formal/coq/math-comp/solvable/center.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 seq div.
From mathcomp Require Import fintype bigop finset fingroup morphism perm.
From mathcomp Require Import automorphism quotient action gproduct gfunctor.
From mathcomp Require Import cyclic.

(******************************************************************************)
(* Definition of the center of a group and of external central products:      *)
(*           'Z(G) == the center of the group G, i.e., 'C_G(G).               *)
(*   cprod_by isoZ == the finGroupType for the central product of H and K     *)
(*                    with centers identified by the isomorphism gz on 'Z(H); *)
(*                    here isoZ : isom 'Z(H) 'Z(K) gz. Note that the actual   *)
(*                    central product is [set: cprod_by isoZ].                *)
(*    cpairg1 isoZ == the isomorphism from H to cprod_by isoZ, isoZ as above. *)
(*    cpair1g isoZ == the isomorphism from K to cprod_by isoZ, isoZ as above. *)
(*      xcprod H K == the finGroupType for the external central product of H  *)
(*                    and K with identified centers, provided the dynamically *)
(*                    tested condition 'Z(H) \isog 'Z(K) holds.               *)
(*      ncprod H n == the finGroupType for the central product of n copies of *)
(*                    H with their centers identified; [set: ncprod H 0] is   *)
(*                    isomorphic to 'Z(H).                                    *)
(*  xcprodm cf eqf == the morphism induced on cprod_by isoZ, where as above   *)
(*                    isoZ : isom 'Z(H) 'Z(K) gz, by fH : {morphism H >-> rT} *)
(*                    and fK : {morphism K >-> rT}, given both                *)
(*                    cf : fH @* H \subset 'C(fK @* K) and                    *)
(*                    eqf : {in 'Z(H), fH =1 fK \o gz}.                       *)
(*   Following Aschbacher, we only provide external central products with     *)
(* identified centers, as these are well defined provided the local center    *)
(* isomorphism group of one of the subgroups is full. Nevertheless the        *)
(* entire construction could be carried out under the weaker assumption that  *)
(* gz is an isomorphism between subgroups of 'Z(H) and 'Z(K), and even the    *)
(* uniqueness theorem holds under the weaker assumption that gz map 'Z(H) to  *)
(* a characteristic subgroup of 'Z(K) not isomorphic to any other subgroup of *)
(* 'Z(K), a condition that holds for example when K is cyclic, as in the      *)
(* structure theorem for p-groups of symplectic type.                         *)
(******************************************************************************)

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

Import GroupScope.

Section Defs.

Variable gT : finGroupType.

Definition center (A : {set gT}) := 'C_A(A).

Canonical center_group (G : {group gT}) : {group gT} :=
  Eval hnf in [group of center G].

End Defs.

Arguments center {gT} A%g.
Notation "''Z' ( A )" := (center A) : group_scope.
Notation "''Z' ( H )" := (center_group H) : Group_scope.

Lemma morphim_center : GFunctor.pcontinuous (@center).
Proof. by move=> gT rT G D f; apply: morphim_subcent. Qed.

Canonical center_igFun := [igFun by fun _ _ => subsetIl _ _ & morphim_center].
Canonical center_gFun := [gFun by morphim_center].
Canonical center_pgFun := [pgFun by morphim_center].

Section Center.

Variables gT : finGroupType.
Implicit Type rT : finGroupType.
Implicit Types (x y : gT) (A B : {set gT}) (G H K D : {group gT}).

Lemma subcentP A B x : reflect (x \in A /\ centralises x B) (x \in 'C_A(B)).
Proof.
rewrite inE. case: (x \in A); last by right; case.
by apply: (iffP centP) => [|[]].
Qed.

Lemma subcent_sub A B : 'C_A(B) \subset 'N_A(B).
Proof. by rewrite setIS ?cent_sub. Qed.

Lemma subcent_norm G B : 'N_G(B) \subset 'N('C_G(B)).
Proof. by rewrite normsI ?subIset ?normG // orbC cent_norm.  Qed.

Lemma subcent_normal G B : 'C_G(B) <| 'N_G(B).
Proof. by rewrite /normal subcent_sub subcent_norm. Qed.

Lemma subcent_char G H K : H \char G -> K \char G -> 'C_H(K) \char G.
Proof.
case/charP=> sHG chHG /charP[sKG chKG]; apply/charP.
split=> [|f injf Gf]; first by rewrite subIset ?sHG.
by rewrite injm_subcent ?chHG ?chKG.
Qed.

Lemma centerP A x : reflect (x \in A /\ centralises x A) (x \in 'Z(A)).
Proof. exact: subcentP. Qed.

Lemma center_sub A : 'Z(A) \subset A.
Proof. exact: subsetIl. Qed.

Lemma center1 : 'Z(1) = 1 :> {set gT}.
Proof. exact: gF1. Qed.

Lemma centerC A : {in A, centralised 'Z(A)}.
Proof. by apply/centsP; rewrite centsC subsetIr. Qed.

Lemma center_normal G : 'Z(G) <| G.
Proof. exact: gFnormal. Qed.

Lemma sub_center_normal H G : H \subset 'Z(G) -> H <| G.
Proof. by rewrite subsetI centsC /normal => /andP[-> /cents_norm]. Qed.

Lemma center_abelian G : abelian 'Z(G).
Proof. by rewrite /abelian subIset // centsC subIset // subxx orbT. Qed.

Lemma center_char G : 'Z(G) \char G.
Proof. exact: gFchar. Qed.

Lemma center_idP A : reflect ('Z(A) = A) (abelian A).
Proof. exact: setIidPl. Qed.

Lemma center_class_formula G :
  #|G| = #|'Z(G)| + \sum_(xG in [set x ^: G | x in G :\: 'C(G)]) #|xG|.
Proof.
by rewrite acts_sum_card_orbit ?cardsID // astabsJ normsD ?norms_cent ?normG.
Qed.

Lemma subcent1P A x y : reflect (y \in A /\ commute x y) (y \in 'C_A[x]).
Proof.
rewrite inE; case: (y \in A); last by right; case.
by apply: (iffP cent1P) => [|[]].
Qed.

Lemma subcent1_id x G : x \in G -> x \in 'C_G[x].
Proof. by move=> Gx; rewrite inE Gx; apply/cent1P. Qed.

Lemma subcent1_sub x G : 'C_G[x] \subset G.
Proof. exact: subsetIl. Qed.

Lemma subcent1C x y G : x \in G -> y \in 'C_G[x] -> x \in 'C_G[y].
Proof. by move=> Gx /subcent1P[_ cxy]; apply/subcent1P. Qed.

Lemma subcent1_cycle_sub x G : x \in G -> <[x]> \subset 'C_G[x].
Proof. by move=> Gx; rewrite cycle_subG ?subcent1_id. Qed.

Lemma subcent1_cycle_norm x G : 'C_G[x] \subset 'N(<[x]>).
Proof. by rewrite cents_norm // cent_gen cent_set1 subsetIr. Qed.

Lemma subcent1_cycle_normal x G : x \in G -> <[x]> <| 'C_G[x].
Proof.
by move=> Gx; rewrite /normal subcent1_cycle_norm subcent1_cycle_sub.
Qed.

(* Gorenstein. 1.3.4 *)
Lemma cyclic_center_factor_abelian G : cyclic (G / 'Z(G)) -> abelian G.
Proof.
case/cyclicP=> a Ga; case: (cosetP a) => /= z Nz def_a.
have G_Zz: G :=: 'Z(G) * <[z]>.
  rewrite -quotientK ?cycle_subG ?quotient_cycle //=.
  by rewrite -def_a -Ga quotientGK // center_normal.
rewrite G_Zz abelianM cycle_abelian center_abelian centsC /= G_Zz.
by rewrite subIset ?centS ?orbT ?mulG_subr.
Qed.

Lemma cyclic_factor_abelian H G :
  H \subset 'Z(G) -> cyclic (G / H) -> abelian G.
Proof.
move=> sHZ cycGH; apply: cyclic_center_factor_abelian.
have /andP[_ nHG]: H <| G := sub_center_normal sHZ.
have [f <-]:= homgP (homg_quotientS nHG (gFnorm _ G) sHZ).
exact: morphim_cyclic cycGH.
Qed.

Section Injm.

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

Hypothesis injf : 'injm f.

Lemma injm_center G : G \subset D -> f @* 'Z(G) = 'Z(f @* G).
Proof. exact: injm_subcent. Qed.

End Injm.

End Center.

Arguments center_idP {gT A}.

Lemma isog_center (aT rT : finGroupType) (G : {group aT}) (H : {group rT}) :
  G \isog H -> 'Z(G) \isog 'Z(H).
Proof. exact: gFisog. Qed.

Section Product.

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

Lemma center_prod H K : K \subset 'C(H) -> 'Z(H) * 'Z(K) = 'Z(H * K).
Proof.
move=> cHK; apply/setP=> z; rewrite {3}/center centM !inE.
have cKH: H \subset 'C(K) by rewrite centsC.
apply/imset2P/and3P=> [[x y /setIP[Hx cHx] /setIP[Ky cKy] ->{z}]| []].
  by rewrite imset2_f ?groupM // ?(subsetP cHK) ?(subsetP cKH).
case/imset2P=> x y Hx Ky ->{z}.
rewrite groupMr => [cHx|]; last exact: subsetP Ky.
rewrite groupMl => [cKy|]; last exact: subsetP Hx.
by exists x y; rewrite ?inE ?Hx ?Ky.
Qed.

Lemma center_cprod A B G : A \* B = G -> 'Z(A) \* 'Z(B) = 'Z(G).
Proof.
case/cprodP => [[H K -> ->] <- cHK].
rewrite cprodE ?center_prod //= subIset ?(subset_trans cHK) //.
by rewrite centS ?center_sub.
Qed.

Lemma center_bigcprod I r P (F : I -> {set gT}) G :
    \big[cprod/1]_(i <- r | P i) F i = G ->
  \big[cprod/1]_(i <- r | P i) 'Z(F i) = 'Z(G).
Proof.
elim/big_ind2: _ G => [_ <-|A B C D IHA IHB G dG|_ _ G ->]; rewrite ?center1 //.
case/cprodP: dG IHA IHB (dG) => [[H K -> ->] _ _] IHH IHK dG.
by rewrite (IHH H) // (IHK K) // (center_cprod dG).
Qed.

Lemma cprod_center_id G : G \* 'Z(G) = G.
Proof. by rewrite cprodE ?subsetIr // mulGSid ?center_sub. Qed.

Lemma center_dprod A B G : A \x B = G -> 'Z(A) \x 'Z(B) = 'Z(G).
Proof.
case/dprodP=> [[H1 H2 -> ->] defG cH12 trH12].
move: defG; rewrite -cprodE // => /center_cprod/cprodP[_ /= <- cZ12].
by apply: dprodE; rewrite //= setIAC setIA -setIA trH12 (setIidPl _) ?sub1G.
Qed.

Lemma center_bigdprod I r P (F: I -> {set gT}) G :
    \big[dprod/1]_(i <- r | P i) F i = G ->
  \big[dprod/1]_(i <- r | P i) 'Z(F i) = 'Z(G).
Proof.
elim/big_ind2: _ G => [_ <-|A B C D IHA IHB G dG|_ _ G ->]; rewrite ?center1 //.
case/dprodP: dG IHA IHB (dG) => [[H K -> ->] _ _ _] IHH IHK dG.
by rewrite (IHH H) // (IHK K) // (center_dprod dG).
Qed.

Lemma Aut_cprod_full G H K :
    H \* K = G -> 'Z(H) = 'Z(K) ->
    Aut_in (Aut H) 'Z(H) \isog Aut 'Z(H) ->
    Aut_in (Aut K) 'Z(K) \isog Aut 'Z(K) ->
  Aut_in (Aut G) 'Z(G) \isog Aut 'Z(G).
Proof.
move=> defG eqZHK; have [_ defHK cHK] := cprodP defG.
have defZ: 'Z(G) = 'Z(H) by rewrite -defHK -center_prod // eqZHK mulGid.
have ziHK: H :&: K = 'Z(K).
  by apply/eqP; rewrite eqEsubset subsetI -{1 2}eqZHK !center_sub setIS.
have AutZP := Aut_sub_fullP (@center_sub gT _).
move/AutZP=> AutZHfull /AutZP AutZKfull; apply/AutZP=> g injg gZ.
have [gH [def_gH ker_gH _ im_gH]] := domP g defZ.
have [gK [def_gK ker_gK _ im_gK]] := domP g (etrans defZ eqZHK).
have [injgH injgK]: 'injm gH /\ 'injm gK by rewrite ker_gH ker_gK.
have [gHH gKK]: gH @* 'Z(H) = 'Z(H) /\ gK @* 'Z(K) = 'Z(K).
  by rewrite im_gH im_gK -eqZHK -defZ.
have [|fH [injfH im_fH fHZ]] := AutZHfull gH injgH.
  by rewrite im_gH /= -defZ.
have [|fK [injfK im_fK fKZ]] := AutZKfull gK injgK.
  by rewrite im_gK /= -eqZHK -defZ.
have cfHK: fK @* K \subset 'C(fH @* H) by rewrite im_fH im_fK.
have eq_fHK: {in H :&: K, fH =1 fK}.
  by move=> z; rewrite ziHK => Zz; rewrite fHZ ?fKZ /= ?eqZHK // def_gH def_gK.
exists (cprodm_morphism defG cfHK eq_fHK).
rewrite injm_cprodm injfH injfK im_cprodm im_fH im_fK defHK.
rewrite -morphimIdom ziHK -eqZHK injm_center // im_fH eqxx.
split=> //= z; rewrite {1}defZ => Zz; have [Hz _] := setIP Zz.
by rewrite cprodmEl // fHZ // def_gH.
Qed.

End Product.

Section CprodBy.

Variables gTH gTK : finGroupType.
Variables (H : {group gTH}) (K : {group gTK}) (gz : {morphism 'Z(H) >-> gTK}).

Definition ker_cprod_by of isom 'Z(H) 'Z(K) gz :=
  [set xy | let: (x, y) := xy in (x \in 'Z(H)) && (y == (gz x)^-1)].

Hypothesis isoZ : isom 'Z(H) 'Z(K) gz.
Let kerHK := ker_cprod_by isoZ.

Let injgz : 'injm gz. Proof. by case/isomP: isoZ. Qed.
Let gzZ : gz @* 'Z(H) = 'Z(K). Proof. by case/isomP: isoZ. Qed.
Let gzZchar : gz @* 'Z(H) \char 'Z(K). Proof. by rewrite gzZ. Qed.
Let sgzZZ : gz @* 'Z(H) \subset 'Z(K) := char_sub gzZchar.
Let sZH := center_sub H.
Let sZK := center_sub K.
Let sgzZG : gz @* 'Z(H) \subset K := subset_trans sgzZZ sZK.

Lemma ker_cprod_by_is_group : group_set kerHK.
Proof.
apply/group_setP; rewrite inE /= group1 morph1 invg1 /=.
split=> // [[x1 y1] [x2 y2]].
rewrite inE /= => /andP[Zx1 /eqP->]; have [_ cGx1] := setIP Zx1.
rewrite inE /= => /andP[Zx2 /eqP->]; have [Gx2 _] := setIP Zx2.
by rewrite inE /= groupM //= -invMg (centP cGx1) // morphM.
Qed.
Canonical ker_cprod_by_group := Group ker_cprod_by_is_group.

Lemma ker_cprod_by_central : kerHK \subset 'Z(setX H K).
Proof.
rewrite -(center_dprod (setX_dprod H K)) -morphim_pairg1 -morphim_pair1g.
rewrite -!injm_center ?subsetT ?injm_pair1g ?injm_pairg1 //=.
rewrite morphim_pairg1 morphim_pair1g setX_dprod.
apply/subsetP=> [[x y]] /[1!inE] /andP[Zx /eqP->].
by rewrite inE /= Zx groupV (subsetP sgzZZ) ?mem_morphim.
Qed.

Fact cprod_by_key : unit. Proof. by []. Qed.
Definition cprod_by_def := subFinGroupType [group of setX H K / kerHK].
Definition cprod_by := locked_with cprod_by_key cprod_by_def.
Local Notation C := [set: FinGroup.arg_sort (FinGroup.base cprod_by)].

Definition in_cprod : gTH * gTK -> cprod_by :=
  let: tt as k := cprod_by_key return _ -> locked_with k cprod_by_def in
  subg _ \o coset kerHK.

Lemma in_cprodM : {in setX H K &, {morph in_cprod : u v / u * v}}.
Proof.
rewrite /in_cprod /cprod_by; case: cprod_by_key => /= u v Gu Gv.
have nkerHKG := normal_norm (sub_center_normal ker_cprod_by_central).
by rewrite -!morphM ?mem_quotient // (subsetP nkerHKG).
Qed.
Canonical in_cprod_morphism := Morphism in_cprodM.

Lemma ker_in_cprod : 'ker in_cprod = kerHK.
Proof.
transitivity ('ker (subg [group of setX H K / kerHK] \o coset kerHK)).
  rewrite /ker /morphpre /= /in_cprod /cprod_by; case: cprod_by_key => /=.
  by rewrite ['N(_) :&: _]quotientGK ?sub_center_normal ?ker_cprod_by_central.
by rewrite ker_comp ker_subg -kerE ker_coset.
Qed.

Lemma cpairg1_dom : H \subset 'dom (in_cprod \o @pairg1 gTH gTK).
Proof. by rewrite -sub_morphim_pre ?subsetT // morphim_pairg1 setXS ?sub1G. Qed.

Lemma cpair1g_dom : K \subset 'dom (in_cprod \o @pair1g gTH gTK).
Proof. by rewrite -sub_morphim_pre ?subsetT // morphim_pair1g setXS ?sub1G. Qed.

Definition cpairg1 := tag (restrmP _ cpairg1_dom).
Definition cpair1g := tag (restrmP _ cpair1g_dom).

Local Notation CH := (mfun cpairg1 @* gval H).
Local Notation CK := (mfun cpair1g @* gval K).

Lemma injm_cpairg1 : 'injm cpairg1.
Proof.
rewrite /cpairg1; case: restrmP => _ [_ -> _ _].
rewrite ker_comp ker_in_cprod; apply/subsetP=> x; rewrite 5!inE /=.
by case/and3P=> _ Zx; rewrite inE eq_sym (inv_eq invgK) invg1 morph_injm_eq1.
Qed.
Let injH := injm_cpairg1.

Lemma injm_cpair1g : 'injm cpair1g.
Proof.
rewrite /cpair1g; case: restrmP => _ [_ -> _ _].
rewrite ker_comp ker_in_cprod; apply/subsetP=> y; rewrite !inE /= morph1 invg1.
by case/and3P.
Qed.
Let injK := injm_cpair1g.

Lemma im_cpair_cent : CK \subset 'C(CH).
Proof.
rewrite /cpairg1 /cpair1g; do 2!case: restrmP => _ [_ _ _ -> //].
rewrite !morphim_comp morphim_cents // morphim_pair1g morphim_pairg1.
by case/dprodP: (setX_dprod H K).
Qed.
Hint Resolve im_cpair_cent : core.

Lemma im_cpair : CH * CK = C.
Proof.
rewrite /cpairg1 /cpair1g; do 2!case: restrmP => _ [_ _ _ -> //].
rewrite !morphim_comp -morphimMl morphim_pairg1 ?setXS ?sub1G //.
rewrite morphim_pair1g setX_prod morphimEdom /= /in_cprod /cprod_by.
by case: cprod_by_key; rewrite /= imset_comp imset_coset -morphimEdom im_subg.
Qed.

Lemma im_cpair_cprod : CH \* CK = C. Proof. by rewrite cprodE ?im_cpair. Qed.

Lemma eq_cpairZ : {in 'Z(H), cpairg1 =1 cpair1g \o gz}.
Proof.
rewrite /cpairg1 /cpair1g => z1 Zz1; set z2 := gz z1.
have Zz2: z2 \in 'Z(K) by rewrite (subsetP sgzZZ) ?mem_morphim.
have [[Gz1 _] [/= Gz2 _]]:= (setIP Zz1, setIP Zz2).
do 2![case: restrmP => f /= [df _ _ _]; rewrite {f}df].
apply/rcoset_kerP; rewrite ?inE ?group1 ?andbT //.
by rewrite ker_in_cprod mem_rcoset inE /= invg1 mulg1 mul1g Zz1 /=.
Qed.

Lemma setI_im_cpair : CH :&: CK = 'Z(CH).
Proof.
apply/eqP; rewrite eqEsubset setIS //=.
rewrite subsetI center_sub -injm_center //.
rewrite (eq_in_morphim _ eq_cpairZ); first by rewrite morphim_comp morphimS.
by rewrite !(setIidPr _) // -sub_morphim_pre.
Qed.

Lemma cpair1g_center : cpair1g @* 'Z(K) = 'Z(C).
Proof.
case/cprodP: (center_cprod im_cpair_cprod) => _ <- _.
by rewrite injm_center // -setI_im_cpair mulSGid //= setIC setIS 1?centsC.
Qed.

(* Uses gzZ. *)
Lemma cpair_center_id : 'Z(CH) = 'Z(CK).
Proof.
rewrite -!injm_center // -gzZ -morphim_comp; apply: eq_in_morphim eq_cpairZ.
by rewrite !(setIidPr _) // -sub_morphim_pre.
Qed.

(* Uses gzZ. *)
Lemma cpairg1_center : cpairg1 @* 'Z(H) = 'Z(C).
Proof. by rewrite -cpair1g_center !injm_center // cpair_center_id. Qed.

Section ExtCprodm.

Variable rT : finGroupType.
Variables (fH : {morphism H >-> rT}) (fK : {morphism K >-> rT}).
Hypothesis cfHK : fK @* K \subset 'C(fH @* H).
Hypothesis eq_fHK : {in 'Z(H), fH =1 fK \o gz}.

Let gH := ifactm fH injm_cpairg1.
Let gK := ifactm fK injm_cpair1g.

Lemma xcprodm_cent : gK @* CK \subset 'C(gH @* CH).
Proof. by rewrite !im_ifactm. Qed.

Lemma xcprodmI : {in CH :&: CK, gH =1 gK}.
Proof.
rewrite setI_im_cpair -injm_center // => fHx; case/morphimP=> x Gx Zx ->{fHx}.
by rewrite {2}eq_cpairZ //= ?ifactmE ?eq_fHK //= (subsetP sgzZG) ?mem_morphim.
Qed.

Definition xcprodm := cprodm im_cpair_cprod xcprodm_cent xcprodmI.
Canonical xcprod_morphism := [morphism of xcprodm].

Lemma xcprodmEl : {in H, forall x, xcprodm (cpairg1 x) = fH x}.
Proof. by move=> x Hx; rewrite /xcprodm cprodmEl ?mem_morphim ?ifactmE. Qed.

Lemma xcprodmEr : {in K, forall y, xcprodm (cpair1g y) = fK y}.
Proof. by move=> y Ky; rewrite /xcprodm cprodmEr ?mem_morphim ?ifactmE. Qed.

Lemma xcprodmE :
  {in H & K, forall x y, xcprodm (cpairg1 x * cpair1g y) = fH x * fK y}.
Proof.
by move=> x y Hx Ky; rewrite /xcprodm cprodmE ?mem_morphim ?ifactmE.
Qed.

Lemma im_xcprodm : xcprodm @* C = fH @* H * fK @* K.
Proof. by rewrite -im_cpair morphim_cprodm // !im_ifactm. Qed.

Lemma im_xcprodml A : xcprodm @* (cpairg1 @* A) = fH @* A.
Proof.
rewrite -!(morphimIdom _ A) morphim_cprodml ?morphimS ?subsetIl //.
by rewrite morphim_ifactm ?subsetIl.
Qed.

Lemma im_xcprodmr A : xcprodm @* (cpair1g @* A) = fK @* A.
Proof.
rewrite -!(morphimIdom _ A) morphim_cprodmr ?morphimS ?subsetIl //.
by rewrite morphim_ifactm ?subsetIl.
Qed.

Lemma injm_xcprodm : 'injm xcprodm = 'injm fH && 'injm fK.
Proof.
rewrite injm_cprodm !ker_ifactm !subG1 !morphim_injm_eq1 ?subsetIl // -!subG1.
apply: andb_id2l => /= injfH; apply: andb_idr => _.
rewrite !im_ifactm // -(morphimIdom gH) setI_im_cpair -injm_center //.
rewrite morphim_ifactm // eqEsubset subsetI morphimS //=.
rewrite {1}injm_center // setIS //=.
rewrite (eq_in_morphim _ eq_fHK); first by rewrite morphim_comp morphimS.
by rewrite !(setIidPr _) // -sub_morphim_pre.
Qed.

End ExtCprodm.

(* Uses gzZchar. *)
Lemma Aut_cprod_by_full :
    Aut_in (Aut H) 'Z(H) \isog Aut 'Z(H) ->
    Aut_in (Aut K) 'Z(K) \isog Aut 'Z(K) ->
  Aut_in (Aut C) 'Z(C) \isog Aut 'Z(C).
Proof.
move=> AutZinH AutZinK.
have Cfull:= Aut_cprod_full im_cpair_cprod cpair_center_id.
by rewrite Cfull // -injm_center // injm_Aut_full ?center_sub.
Qed.

Section Isomorphism.

Let gzZ_lone (Y : {group gTK}) :
  Y \subset 'Z(K) -> gz @* 'Z(H) \isog Y -> gz @* 'Z(H) = Y.
Proof.
move=> sYZ isoY; apply/eqP.
by rewrite eq_sym eqEcard (card_isog isoY) gzZ sYZ /=.
Qed.

Variables (rT : finGroupType) (GH GK G : {group rT}).
Hypotheses (defG : GH \* GK = G) (ziGHK : GH :&: GK = 'Z(GH)).
Hypothesis AutZHfull : Aut_in (Aut H) 'Z(H) \isog Aut 'Z(H).
Hypotheses (isoGH : GH \isog H) (isoGK : GK \isog K).

(* Uses gzZ_lone *)
Lemma cprod_by_uniq :
  exists f : {morphism G >-> cprod_by},
    [/\ isom G C f, f @* GH = CH & f @* GK = CK].
Proof.
have [_ defGHK cGKH] := cprodP defG.
have AutZinH := Aut_sub_fullP sZH AutZHfull.
have [fH injfH defGH]:= isogP (isog_symr isoGH).
have [fK injfK defGK]:= isogP (isog_symr isoGK).
have sfHZfK: fH @* 'Z(H) \subset fK @* K.
  by rewrite injm_center //= defGH defGK -ziGHK subsetIr.
have gzZ_id: gz @* 'Z(H) = invm injfK @* (fH @* 'Z(H)).
  apply: gzZ_lone => /=.
    rewrite injm_center // defGH -ziGHK sub_morphim_pre /= ?defGK ?subsetIr //.
    by rewrite setIC morphpre_invm injm_center // defGK setIS 1?centsC.
  rewrite -morphim_comp.
  apply: isog_trans (sub_isog _ _); first by rewrite isog_sym sub_isog.
    by rewrite -sub_morphim_pre.
  by rewrite !injm_comp ?injm_invm.
have: 'dom (invm injfH \o fK \o gz) = 'Z(H).
  rewrite /dom /= -(morphpreIdom gz); apply/setIidPl.
  by rewrite -2?sub_morphim_pre // gzZ_id morphim_invmE morphpreK ?morphimS.
case/domP=> gzH [def_gzH ker_gzH _ im_gzH].
have{ker_gzH} injgzH: 'injm gzH by rewrite ker_gzH !injm_comp ?injm_invm.
have{AutZinH} [|gH [injgH gH_H def_gH]] := AutZinH _ injgzH.
  by rewrite im_gzH !morphim_comp /= gzZ_id !morphim_invmE morphpreK ?injmK.
have: 'dom (fH \o gH) = H by rewrite /dom /= -{3}gH_H injmK.
case/domP=> gfH [def_gfH ker_gfH _ im_gfH].
have{im_gfH} gfH_H: gfH @* H = GH by rewrite im_gfH morphim_comp gH_H.
have cgfHfK: fK @* K \subset 'C(gfH @* H) by rewrite gfH_H defGK.
have eq_gfHK: {in 'Z(H), gfH =1 fK \o gz}.
  move=> z Zz; rewrite def_gfH /= def_gH //= def_gzH /= invmK //.
  have {Zz}: gz z \in gz @* 'Z(H) by rewrite mem_morphim.
  rewrite gzZ_id morphim_invmE; case/morphpreP=> _.
  exact: (subsetP (morphimS _ _)).
pose f := xcprodm cgfHfK eq_gfHK.
have injf: 'injm f by rewrite injm_xcprodm ker_gfH injm_comp.
have fCH: f @* CH = GH by rewrite im_xcprodml gfH_H.
have fCK: f @* CK = GK by rewrite im_xcprodmr defGK.
have fC: f @* C = G by rewrite im_xcprodm gfH_H defGK defGHK.
have [f' [_ ker_f' _ im_f']] := domP (invm_morphism injf) fC.
exists f'; rewrite -fCH -fCK !{1}im_f' !{1}morphim_invm ?subsetT //.
by split=> //; apply/isomP; rewrite ker_f' injm_invm im_f' -fC im_invm.
Qed.

Lemma isog_cprod_by : G \isog C.
Proof. by have [f [isoG _ _]] := cprod_by_uniq; apply: isom_isog isoG. Qed.

End Isomorphism.

End CprodBy.

Section ExtCprod.
Import finfun.

Variables gTH gTK : finGroupType.
Variables (H : {group gTH}) (K : {group gTK}).

Let gt_ b := if b then gTK else gTH.
Local Notation isob := ('Z(H) \isog 'Z(K)) (only parsing).
Let G_ b := if b as b' return {group gt_ b'} then K else H.

Lemma xcprod_subproof :
  {gz : {morphism 'Z(H) >-> gt_ isob} | isom 'Z(H) 'Z(G_ isob) gz}.
Proof.
case: (pickP [pred f : {ffun _} | misom 'Z(H) 'Z(K) f]) => [f isoZ | no_f].
  rewrite (misom_isog isoZ); case/andP: isoZ => fM isoZ.
  by exists [morphism of morphm fM].
move/pred0P: no_f => not_isoZ; rewrite [isob](congr1 negb not_isoZ).
by exists (idm_morphism  _); apply/isomP; rewrite injm_idm im_idm.
Qed.

Definition xcprod := cprod_by (svalP xcprod_subproof).

Inductive xcprod_spec : finGroupType -> Prop :=
  XcprodSpec gz isoZ : xcprod_spec (@cprod_by gTH gTK H K gz isoZ).

Lemma xcprodP : 'Z(H) \isog 'Z(K) -> xcprod_spec xcprod.
Proof. by rewrite /xcprod => isoZ; move: xcprod_subproof; rewrite isoZ. Qed.

Lemma isog_xcprod (rT : finGroupType) (GH GK G : {group rT}) :
    Aut_in (Aut H) 'Z(H) \isog Aut 'Z(H) ->
    GH \isog H -> GK \isog K -> GH \* GK = G -> 'Z(GH) = 'Z(GK) ->
  G \isog [set: xcprod].
Proof.
move=> AutZinH isoGH isoGK defG eqZGHK; have [_ _ cGHK] := cprodP defG.
have [|gz isoZ] := xcprodP.
  have [[fH injfH <-] [fK injfK <-]] := (isogP isoGH, isogP isoGK).
  rewrite -!injm_center -?(isog_transl _ (sub_isog _ _)) ?center_sub //=.
  by rewrite eqZGHK sub_isog ?center_sub.
rewrite (isog_cprod_by _ defG) //.
by apply/eqP; rewrite eqEsubset setIS // subsetI {2}eqZGHK !center_sub.
Qed.

End ExtCprod.

Section IterCprod.

Variables (gT : finGroupType) (G : {group gT}).

Fixpoint ncprod_def n : finGroupType :=
  if n is n'.+1 then xcprod G [set: ncprod_def n']
  else [finGroupType of subg_of 'Z(G)].
Fact ncprod_key : unit. Proof. by []. Qed.
Definition ncprod := locked_with ncprod_key ncprod_def.

Local Notation G_ n := [set: gsort (ncprod n)].

Lemma ncprod0 : G_ 0 \isog 'Z(G).
Proof. by rewrite [ncprod]unlock isog_sym isog_subg. Qed.

Lemma center_ncprod0 : 'Z(G_ 0) = G_ 0.
Proof. by apply: center_idP; rewrite (isog_abelian ncprod0) center_abelian. Qed.

Lemma center_ncprod n : 'Z(G_ n) \isog 'Z(G).
Proof.
elim: n => [|n]; first by rewrite center_ncprod0 ncprod0.
rewrite [ncprod]unlock=> /isog_symr/xcprodP[gz isoZ] /=.
by rewrite -cpairg1_center isog_sym sub_isog ?center_sub ?injm_cpairg1.
Qed.

Lemma ncprodS n : xcprod_spec G [set: ncprod n] (ncprod n.+1).
Proof.
by have:= xcprodP (isog_symr (center_ncprod n)); rewrite [ncprod]unlock.
Qed.

Lemma ncprod1 : G_ 1 \isog G.
Proof.
case: ncprodS => gz isoZ; rewrite isog_sym /= -im_cpair.
rewrite mulGSid /=; first by rewrite sub_isog ?injm_cpairg1.
rewrite -{3}center_ncprod0 injm_center ?injm_cpair1g //.
by rewrite -cpair_center_id center_sub.
Qed.

Lemma Aut_ncprod_full n :
    Aut_in (Aut G) 'Z(G) \isog Aut 'Z(G) ->
  Aut_in (Aut (G_ n)) 'Z(G_ n) \isog Aut 'Z(G_ n).
Proof.
move=> AutZinG; elim: n => [|n IHn].
  by rewrite center_ncprod0; apply/Aut_sub_fullP=> // g injg gG0; exists g.
by case: ncprodS => gz isoZ; apply: Aut_cprod_by_full.
Qed.

End IterCprod.