomM : {in Aut G &, {morph Aut_isom: x y / x * y}}.
Proof.
move=> a b AutGa AutGb.
apply: (eq_Aut (Aut_Aut_isom _)); rewrite ?groupM ?Aut_Aut_isom // => fx.
case/morphimP=> x Dx Gx ->{fx}.
by rewrite permM !Aut_isomE ?groupM /= ?permM ?Aut_closed.
Qed.
Canonical Aut_isom_morphism := Morphism Aut_isomM.

Lemma injm_Aut_isom : 'injm Aut_isom.
Proof.
apply/injmP=> a b AutGa AutGb eq_ab'; apply: (eq_Aut AutGa AutGb) => x Gx.
by apply: (injmP injf); rewrite ?domG ?Aut_closed // -!Aut_isomE //= eq_ab'.
Qed.

End AutIsom.

Section InjmAut.

Variables (gT rT : finGroupType) (G D : {group gT}) (f : {morphism D >-> rT}).

Hypotheses (injf : 'injm f) (sGD : G \subset D).
Let domG := subsetP sGD.

Lemma im_Aut_isom : Aut_isom injf sGD @* Aut G = Aut (f @* G).
Proof.
apply/eqP; rewrite eqEcard; apply/andP; split.
  by apply/subsetP=> _ /morphimP[a _ AutGa ->]; apply: Aut_Aut_isom.
have inj_isom' := injm_Aut_isom (injm_invm injf) (morphimS _ sGD).
rewrite card_injm ?injm_Aut_isom // -(card_injm inj_isom') ?subset_leq_card //.
apply/subsetP=> a /morphimP[a' _ AutfGa' def_a].
by rewrite -(morphim_invm injf sGD) def_a Aut_Aut_isom.
Qed.

Lemma Aut_isomP : isom (Aut G) (Aut (f @* G)) (Aut_isom injf sGD).
Proof. by apply/isomP; split; [apply: injm_Aut_isom | apply: im_Aut_isom]. Qed.

Lemma injm_Aut : Aut (f @* G) \isog Aut G.
Proof. by rewrite isog_sym (isom_isog _ _ Aut_isomP). Qed.

End InjmAut.

(* conjugation automorphism *)

Section ConjugationMorphism.

Variable gT : finGroupType.
Implicit Type A : {set gT}.

Definition conjgm of {set gT} := fun x y : gT => y ^ x.

Lemma conjgmE A x y : conjgm A x y = y ^ x. Proof. by []. Qed.

Canonical conjgm_morphism A x :=
  @Morphism _ _ A (conjgm A x) (in2W (fun y z => conjMg y z x)).

Lemma morphim_conj A x B : conjgm A x @* B = (A :&: B) :^ x.
Proof. by []. Qed.

Variable G : {group gT}.

Lemma injm_conj x : 'injm (conjgm G x).
Proof. by apply/injmP; apply: in2W; apply: conjg_inj. Qed.

Lemma conj_isom x : isom G (G :^ x) (conjgm G x).
Proof. by apply/isomP; rewrite morphim_conj setIid injm_conj. Qed.

Lemma conj_isog x : G \isog G :^ x.
Proof. exact: isom_isog (conj_isom x). Qed.

Lemma norm_conjg_im x : x \in 'N(G) -> conjgm G x @* G = G.
Proof. by rewrite morphimEdom; apply: normP. Qed.

Lemma norm_conj_isom x : x \in 'N(G) -> isom G G (conjgm G x).
Proof. by move/norm_conjg_im/restr_isom_to/(_ (conj_isom x))->. Qed.

Definition conj_aut x := aut (injm_conj _) (norm_conjg_im (subgP (subg _ x))).

Lemma norm_conj_autE : {in 'N(G) & G, forall x y, conj_aut x y = y ^ x}.
Proof. by move=> x y nGx Gy; rewrite /= autE //= subgK. Qed.

Lemma conj_autE : {in G &, forall x y, conj_aut x y = y ^ x}.
Proof. by apply: sub_in11 norm_conj_autE => //; apply: subsetP (normG G). Qed.

Lemma conj_aut_morphM : {in 'N(G) &, {morph conj_aut : x y / x * y}}.
Proof.
move=> x y nGx nGy; apply/permP=> z /=; rewrite permM.
case Gz: (z \in G); last by rewrite !permE /= !Gz.
by rewrite !norm_conj_autE // (conjgM, memJ_norm, groupM).
Qed.

Canonical conj_aut_morphism := Morphism conj_aut_morphM.

Lemma ker_conj_aut : 'ker conj_aut = 'C(G).
Proof.
apply/setP=> x /[1!inE]; case nGx: (x \in 'N(G)); last first.
  by symmetry; apply/idP=> cGx; rewrite (subsetP (cent_sub G)) in nGx.
rewrite 2!inE /=; apply/eqP/centP=> [cx1 y Gy | cGx].
  by rewrite /commute (conjgC y) -norm_conj_autE // cx1 perm1.
apply/permP=> y; case Gy: (y \in G); last by rewrite !permE Gy.
by rewrite perm1 norm_conj_autE // conjgE -cGx ?mulKg.
Qed.

Lemma Aut_conj_aut A : conj_aut @* A \subset Aut G.
Proof. by apply/subsetP=> _ /imsetP[x _ ->]; apply: Aut_aut. Qed.

End ConjugationMorphism.

Arguments conjgm _ _%g.
Prenex Implicits conjgm conj_aut.

Reserved Notation "G \char H" (at level 70).

(* Characteristic subgroup *)

Section Characteristicity.

Variable gT : finGroupType.
Implicit Types A B : {set gT}.
Implicit Types G H K L : {group gT}.

Definition characteristic A B :=
  (A \subset B) && [forall f in Aut B, f @: A \subset A].

Infix "\char" := characteristic.

Lemma charP H G :
  let fixH (f : {morphism G >-> gT}) := 'injm f -> f @* G = G -> f @* H = H in
  reflect [/\ H \subset G & forall f, fixH f] (H \char G).
Proof.
do [apply: (iffP andP) => -[sHG chHG]; split] => // [f injf Gf|].
  by apply/morphim_fixP; rewrite // -imset_autE ?(forall_inP chHG) ?Aut_aut.
apply/forall_inP=> f Af; rewrite -(autmE Af) -morphimEsub //.
by rewrite chHG ?injm_autm ?im_autm.
Qed.

(* Characteristic subgroup properties : composition, relational properties *)

Lemma char1 G : 1 \char G.
Proof. by apply/charP; split=> [|f _ _]; rewrite (sub1G, morphim1). Qed.

Lemma char_refl G : G \char G.
Proof. exact/charP. Qed.

Lemma char_trans H G K : K \char H -> H \char G -> K \char G.
Proof.
case/charP=> sKH chKH; case/charP=> sHG chHG.
apply/charP; split=> [|f injf Gf]; first exact: subset_trans sHG.
rewrite -{1}(setIidPr sKH) -(morphim_restrm sHG) chKH //.
  by rewrite ker_restrm; move/trivgP: injf => ->; apply: subsetIr.
by rewrite morphim_restrm setIid chHG.
Qed.

Lemma char_norms H G : H \char G -> 'N(G) \subset 'N(H).
Proof.
case/charP=> sHG chHG; apply/normsP=> x /normP-Nx.
have:= chHG [morphism of conjgm G x] => /=.
by rewrite !morphimEsub //=; apply; rewrite // injm_conj.
Qed.

Lemma char_sub A B : A \char B -> A \subset B.
Proof. by case/andP. Qed.

Lemma char_norm_trans H G A : H \char G -> A \subset 'N(G) -> A \subset 'N(H).
Proof. by move/char_norms=> nHnG nGA; apply: subset_trans nHnG. Qed.

Lemma char_normal_trans H G K : K \char H -> H <| G -> K <| G.
Proof.
move=> chKH /andP[sHG nHG].
by rewrite /normal (subset_trans (char_sub chKH)) // (char_norm_trans chKH).
Qed.

Lemma char_normal H G : H \char G -> H <| G.
Proof. by move/char_normal_trans; apply; apply/andP; rewrite normG. Qed.

Lemma char_norm H G : H \char G -> G \subset 'N(H).
Proof. by case/char_normal/andP. Qed.

Lemma charI G H K : H \char G -> K \char G -> H :&: K \char G.
Proof.
case/charP=> sHG chHG; case/charP=> _ chKG.
apply/charP; split=> [|f injf Gf]; first by rewrite subIset // sHG.
by rewrite morphimGI ?(chHG, chKG) //; apply: subset_trans (sub1G H).
Qed.

Lemma charY G H K : H \char G -> K \char G -> H <*> K \char G.
Proof.
case/charP=> sHG chHG; case/charP=> sKG chKG.
apply/charP; split=> [|f injf Gf]; first by rewrite gen_subG subUset sHG.
by rewrite morphim_gen ?(morphimU, subUset, sHG, chHG, chKG).
Qed.

Lemma charM G H K : H \char G -> K \char G -> H * K \char G.
Proof.
move=> chHG chKG; rewrite -norm_joinEl ?charY //.
exact: subset_trans (char_sub chHG) (char_norm chKG).
Qed.

Lemma lone_subgroup_char G H :
  H \subset G -> (forall K, K \subset G -> K \isog H -> K \subset H) ->
  H \char G.
Proof.
move=> sHG Huniq; apply/charP; split=> // f injf Gf; apply/eqP.
have{} injf: {in H &, injective f}.
  by move/injmP: injf; apply: sub_in2; apply/subsetP.
have fH: f @* H = f @: H by rewrite /morphim (setIidPr sHG).
rewrite eqEcard {2}fH card_in_imset ?{}Huniq //=.
  by rewrite -{3}Gf morphimS.
rewrite isog_sym; apply/isogP.
exists [morphism of restrm sHG f] => //=; first exact/injmP.
by rewrite morphimEdom fH.
Qed.

End Characteristicity.

Arguments characteristic _ _%g _%g.
Notation "H \char G" := (characteristic H G) : group_scope.
#[global] Hint Resolve char_refl : core.

Section InjmChar.

Variables (aT rT : finGroupType) (D : {group aT}) (f : {morphism D >-> rT}).

Hypothesis injf : 'injm f.

Lemma injm_char (G H : {group aT}) :
  G \subset D -> H \char G -> f @* H \char f @* G.
Proof.
move=> sGD /charP[sHG charH].
apply/charP; split=> [|g injg gfG]; first exact: morphimS.
have /domP[h [_ ker_h _ im_h]]: 'dom (invm injf \o g \o f) = G.
  by rewrite /dom /= -(morphpreIim g) (setIidPl _) ?injmK // gfG morphimS.
have hH: h @* H = H.
  apply: charH; first by rewrite ker_h !injm_comp ?injm_invm.
  by rewrite im_h !morphim_comp gfG morphim_invm.
rewrite /= -{2}hH im_h !morphim_comp morphim_invmE morphpreK //.
by rewrite (subset_trans _ (morphimS f sGD)) //= -{3}gfG !morphimS.
Qed.

End InjmChar.

Section CharInjm.

Variables (aT rT : finGroupType) (D : {group aT}) (f : {morphism D >-> rT}).
Hypothesis injf : 'injm f.

Lemma char_injm (G H : {group aT}) :
  G \subset D -> H \subset D -> (f @* H \char f @* G) = (H \char G).
Proof.
move=> sGD sHD; apply/idP/idP; last exact: injm_char.
by move/(injm_char (injm_invm injf)); rewrite !morphim_invm ?morphimS // => ->.
Qed.

End CharInjm.

Unset Implicit Arguments.