(* 071_order_of_a_subgroup__lagrange_s_theorem — rocq *)

(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From HB Require Import structures.
From mathcomp Require Import ssreflect ssrbool ssrfun eqtype ssrnat seq choice.
From mathcomp Require Import fintype div path tuple bigop prime finset.
From mathcomp Require Export monoid.

(******************************************************************************)
(*                               Finite groups                                *)
(*                                                                            *)
(* NB: See CONTRIBUTING.md for an introduction to HB concepts and commands.   *)
(*                                                                            *)
(* This file defines the main interface for finite groups:                    *)
(*          finGroupType == the structure for finite types with a group law   *)
(*                          The HB class is called FinGroup.                  *)
(*           {group gT}  == type of groups with elements of type gT           *)
(*     finStarMonoidType == the structure for finite star-monoids             *)
(*                          The HB class is called FinStarMonoid.             *)
(*    FinGroupType mulVg == the finGroupType structure for an existing        *)
(*                          finStarMonoidType structure, built from a proof of*)
(*                          the left inverse group axiom for that structure's *)
(*                          operations                                        *)
(*          [group of G] == a clone for an existing {group gT} structure on   *)
(*                          G : {set gT} (the existing structure might be for *)
(*                          some delta-expansion of G)                        *)
(* If gT implements finGroupType, then we can form {set gT}, the type of      *)
(* finite sets with elements of type gT (as finGroupType extends finType).    *)
(* The group law extends pointwise to {set gT}, which thus implements a sub-  *)
(* interface finStarMonoidType of finGroupType. To be consistent with the     *)
(* predType interface, this is done by coercion to FinGroup.arg_sort, an      *)
(* alias for FinGroup.sort. Accordingly, all pointwise group operations below *)
(* have arguments of type (FinGroup.arg_sort) gT and return results of type   *)
(* FinGroup.sort gT.                                                          *)
(*   The notations below are declared in two scopes:                          *)
(*      group_scope (delimiter %g) for point operations and set constructs.   *)
(*      Group_scope (delimiter %G) for explicit {group gT} structures.        *)
(* These scopes should not be opened globally, although group_scope is often  *)
(* opened locally in group-theory files.                                      *)
(*   As {group gT} is both a subtype and an interface structure for {set gT}, *)
(* the fact that a given G : {set gT} is a group can (and usually should) be  *)
(* inferred by type inference with canonical structures. This means that all  *)
(* `group' constructions (e.g., the normaliser 'N_G(H)) actually define sets  *)
(* with a canonical {group gT} structure; the %G delimiter can be used to     *)
(* specify the actual {group gT} structure (e.g., 'N_G(H)%G).                 *)
(*  Operations on elements of a group:                                        *)
(*                x * y == the group product of x and y                       *)
(*               x ^+ n == the nth power of x, i.e., x * ... * x (n times)    *)
(*                 x^-1 == the group inverse of x                             *)
(*               x ^- n == the inverse of x ^+ n (notation for (x ^+ n)^-1)   *)
(*                    1 == the unit element                                   *)
(*                x ^ y == the conjugate of x by y (i.e., y^-1 * (x * y))     *)
(*            [~ x, y]  == the commutator of x and y (i.e., x^-1 * x ^ y)     *)
(*     [~ x1, ..., xn]  == the commutator of x1, ..., xn (associating left)   *)
(*    \prod_(i ...) x i == the product of the x i (order-sensitive)           *)
(*         commute x y  <-> x and y commute                                   *)
(*      centralises x A <-> x centralises A                                   *)
(*                'C[x] == the set of elements that commute with x            *)
(*              'C_G[x] == the set of elements of G that commute with x       *)
(*                <[x]> == the cyclic subgroup generated by the element x     *)
(*                 #[x] == the order of the element x, i.e., #|<[x]>|         *)
(*  Operations on subsets/subgroups of a finite group:                        *)
(*                H * G == {xy | x \in H, y \in G}                            *)
(*   1 or [1] or [1 gT] == the unit group                                     *)
(*          [set: gT]%G == the group of all x : gT (in Group_scope)           *)
(*          group_set G == G contains 1 and is closed under binary product;   *)
(*                         this is the characteristic property of the         *)
(*                         {group gT} subtype of {set gT}                     *)
(*             [subg G] == the subtype, set, or group of all x \in G: this    *)
(*                         notation is defined simultaneously in %type, %g    *)
(*                         and %G scopes, and G must denote a {group gT}      *)
(*                         structure (G is in the %G scope)                   *)
(*          subg, sgval == the projection into and injection from [subg G]    *)
(*                  H^# == the set H minus the unit element                   *)
(*               repr H == some element of H if 1 \notin H != set0, else 1    *)
(*                         (repr is defined over sets of a finStarMonoidType, *)
(*                         so it can be used, e.g., to pick right cosets.)    *)
(*               x *: H == left coset of H by x                               *)
(*          lcosets H G == the set of the left cosets of H by elements of G   *)
(*               H :* x == right coset of H by x                              *)
(*          rcosets H G == the set of the right cosets of H by elements of G  *)
(*             #|G : H| == the index of H in G, i.e., #|rcosets G H|          *)
(*               H :^ x == the conjugate of H by x                            *)
(*               x ^: H == the conjugate class of x in H                      *)
(*            classes G == the set of all conjugate classes of G              *)
(*              G :^: H == {G :^ x | x \in H}                                 *)
(*    class_support G H == {x ^ y | x \in G, y \in H}                         *)
(*        commg_set G H == {[~ x, y] | x \in G, y \in H}; NOT the commutator! *)
(*                <<H>> == the subgroup generated by the set H                *)
(*            [~: G, H] == the commmutator subgroup of G and H, i.e.,         *)
(*                         <<commg_set G H>>>                                 *)
(*     [~: H1, ..., Hn] == commutator subgroup of H1, ..., Hn (left assoc.)   *)
(*              H <*> G == the subgroup generated by sets H and G (H join G)  *)
(*            (H * G)%G == the join of G H : {group gT} (convertible, but not *)
(*                         identical to (G <*> H)%G)                          *)
(* (\prod_(i ...) H i)%G == the group generated by the H i                    *)
(* {in G, centralised H} <-> G centralises H                                  *)
(* {in G, normalised H} <-> G normalises H                                    *)
(*                      <-> forall x, x \in G -> H :^ x = H                   *)
(*                'N(H) == the normaliser of H                                *)
(*              'N_G(H) == the normaliser of H in G                           *)
(*               H <| G <=> H is a normal subgroup of G                       *)
(*                'C(H) == the centraliser of H                               *)
(*              'C_G(H) == the centraliser of H in G                          *)
(*            gcore H G == the largest subgroup of H normalised by G          *)
(*                         If H is a subgroup of G, this is the largest       *)
(*                         normal subgroup of G contained in H).              *)
(*            abelian H <=> H is abelian                                      *)
(*          subgroups G == the set of subgroups of G, i.e., the set of all    *)
(*                         H : {group gT} such that H \subset G               *)
(* In the notation below G is a variable that is bound in P.                  *)
(*          [max G | P] <=> G is the largest group such that P holds          *)
(*     [max H of G | P] <=> H is the largest group G such that P holds        *)
(*      [max G | P & Q] := [max G | P && Q], likewise [max H of G | P & Q]    *)
(*          [min G | P] <=> G is the smallest group such that P holds         *)
(*      [min G | P & Q] := [min G | P && Q], likewise [min H of G | P & Q]    *)
(*     [min H of G | P] <=> H is the smallest group G such that P holds       *)
(* In addition to the generic suffixes described in ssrbool.v and finset.v,   *)
(* we associate the following suffixes to group operations:                   *)
(*   1 - identity element, as in group1 : 1 \in G                             *)
(*   M - multiplication, as is invMg : (x * y)^-1 = y^-1 * x^-1               *)
(*       Also nat multiplication, for expgM : x ^+ (m * n) = x ^+ m ^+ n      *)
(*   D - (nat) addition, for expgD : x ^+ (m + n) = x ^+ m * x ^+ n           *)
(*   V - inverse, as in mulgV : x * x^-1 = 1                                  *)
(*   X - exponentiation, as in conjXg : (x ^+ n) ^ y = (x ^ y) ^+ n           *)
(*   J - conjugation, as in orderJ : #[x ^ y] = #[x]                          *)
(*   R - commutator, as in conjRg : [~ x, y] ^ z = [~ x ^ z, y ^ z]           *)
(*   Y - join, as in centY : 'C(G <*> H) = 'C(G) :&: 'C(H)                    *)
(* We sometimes prefix these with an `s' to indicate a set-lifted operation,  *)
(* e.g., conjsMg : (A * B) :^ x = A :^ x * B :^ x.                            *)
(******************************************************************************)

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

Declare Scope Group_scope.

Delimit Scope Group_scope with G.

Module GroupScope.
Open Scope group_scope.
End GroupScope.
Local Open Scope group_scope.

(* These are the operation notations introduced by this file. *)
Reserved Notation "[ ~ x1 , x2 , .. , xn ]"
  (format  "'[ ' [ ~  x1 , '/'  x2 , '/'  .. , '/'  xn ] ']'").
Reserved Notation "[ 1 gT ]" (format "[ 1  gT ]").
Reserved Notation "[ 1 ]" (format "[ 1 ]").
Reserved Notation "[ 'subg' G ]" (format "[ 'subg'  G ]").
#[warning="-postfix-notation-not-level-1"]
Reserved Notation "A ^#" (at level 3, format "A ^#").

Reserved Notation "A :^ x" (at level 35, right associativity).
Reserved Notation "x ^: B" (at level 35, right associativity).
Reserved Notation "A :^: B" (at level 35, right associativity).
Reserved Notation "#| B : A |" (A at level 99, format "#| B  :  A |").
Reserved Notation "''N' ( A )" (format "''N' ( A )").
Reserved Notation "''N_' G ( A )" (G at level 2, format "''N_' G ( A )").
Reserved Notation "A <| B" (at level 70, no associativity).
Reserved Notation "A <*> B" (at level 40, left associativity).
Reserved Notation "[ ~: A1 , A2 , .. , An ]"
  (format "[ ~: '['  A1 , '/'  A2 , '/'  .. , '/'  An ']' ]").
Reserved Notation "[ 'max' A 'of' G | gP ]"
  (format "[ '[hv' 'max'  A  'of'  G '/ '  |  gP ']' ]").
Reserved Notation "[ 'max' G | gP ]"
  (format "[ '[hv' 'max'  G '/ '  |  gP ']' ]").
Reserved Notation "[ 'max' A 'of' G | gP & gQ ]"
  (format "[ '[hv' 'max'  A  'of'  G '/ '  |  gP '/ '  &  gQ ']' ]").
Reserved Notation "[ 'max' G | gP & gQ ]"
  (format "[ '[hv' 'max'  G '/ '  |  gP '/ '  &  gQ ']' ]").
Reserved Notation "[ 'min' A 'of' G | gP ]"
  (format "[ '[hv' 'min'  A  'of'  G '/ '  |  gP ']' ]").
Reserved Notation "[ 'min' G | gP ]"
  (format "[ '[hv' 'min'  G '/ '  |  gP ']' ]").
Reserved Notation "[ 'min' A 'of' G | gP & gQ ]"
  (format "[ '[hv' 'min'  A  'of'  G '/ '  |  gP '/ '  &  gQ ']' ]").
Reserved Notation "[ 'min' G | gP & gQ ]"
  (format "[ '[hv' 'min'  G '/ '  |  gP '/ '  &  gQ ']' ]").

Module isMulBaseGroup.
#[deprecated(since="mathcomp 2.5.0", use=isStarMonoid.Build)]
Notation Build G := (isStarMonoid.Build G) (only parsing).
End isMulBaseGroup.

#[deprecated(since="mathcomp 2.5.0", use=isStarMonoid)]
Notation isMulBaseGroup G := (isStarMonoid G) (only parsing).

Module BaseFinGroup_isGroup.
#[deprecated(since="mathcomp 2.5.0", use=StarMonoid_isGroup.Build)]
Notation Build G := (StarMonoid_isGroup.Build G) (only parsing).
End BaseFinGroup_isGroup.

#[deprecated(since="mathcomp 2.5.0", use=StarMonoid_isGroup)]
Notation BaseFinGroup_isGroup G := (StarMonoid_isGroup G) (only parsing).

#[arg_sort, short(type="finStarMonoidType")]
HB.structure Definition FinStarMonoid := { G of StarMonoid G & Finite G }.

#[deprecated(since="mathcomp 2.5.0", use=finStarMonoidType)]
Notation baseFinGroupType := finStarMonoidType (only parsing).

Module BaseFinGroup.
#[deprecated(since="mathcomp 2.5.0", use=FinStarMonoid.sort)]
Notation sort := (FinStarMonoid.sort) (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=FinStarMonoid.arg_sort)]
Notation arg_sort := (FinStarMonoid.arg_sort) (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=FinStarMonoid.on)]
Notation on M := (FinStarMonoid.on M) (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=FinStarMonoid.copy)]
Notation copy M N := (FinStarMonoid.copy M N) (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=FinStarMonoid.clone)]
Notation clone M N := (FinStarMonoid.clone M N) (only parsing).
End BaseFinGroup.

#[deprecated(since="mathcomp 2.4.0", use=FinStarMonoid)]
Notation BaseFinGroup R := (FinStarMonoid R) (only parsing).

Module FinStarMonoidExports.
Bind Scope group_scope with FinStarMonoid.arg_sort.
Bind Scope group_scope with FinStarMonoid.sort.
End FinStarMonoidExports.
HB.export FinStarMonoidExports.

#[deprecated(since="mathcomp 2.5.0", use=one)]
Notation oneg := one (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=mul)]
Notation mulg := mul (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=inv)]
Notation invg := inv (only parsing).
#[deprecated(since="mathcomp 2.5.0", use=natexp)]
Notation expgn := natexp (only parsing).

#[short(type="finGroupType")]
HB.structure Definition FinGroup :=
  { G of Group G & Finite G }.

Module FinGroupExports.
Bind Scope group_scope with FinGroup.sort.
End FinGroupExports.
HB.export FinGroupExports.

HB.factory Record Finite_isGroup G & Finite G := {
  mul : G -> G -> G;
  one : G;
  inv : G -> G;
  mulgA : associative mul;
  mul1g : left_id one mul;
  mulVg : left_inverse one inv mul;
}.

Module isMulGroup.
#[deprecated(since="mathcomp 2.5.0", use=Finite_isGroup.Build)]
Notation Build G := (Finite_isGroup.Build G) (only parsing).
End isMulGroup.
#[deprecated(since="mathcomp 2.5.0", use=Finite_isGroup)]
Notation isMulGroup G := (Finite_isGroup G) (only parsing).

HB.builders Context G & Finite_isGroup G.

Notation "1" := one.
Infix "*" := mul.
Notation "x ^-1" := (inv x).

Lemma invgK : involutive inv.
Proof.
have mulV21 x: x^-1^-1 * 1 = x by rewrite -(mulVg x) mulgA mulVg mul1g.
by move=> x; rewrite -[_ ^-1]mulV21 -(mul1g 1) mulgA !mulV21.
Qed.

Lemma invMg : {morph inv : x y / x * y >-> y * x}.
Proof.
have mulgV x: x * x^-1 = 1 by rewrite -{1}[x]invgK mulVg.
move=> x y /=; rewrite -[y^-1 * _]mul1g -(mulVg (x * y)) -2!mulgA (mulgA y).
by rewrite mulgV mul1g mulgV -(mulgV (x * y)) mulgA mulVg mul1g.
Qed.

HB.instance Definition _ :=
  isStarMonoid.Build G mulgA mul1g invgK invMg.
HB.instance Definition _ := StarMonoid_isGroup.Build G mulVg.

HB.end.

#[compress_coercions]
HB.instance Definition _ (T : finStarMonoidType) :
    Finite (FinStarMonoid.arg_sort T) := Finite.class T.

Notation conjg := conjg (only parsing).
Notation commg := commg (only parsing).
Notation mulgA := mulgA (only parsing).
Notation mul1g := mul1g (only parsing).
Notation invgK := invgK (only parsing).
Notation invMg := invgM (only parsing).
Notation invg_inj := invg_inj (only parsing).
Notation eq_invg_sym := eqg_invLR (only parsing).
Notation invg1 := invg1 (only parsing).
Notation eq_invg1 := invg_eq1 (only parsing).
Notation mulg1 := mulg1 (only parsing).
Notation expgnE := expgnE (only parsing).
Notation expg0 := expg0 (only parsing).
Notation expg1 := expg1 (only parsing).
Notation expg1n := expg1n (only parsing).
Notation expgD := expgnDr (only parsing).
Notation expgSr := expgSr (only parsing).
Notation expgM := expgnA (only parsing).
Notation expgAC := expgnAC (only parsing).
Notation commute := commute (only parsing).
Notation commute_refl := commute_refl (only parsing).
Notation commute_sym := commute_sym (only parsing).
Notation commute1 := commute1 (only parsing).
Notation commuteM := commuteM (only parsing).
Notation commuteX := commuteX (only parsing).
Notation commuteX2 := commuteX2 (only parsing).
Notation expgVn := expVgn (only parsing).
Notation expgMn := expgMn (only parsing).
Notation mulVg := mulVg (only parsing).
Notation mulgV := mulgV (only parsing).
Notation mulKg := mulKg (only parsing).
Notation mulKVg := mulVKg (only parsing).
Notation mulgI := mulgI (only parsing).
Notation mulgK := mulgK (only parsing).
Notation mulgKV := mulgVK (only parsing).
Lemma eq_invg_mul (T : finGroupType) x y : (x^-1 == y :> T) = (x * y == 1).
Proof. by rewrite mulg_eq1 eqg_invLR. Qed.
Lemma eq_mulgV1 (T : finGroupType) x y : (x == y) = (x * y^-1 == 1 :> T).
Proof. exact/esym/divg_eq1. Qed.
Lemma eq_mulVg1 (T : finGroupType) x y : (x == y) = (x^-1 * y == 1 :> T).
Proof. by rewrite mulg_eq1 eqg_inv. Qed.
Notation commuteV := commuteV (only parsing).
Notation conjgE := conjgE (only parsing).
Notation conjgC := conjgC (only parsing).
Notation conjgCV := conjgCV (only parsing).
Notation conjg1 := conjg1 (only parsing).
Notation conj1g := conj1g (only parsing).
Notation conjMg := conjMg (only parsing).
Notation conjgM := conjgM (only parsing).
Notation conjVg := conjVg (only parsing).
Notation conjJg := conjJg (only parsing).
Notation conjXg := conjXg (only parsing).
Notation conjgK := conjgK (only parsing).
Notation conjgKV := conjgKV (only parsing).
Notation conjg_inj := conjg_inj (only parsing).
Notation conjg_eq1 := conjg_eq1 (only parsing).
Notation conjg_prod := conjg_prod (only parsing).
Notation commgEl := commgEl (only parsing).
Notation commgEr := commgEr (only parsing).
Notation commgC := commgC (only parsing).
Notation commgCV := commgCV (only parsing).
Notation conjRg := conjRg (only parsing).
Notation invg_comm := invgR (only parsing).
Notation commgP := commgP (only parsing).
Notation conjg_fixP := conjg_fixP (only parsing).
Notation commg1_sym := commg1_sym (only parsing).
Notation commg1 := commg1 (only parsing).
Notation comm1g := comm1g (only parsing).
Notation commgg := commgg (only parsing).
Notation commgXg := commgXg (only parsing).
Notation commgVg := commgVg (only parsing).
Notation commgXVg := commgXVg (only parsing).

Section Repr.
(* Plucking a set representative. *)

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

Definition repr A := if 1 \in A then 1 else odflt 1 [pick x in A].

Lemma mem_repr A x : x \in A -> repr A \in A.
Proof.
by rewrite /repr; case: ifP => // _; case: pickP => // A0; rewrite [x \in A]A0.
Qed.

Lemma card_mem_repr A : #|A| > 0 -> repr A \in A.
Proof. by rewrite lt0n => /existsP[x]; apply: mem_repr. Qed.

Lemma repr_set1 x : repr [set x] = x.
Proof. by apply/set1P/card_mem_repr; rewrite cards1. Qed.

Lemma repr_set0 : repr set0 = 1.
Proof. by rewrite /repr; case: pickP => [x|_] /[!inE]. Qed.

End Repr.

Arguments mem_repr [gT A].

Section BaseSetMulDef.
(* We only assume a finStarMonoidType to allow this construct to be iterated. *)
Variable gT : finStarMonoidType.
Implicit Types A B : {set gT}.

(* Set-lifted group operations. *)

Definition set_mulg A B := mul @2: (A, B).
Definition set_invg A := inv @^-1: A.

(* The pre-group structure of group subsets. *)

Lemma set_mul1g : left_id [set 1] set_mulg.
Proof.
move=> A; apply/setP=> y; apply/imset2P/idP=> [[_ x /set1P-> Ax ->] | Ay].
  by rewrite mul1g.
by exists (1 : gT) y; rewrite ?(set11, mul1g).
Qed.

Lemma set_mulgA : associative set_mulg.
Proof.
move=> A B C; apply/setP=> y.
apply/imset2P/imset2P=> [[x1 z Ax1 /imset2P[x2 x3 Bx2 Cx3 ->] ->]| [z x3]].
  by exists (x1 * x2) x3; rewrite ?mulgA //; apply/imset2P; exists x1 x2.
case/imset2P=> x1 x2 Ax1 Bx2 -> Cx3 ->.
by exists x1 (x2 * x3); rewrite ?mulgA //; apply/imset2P; exists x2 x3.
Qed.

Lemma set_invgK : involutive set_invg.
Proof. by move=> A; apply/setP=> x; rewrite !inE invgK. Qed.

Lemma set_invgM : {morph set_invg : A B / set_mulg A B >-> set_mulg B A}.
Proof.
move=> A B; apply/setP=> z; rewrite inE.
apply/imset2P/imset2P=> [[x y Ax By /(canRL invgK)->] | [y x]].
  by exists y^-1 x^-1; rewrite ?invMg // inE invgK.
by rewrite !inE => By1 Ax1 ->; exists x^-1 y^-1; rewrite ?invMg.
Qed.

HB.instance Definition set_base_group := isStarMonoid.Build (set_type gT)
  set_mulgA set_mul1g set_invgK set_invgM.
HB.instance Definition _ : isStarMonoid {set gT} := set_base_group.

End BaseSetMulDef.

(* Time to open the bag of dirty tricks. When we define groups down below *)
(* as a subtype of {set gT}, we need them to be able to coerce to sets in *)
(* both set-style contexts (x \in G) and monoid-style contexts (G * H),   *)
(* and we need the coercion function to be EXACTLY the structure          *)
(* projection in BOTH cases -- otherwise the canonical unification breaks.*)
(*   Alas, Coq doesn't let us use the same coercion function twice, even  *)
(* when the targets are convertible. Our workaround (ab)uses the module   *)
(* system to declare two different identity coercions on an alias class.  *)

Module GroupSet.
Definition sort (gT : finStarMonoidType) := {set gT}.
End GroupSet.
Identity Coercion GroupSet_of_sort : GroupSet.sort >-> set_of.

Module Type GroupSetFinStarMonoidSig.
Definition sort (gT : finStarMonoidType) := FinStarMonoid.arg_sort {set gT}.
End GroupSetFinStarMonoidSig.

Module MakeGroupSetFinStarMonoid (Gset_base : GroupSetFinStarMonoidSig).
Identity Coercion of_sort : Gset_base.sort >-> FinStarMonoid.arg_sort.
End MakeGroupSetFinStarMonoid.

Module Export GroupSetFinStarMonoid := MakeGroupSetFinStarMonoid GroupSet.

Module Type GroupSetMagmaSig.
Definition sort (gT : finStarMonoidType) := Magma.sort {set gT}.
End GroupSetMagmaSig.

Module MakeGroupSetMagma (Gset_base : GroupSetMagmaSig).
Identity Coercion of_sort : Gset_base.sort >-> Magma.sort.
End MakeGroupSetMagma.

Module Export GroupSetMagma := MakeGroupSetMagma GroupSet.

Module Type GroupSetBaseGroupSig.
Definition sort (gT : finStarMonoidType) := BaseGroup.sort {set gT}.
End GroupSetBaseGroupSig.

Module MakeGroupSetBaseGroup (Gset_base : GroupSetBaseGroupSig).
Identity Coercion of_sort : Gset_base.sort >-> BaseGroup.sort.
End MakeGroupSetBaseGroup.

Module Export GroupSetBaseGroup := MakeGroupSetBaseGroup GroupSet.

HB.instance Definition _ gT : Finite (GroupSet.sort gT) :=
   Finite.class {set gT}.

Section GroupSetMulDef.
(* Some of these constructs could be defined on a finStarMonoidType. *)
(* We restrict them to proper finGroupType because we only develop   *)
(* the theory for that case.                                         *)
Variable gT : finGroupType.
Implicit Types A B : {set gT}.
Implicit Type x y : gT.

Definition lcoset A x := mul x @: A.
Definition rcoset A x := mul^~ x @: A.
Definition lcosets A B := lcoset A @: B.
Definition rcosets A B := rcoset A @: B.
Definition indexg B A := #|rcosets A B|.

Definition conjugate A x := conjg^~ x @: A.
Definition conjugates A B := conjugate A @: B.
Definition class x B := conjg x @: B.
Definition classes A := class^~ A @: A.
Definition class_support A B := conjg @2: (A, B).

Definition commg_set A B := commg @2: (A, B).

(* These will only be used later, but are defined here so that we can *)
(* keep all the Notation together.                                    *)
Definition normaliser A := [set x | conjugate A x \subset A].
Definition centraliser A := \bigcap_(x in A) normaliser [set x].
Definition abelian A := A \subset centraliser A.
Definition normal A B := (A \subset B) && (B \subset normaliser A).

(* "normalised" and "centralise[s|d]" are intended to be used with   *)
(* the {in ...} form, as in abelian below.                           *)
Definition normalised A := forall x, conjugate A x = A.
Definition centralises x A := forall y, y \in A -> commute x y.
Definition centralised A := forall x, centralises x A.

End GroupSetMulDef.

Arguments lcoset _ _%_g _%_g.
Arguments rcoset _ _%_g _%_g.
Arguments rcosets _ _%_g _%_g.
Arguments lcosets _ _%_g _%_g.
Arguments indexg _ _%_g _%_g.
Arguments conjugate _ _%_g _%_g.
Arguments conjugates _ _%_g _%_g.
Arguments class _ _%_g _%_g.
Arguments classes _ _%_g.
Arguments class_support _ _%_g _%_g.
Arguments commg_set _ _%_g _%_g.
Arguments normaliser _ _%_g.
Arguments centraliser _ _%_g.
Arguments abelian _ _%_g.
Arguments normal _ _%_g _%_g.
Arguments normalised _ _%_g.
Arguments centralises _ _%_g _%_g.
Arguments centralised _ _%_g.

Notation "[ 1 gT ]" := (1 : {set gT}) : group_scope.
Notation "[ 1 ]" := [1 FinGroup.sort _] : group_scope.

Notation "A ^#" := (A :\ 1) : group_scope.

Notation "x *: A" := ([set x%g] * A) : group_scope.
Notation "A :* x" := (A * [set x%g]) : group_scope.
Notation "A :^ x" := (conjugate A x) : group_scope.
Notation "x ^: B" := (class x B) : group_scope.
Notation "A :^: B" := (conjugates A B) : group_scope.

Notation "#| B : A |" := (indexg B A) : group_scope.

(* No notation for lcoset and rcoset, which are to be used mostly  *)
(* in curried form; x *: B and A :* 1 denote singleton products,   *)
(* so we can use mulgA, mulg1, etc, on, say, A :* 1 * B :* x.      *)
(* No notation for the set commutator generator set commg_set.     *)

Notation "''N' ( A )" := (normaliser A) : group_scope.
Notation "''N_' G ( A )" := (G%g :&: 'N(A)) : group_scope.
Notation "A <| B" := (normal A B) : group_scope.
Notation "''C' ( A )" := (centraliser A) : group_scope.
Notation "''C_' G ( A )" := (G%g :&: 'C(A)) : group_scope.
Notation "''C_' ( G ) ( A )" := 'C_G(A) (only parsing) : group_scope.
Notation "''C' [ x ]" := 'N([set x%g]) : group_scope.
Notation "''C_' G [ x ]" := 'N_G([set x%g]) : group_scope.
Notation "''C_' ( G ) [ x ]" := 'C_G[x] (only parsing) : group_scope.

Prenex Implicits repr lcoset rcoset lcosets rcosets normal.
Prenex Implicits conjugate conjugates class classes class_support.
Prenex Implicits commg_set normalised centralised abelian.

Section BaseSetMulProp.
(* Properties of the purely multiplicative structure. *)
Variable gT : finStarMonoidType.
Implicit Types A B C D : {set gT}.
Implicit Type x y z : gT.

(* Set product. We already have all the pregroup identities, so we *)
(* only need to add the monotonicity rules.                        *)

Lemma mulsgP A B x :
  reflect (imset2_spec mul (mem A) (fun _ => mem B) x) (x \in A * B).
Proof. exact: imset2P. Qed.

Lemma mem_mulg A B x y : x \in A -> y \in B -> x * y \in A * B.
Proof. by move=> Ax By; apply/mulsgP; exists x y. Qed.

Lemma prodsgP (I : finType) (P : pred I) (A : I -> {set gT}) x :
  reflect (exists2 c, forall i, P i -> c i \in A i & x = \prod_(i | P i) c i)
          (x \in \prod_(i | P i) A i).
Proof.
have [r big_r [Ur mem_r] _] := big_enumP P.
pose inA c := all (fun i => c i \in A i); rewrite -big_r; set piAx := x \in _.
suffices{big_r} IHr: reflect (exists2 c, inA c r & x = \prod_(i <- r) c i) piAx.
  apply: (iffP IHr) => -[c inAc ->]; do [exists c; last by rewrite big_r].
    by move=> i Pi; rewrite (allP inAc) ?mem_r.
  by apply/allP=> i; rewrite mem_r => /inAc.
elim: {P mem_r}r x @piAx Ur => /= [x _ | i r IHr x /andP[r'i /IHr{}IHr]].
  by rewrite unlock; apply: (iffP set1P) => [-> | [] //]; exists (fun=> x).
rewrite big_cons; apply: (iffP idP) => [|[c /andP[Aci Ac] ->]]; last first.
  by rewrite big_cons mem_mulg //; apply/IHr=> //; exists c.
case/mulsgP=> c_i _ Ac_i /IHr[c /allP-inAcr ->] ->{x}.
exists [eta c with i |-> c_i]; rewrite /= ?big_cons eqxx ?Ac_i.
  by apply/allP=> j rj; rewrite /= ifN ?(memPn r'i) ?inAcr.
by congr (_ * _); apply: eq_big_seq => j rj; rewrite ifN ?(memPn r'i).
Qed.

Lemma mem_prodg (I : finType) (P : pred I) (A : I -> {set gT}) c :
  (forall i, P i -> c i \in A i) -> \prod_(i | P i) c i \in \prod_(i | P i) A i.
Proof. by move=> Ac; apply/prodsgP; exists c. Qed.

Lemma mulSg A B C : A \subset B -> A * C \subset B * C.
Proof. exact: imset2Sl. Qed.

Lemma mulgS A B C : B \subset C -> A * B \subset A * C.
Proof. exact: imset2Sr. Qed.

Lemma mulgSS A B C D : A \subset B -> C \subset D -> A * C \subset B * D.
Proof. exact: imset2S. Qed.

Lemma mulg_subl A B : 1 \in B -> A \subset A * B.
Proof. by move=> B1; rewrite -{1}(mulg1 A) mulgS ?sub1set. Qed.

Lemma mulg_subr A B : 1 \in A -> B \subset A * B.
Proof. by move=> A1; rewrite -{1}(mul1g B) mulSg ?sub1set. Qed.

Lemma mulUg A B C : (A :|: B) * C = (A * C) :|: (B * C).
Proof. exact: imset2Ul. Qed.

Lemma mulgU A B C : A * (B :|: C) = (A * B) :|: (A * C).
Proof. exact: imset2Ur. Qed.

(* Set (pointwise) inverse. *)

Lemma invUg A B : (A :|: B)^-1 = A^-1 :|: B^-1.
Proof. exact: preimsetU. Qed.

Lemma invIg A B : (A :&: B)^-1 = A^-1 :&: B^-1.
Proof. exact: preimsetI. Qed.

Lemma invDg A B : (A :\: B)^-1 = A^-1 :\: B^-1.
Proof. exact: preimsetD. Qed.

Lemma invCg A : (~: A)^-1 = ~: A^-1.
Proof. exact: preimsetC. Qed.

Lemma invSg A B : (A^-1 \subset B^-1) = (A \subset B).
Proof. by rewrite !(sameP setIidPl eqP) -invIg (inj_eq invg_inj). Qed.

Lemma mem_invg x A : (x \in A^-1) = (x^-1 \in A).
Proof. by rewrite inE. Qed.

Lemma memV_invg x A : (x^-1 \in A^-1) = (x \in A).
Proof. by rewrite inE invgK. Qed.

Lemma card_invg A : #|A^-1| = #|A|.
Proof. exact/card_preimset/invg_inj. Qed.

(* Product with singletons. *)

Lemma set1gE : 1 = [set 1] :> {set gT}. Proof. by []. Qed.
Lemma set1gP x : reflect (x = 1) (x \in [1 gT]).
Proof. exact: set1P. Qed.

Lemma mulg_set1 x y : [set x] :* y = [set x * y].
Proof. by rewrite [_ * _]imset2_set1l imset_set1. Qed.

Lemma invg_set1 x : [set x]^-1 = [set x^-1].
Proof. by apply/setP=> y; rewrite !inE inv_eq //; apply: invgK. Qed.

End BaseSetMulProp.

Arguments set1gP {gT x}.
Arguments mulsgP {gT A B x}.
Arguments prodsgP {gT I P A x}.

Section GroupSetMulProp.
(* Constructs that need a finGroupType *)
Variable gT : finGroupType.
Implicit Types A B C D : {set gT}.
Implicit Type x y z : gT.

(* Left cosets. *)

Lemma lcosetE A x : lcoset A x = x *: A.
Proof. by rewrite [_ * _]imset2_set1l. Qed.

Lemma card_lcoset A x : #|x *: A| = #|A|.
Proof. by rewrite -lcosetE (card_imset _ (mulgI _)). Qed.

Lemma mem_lcoset A x y : (y \in x *: A) = (x^-1 * y \in A).
Proof. by rewrite -lcosetE [_ x](can_imset_pre _ (mulKg _)) inE. Qed.

Lemma lcosetP A x y : reflect (exists2 a, a \in A & y = x * a) (y \in x *: A).
Proof. by rewrite -lcosetE; apply: imsetP. Qed.

Lemma lcosetsP A B C :
  reflect (exists2 x, x \in B & C = x *: A) (C \in lcosets A B).
Proof. by apply: (iffP imsetP) => [] [x Bx ->]; exists x; rewrite ?lcosetE. Qed.

Lemma lcosetM A x y : (x * y) *: A = x *: (y *: A).
Proof. by rewrite -mulg_set1 mulgA. Qed.

Lemma lcoset1 A : 1 *: A = A.
Proof. exact: mul1g. Qed.

Lemma lcosetK : left_loop inv (fun x A => x *: A).
Proof. by move=> x A; rewrite -lcosetM mulVg mul1g. Qed.

Lemma lcosetKV : rev_left_loop inv (fun x A => x *: A).
Proof. by move=> x A; rewrite -lcosetM mulgV mul1g. Qed.

Lemma lcoset_inj : right_injective (fun x A => x *: A).
Proof. by move=> x; apply: can_inj (lcosetK x). Qed.

Lemma lcosetS x A B : (x *: A \subset x *: B) = (A \subset B).
Proof.
apply/idP/idP=> sAB; last exact: mulgS.
by rewrite -(lcosetK x A) -(lcosetK x B) mulgS.
Qed.

Lemma sub_lcoset x A B : (A \subset x *: B) = (x^-1 *: A \subset B).
Proof. by rewrite -(lcosetS x^-1) lcosetK. Qed.

Lemma sub_lcosetV x A B : (A \subset x^-1 *: B) = (x *: A \subset B).
Proof. by rewrite sub_lcoset invgK. Qed.

(* Right cosets. *)

Lemma rcosetE A x : rcoset A x = A :* x.
Proof. by rewrite [_ * _]imset2_set1r. Qed.

Lemma card_rcoset A x : #|A :* x| = #|A|.
Proof. by rewrite -rcosetE (card_imset _ (mulIg _)). Qed.

Lemma mem_rcoset A x y : (y \in A :* x) = (y * x^-1 \in A).
Proof. by rewrite -rcosetE  [_ x](can_imset_pre A (mulgK _)) inE. Qed.

Lemma rcosetP A x y : reflect (exists2 a, a \in A & y = a * x) (y \in A :* x).
Proof. by rewrite -rcosetE; apply: imsetP. Qed.

Lemma rcosetsP A B C :
  reflect (exists2 x, x \in B & C = A :* x) (C \in rcosets A B).
Proof. by apply: (iffP imsetP) => [] [x Bx ->]; exists x; rewrite ?rcosetE. Qed.

Lemma rcosetM A x y : A :* (x * y) = A :* x :* y.
Proof. by rewrite -mulg_set1 mulgA. Qed.

Lemma rcoset1 A : A :* 1 = A.
Proof. exact: mulg1. Qed.

Lemma rcosetK : right_loop inv (fun A x => A :* x).
Proof. by move=> x A; rewrite -rcosetM mulgV mulg1. Qed.

Lemma rcosetKV : rev_right_loop inv (fun A x => A :* x).
Proof. by move=> x A; rewrite -rcosetM mulVg mulg1. Qed.

Lemma rcoset_inj : left_injective (fun A x => A :* x).
Proof. by move=> x; apply: can_inj (rcosetK x). Qed.

Lemma rcosetS x A B : (A :* x \subset B :* x) = (A \subset B).
Proof.
apply/idP/idP=> sAB; last exact: mulSg.
by rewrite -(rcosetK x A) -(rcosetK x B) mulSg.
Qed.

Lemma sub_rcoset x A B : (A \subset B :* x) = (A :* x ^-1 \subset B).
Proof. by rewrite -(rcosetS x^-1) rcosetK. Qed.

Lemma sub_rcosetV x A B : (A \subset B :* x^-1) = (A :* x \subset B).
Proof. by rewrite sub_rcoset invgK. Qed.

(* Inverse maps lcosets to rcosets *)
Lemma invg_lcosets A B : (lcosets A B)^-1 = rcosets A^-1 B^-1.
Proof.
rewrite /A^-1/= -![_^-1](can_imset_pre _ invgK) -[RHS]imset_comp -imset_comp.
by apply: eq_imset => x /=; rewrite lcosetE rcosetE invMg invg_set1.
Qed.

(* Conjugates. *)

Lemma conjg_preim A x : A :^ x = (conjg^~ x^-1) @^-1: A.
Proof. exact: can_imset_pre (conjgK _). Qed.

Lemma mem_conjg A x y : (y \in A :^ x) = (y ^ x^-1 \in A).
Proof. by rewrite conjg_preim inE. Qed.

Lemma mem_conjgV A x y : (y \in A :^ x^-1) = (y ^ x \in A).
Proof. by rewrite mem_conjg invgK. Qed.

Lemma memJ_conjg A x y : (y ^ x \in A :^ x) = (y \in A).
Proof. by rewrite mem_conjg conjgK. Qed.

Lemma conjsgE A x : A :^ x = x^-1 *: (A :* x).
Proof. by apply/setP=> y; rewrite mem_lcoset mem_rcoset -mulgA mem_conjg. Qed.

Lemma conjsg1 A : A :^ 1 = A.
Proof. by rewrite conjsgE invg1 mul1g mulg1. Qed.

Lemma conjsgM A x y : A :^ (x * y) = (A :^ x) :^ y.
Proof. by rewrite !conjsgE invMg -!mulg_set1 !mulgA. Qed.

Lemma conjsgK : @right_loop _ gT inv conjugate.
Proof. by move=> x A; rewrite -conjsgM mulgV conjsg1. Qed.

Lemma conjsgKV : @rev_right_loop _ gT inv conjugate.
Proof. by move=> x A; rewrite -conjsgM mulVg conjsg1. Qed.

Lemma conjsg_inj : @left_injective _ gT _ conjugate.
Proof. by move=> x; apply: can_inj (conjsgK x). Qed.

Lemma cardJg A x : #|A :^ x| = #|A|.
Proof. by rewrite (card_imset _ (conjg_inj x)). Qed.

Lemma conjSg A B x : (A :^ x \subset B :^ x) = (A \subset B).
Proof. by rewrite !conjsgE lcosetS rcosetS. Qed.

Lemma properJ A B x : (A :^ x \proper B :^ x) = (A \proper B).
Proof. by rewrite /proper !conjSg. Qed.

Lemma sub_conjg A B x : (A :^ x \subset B) = (A \subset B :^ x^-1).
Proof. by rewrite -(conjSg A _ x) conjsgKV. Qed.

Lemma sub_conjgV A B x : (A :^ x^-1 \subset B) = (A \subset B :^ x).
Proof. by rewrite -(conjSg _ B x) conjsgKV. Qed.

Lemma conjg_set1 x y : [set x] :^ y = [set x ^ y].
Proof. by rewrite [_ :^ _]imset_set1. Qed.

Lemma conjs1g x : 1 :^ x = 1.
Proof. by rewrite conjg_set1 conj1g. Qed.

Lemma conjsg_eq1 A x : (A :^ x == 1%g) = (A == 1%g).
Proof. by rewrite (canF_eq (conjsgK x)) conjs1g. Qed.

Lemma conjsMg A B x : (A * B) :^ x = A :^ x * B :^ x.
Proof. by rewrite !conjsgE !mulgA rcosetK. Qed.

Lemma conjIg A B x : (A :&: B) :^ x = A :^ x :&: B :^ x.
Proof. by rewrite !conjg_preim preimsetI. Qed.

Lemma conj0g x : set0 :^ x = set0.
Proof. exact: imset0. Qed.

Lemma conjTg x : [set: gT] :^ x = [set: gT].
Proof. by rewrite conjg_preim preimsetT. Qed.

Lemma bigcapJ I r (P : pred I) (B : I -> {set gT}) x :
  \bigcap_(i <- r | P i) (B i :^ x) = (\bigcap_(i <- r | P i) B i) :^ x.
Proof.
by rewrite (big_endo (conjugate^~ x)) => // [B1 B2|]; rewrite (conjTg, conjIg).
Qed.

Lemma conjUg A B x : (A :|: B) :^ x = A :^ x :|: B :^ x.
Proof. by rewrite !conjg_preim preimsetU. Qed.

Lemma bigcupJ I r (P : pred I) (B : I -> {set gT}) x :
  \bigcup_(i <- r | P i) (B i :^ x) = (\bigcup_(i <- r | P i) B i) :^ x.
Proof.
rewrite (big_endo (conjugate^~ x)) => // [B1 B2|]; first by rewrite conjUg.
exact: imset0.
Qed.

Lemma conjCg A x : (~: A) :^ x = ~: A :^ x.
Proof. by rewrite !conjg_preim preimsetC. Qed.

Lemma conjDg A B x : (A :\: B) :^ x = A :^ x :\: B :^ x.
Proof. by rewrite !setDE !(conjCg, conjIg). Qed.

Lemma conjD1g A x : A^# :^ x = (A :^ x)^#.
Proof. by rewrite conjDg conjs1g. Qed.

(* Classes; not much for now. *)

Lemma memJ_class x y A : y \in A -> x ^ y \in x ^: A.
Proof. exact: imset_f. Qed.

Lemma classS x A B : A \subset B -> x ^: A \subset x ^: B.
Proof. exact: imsetS. Qed.

Lemma class_set1 x y :  x ^: [set y] = [set x ^ y].
Proof. exact: imset_set1. Qed.

Lemma class1g x A : x \in A -> 1 ^: A = 1.
Proof.
move=> Ax; apply/setP=> y.
by apply/imsetP/set1P=> [[a Aa]|] ->; last exists x; rewrite ?conj1g.
Qed.

Lemma classVg x A : x^-1 ^: A = (x ^: A)^-1.
Proof.
apply/setP=> xy; rewrite inE; apply/imsetP/imsetP=> [] [y Ay def_xy].
  by rewrite def_xy conjVg invgK; exists y.
by rewrite -[xy]invgK def_xy -conjVg; exists y.
Qed.

Lemma mem_classes x A : x \in A -> x ^: A \in classes A.
Proof. exact: imset_f. Qed.

Lemma memJ_class_support A B x y :
   x \in A -> y \in B -> x ^ y \in class_support A B.
Proof. by move=> Ax By; apply: imset2_f. Qed.

Lemma class_supportM A B C :
  class_support A (B * C) = class_support (class_support A B) C.
Proof.
apply/setP=> x; apply/imset2P/imset2P=> [[a y Aa] | [y c]].
  case/mulsgP=> b c Bb Cc -> ->{x y}.
  by exists (a ^ b) c; rewrite ?(imset2_f, conjgM).
case/imset2P=> a b Aa Bb -> Cc ->{x y}.
by exists a (b * c); rewrite ?(mem_mulg, conjgM).
Qed.

Lemma class_support_set1l A x : class_support [set x] A = x ^: A.
Proof. exact: imset2_set1l. Qed.

Lemma class_support_set1r A x : class_support A [set x] = A :^ x.
Proof. exact: imset2_set1r. Qed.

Lemma classM x A B : x ^: (A * B) = class_support (x ^: A) B.
Proof. by rewrite -!class_support_set1l class_supportM. Qed.

Lemma class_lcoset x y A : x ^: (y *: A) = (x ^ y) ^: A.
Proof. by rewrite classM class_set1 class_support_set1l. Qed.

Lemma class_rcoset x A y : x ^: (A :* y) = (x ^: A) :^ y.
Proof. by rewrite -class_support_set1r classM. Qed.

(* Conjugate set. *)

Lemma conjugatesS A B C : B \subset C -> A :^: B \subset A :^: C.
Proof. exact: imsetS. Qed.

Lemma conjugates_set1 A x : A :^: [set x] = [set A :^ x].
Proof. exact: imset_set1. Qed.

Lemma conjugates_conj A x B : (A :^ x) :^: B = A :^: (x *: B).
Proof.
rewrite /conjugates [x *: B]imset2_set1l -imset_comp.
by apply: eq_imset => y /=; rewrite conjsgM.
Qed.

(* Class support. *)

Lemma class_supportEl A B : class_support A B = \bigcup_(x in A) x ^: B.
Proof. exact: curry_imset2l. Qed.

Lemma class_supportEr A B : class_support A B = \bigcup_(x in B) A :^ x.
Proof. exact: curry_imset2r. Qed.

(* Groups (at last!) *)

Definition group_set A := (1 \in A) && (A * A \subset A).

Lemma group_setP A :
  reflect (1 \in A /\ {in A & A, forall x y, x * y \in A}) (group_set A).
Proof.
apply: (iffP andP) => [] [A1 AM]; split=> {A1}//.
  by move=> x y Ax Ay; apply: (subsetP AM); rewrite mem_mulg.
by apply/subsetP=> _ /mulsgP[x y Ax Ay ->]; apply: AM.
Qed.

Structure group_type : Type := Group {
  gval :> GroupSet.sort gT;
  _ : group_set gval
}.

Definition group_of : predArgType := group_type.
Local Notation groupT := group_of.
Identity Coercion type_of_group : group_of >-> group_type.

HB.instance Definition _ := [isSub for gval].
#[hnf] HB.instance Definition _ := [Finite of group_type by <:].

(* No predType or finStarMonoidType structures, as these would hide the *)
(* group-to-set coercion and thus spoil unification.                    *)
HB.instance Definition _ := SubFinite.copy groupT group_type.

Definition group (A : {set gT}) gA : groupT := @Group A gA.

Definition clone_group G :=
  let: Group _ gP := G return {type of Group for G} -> groupT in fun k => k gP.

Lemma group_inj : injective gval. Proof. exact: val_inj. Qed.
Lemma groupP (G : groupT) : group_set G. Proof. by case: G. Qed.

Lemma congr_group (H K : groupT) : H = K -> H :=: K.
Proof. exact: congr1. Qed.

Lemma isgroupP A : reflect (exists G : groupT, A = G) (group_set A).
Proof. by apply: (iffP idP) => [gA | [[B gB] -> //]]; exists (Group gA). Qed.

Lemma group_set_one : group_set 1.
Proof. by rewrite /group_set set11 mulg1 subxx. Qed.

Canonical one_group := group group_set_one.
Canonical set1_group := @group [set 1] group_set_one.

Lemma group_setT : group_set (setTfor gT).
Proof. by apply/group_setP; split=> [|x y _ _]; rewrite inE. Qed.

Canonical setT_group := group group_setT.

End GroupSetMulProp.

Arguments group_of gT%_type.
Arguments lcosetP {gT A x y}.
Arguments lcosetsP {gT A B C}.
Arguments rcosetP {gT A x y}.
Arguments rcosetsP {gT A B C}.
Arguments group_setP {gT A}.
Arguments setT_group gT%_type.
Prenex Implicits group_set mulsgP set1gP.

Notation "{ 'group' gT }" := (group_of gT)
  (format "{ 'group'  gT }") : type_scope.

Notation "[ 'group' 'of' G ]" := (clone_group (@group _ G))
  (format "[ 'group'  'of'  G ]") : form_scope.

Bind Scope Group_scope with group_type.
Bind Scope Group_scope with group_of.
Notation "1" := (one_group _) : Group_scope.
Notation "[ 1 gT ]" := (1%G : {group gT}) : Group_scope.
Notation "[ 'set' : gT ]" := (setT_group gT) : Group_scope.

(* These definitions come early so we can establish the Notation. *)
HB.lock
Definition generated (gT : finGroupType) (A : {set gT}) :=
  \bigcap_(G : {group gT} | A \subset G) G.
Canonical generated_unlockable := Unlockable generated.unlock.

Definition gcore (gT : finGroupType) (A B : {set gT}) := \bigcap_(x in B) A :^ x.
Definition joing (gT : finGroupType) (A B : {set gT}) := generated (A :|: B).
Definition commutator (gT : finGroupType) (A B : {set gT}) := generated (commg_set A B).
Definition cycle (gT : finGroupType) (x : gT) := generated [set x].
Definition order (gT : finGroupType) (x : gT) := #|cycle x|.

Arguments commutator _ _%_g _%_g.
Arguments joing _ _%_g _%_g.
Arguments generated _ _%_g.

(* Helper notation for defining new groups that need a bespoke finGroupType. *)
(* The actual group for such a type (say, my_gT) will be the full group,     *)
(* i.e., [set: my_gT] or [set: my_gT]%G, but Coq will not recognize          *)
(* specific notation for these because of the coercions inserted during type *)
(* inference, unless they are defined as [set: gsort my_gT] using the        *)
(* Notation below.                                                           *)
Notation gsort gT := (FinStarMonoid.arg_sort gT%type) (only parsing).
Notation "<< A >>"  := (generated A) : group_scope.
Notation "<[ x ] >"  := (cycle x) : group_scope.
Notation "#[ x ]"  := (order x) : group_scope.
Notation "A <*> B" := (joing A B) : group_scope.
Notation "[ ~: A1 , A2 , .. , An ]" :=
  (commutator .. (commutator A1 A2) .. An) : group_scope.

Prenex Implicits order cycle gcore.

Section GroupProp.

Variable gT : finGroupType.
Notation sT := {set gT}.
Implicit Types A B C D : sT.
Implicit Types x y z : gT.
Implicit Types G H K : {group gT}.

Section OneGroup.

Variable G : {group gT}.

Lemma valG : val G = G. Proof. by []. Qed.

(* Non-triviality. *)

Lemma group1 : 1 \in G. Proof. by case/group_setP: (valP G). Qed.
#[local] Hint Resolve group1 : core.

Lemma group1_contra x : x \notin G -> x != 1.
Proof. by apply: contraNneq => ->. Qed.

Lemma sub1G : [1 gT] \subset G. Proof. by rewrite sub1set. Qed.
Lemma subG1 : (G \subset [1]) = (G :==: 1).
Proof. by rewrite eqEsubset sub1G andbT. Qed.

Lemma setI1g : 1 :&: G = 1. Proof. exact: (setIidPl sub1G). Qed.
Lemma setIg1 : G :&: 1 = 1. Proof. exact: (setIidPr sub1G). Qed.

Lemma subG1_contra H : G \subset H -> G :!=: 1 -> H :!=: 1.
Proof. by move=> sGH; rewrite -subG1; apply: contraNneq => <-. Qed.

Lemma repr_group : repr G = 1. Proof. by rewrite /repr group1. Qed.

Lemma cardG_gt0 : 0 < #|G|.
Proof. by rewrite lt0n; apply/existsP; exists (1 : gT). Qed.

Lemma indexg_gt0 A : 0 < #|G : A|.
Proof.
rewrite lt0n; apply/existsP; exists A.
by rewrite -{2}[A]mulg1 -rcosetE; apply: imset_f.
Qed.

Lemma trivgP : reflect (G :=: 1) (G \subset [1]).
Proof. by rewrite subG1; apply: eqP. Qed.

Lemma trivGP : reflect (G = 1%G) (G \subset [1]).
Proof. by rewrite subG1; apply: eqP. Qed.

Lemma proper1G : ([1] \proper G) = (G :!=: 1).
Proof. by rewrite properEneq sub1G andbT eq_sym. Qed.

Lemma in_one_group x : (x \in 1%G) = (x == 1).
Proof. by rewrite -[x \in _]/(x \in [set 1]) !inE. Qed.

Definition inE := (in_one_group, inE).

Lemma trivgPn : reflect (exists2 x, x \in G & x != 1) (G :!=: 1).
Proof.
rewrite -subG1.
by apply: (iffP subsetPn) => [] [x Gx x1]; exists x; rewrite ?inE in x1 *.
Qed.

Lemma trivg_card_le1 : (G :==: 1) = (#|G| <= 1).
Proof. by rewrite eq_sym eqEcard cards1 sub1G. Qed.

Lemma trivg_card1 : (G :==: 1) = (#|G| == 1%N).
Proof. by rewrite trivg_card_le1 eqn_leq cardG_gt0 andbT. Qed.

Lemma cardG_gt1 : (#|G| > 1) = (G :!=: 1).
Proof. by rewrite trivg_card_le1 ltnNge. Qed.

Lemma card_le1_trivg : #|G| <= 1 -> G :=: 1.
Proof. by rewrite -trivg_card_le1; move/eqP. Qed.

Lemma card1_trivg : #|G| = 1%N -> G :=: 1.
Proof. by move=> G1; rewrite card_le1_trivg ?G1. Qed.

(* Inclusion and product. *)

Lemma mulG_subl A : A \subset A * G.
Proof. exact: mulg_subl group1. Qed.

Lemma mulG_subr A : A \subset (G * A).
Proof. exact: mulg_subr group1. Qed.

Lemma mulGid : G * G = G.
Proof.
by apply/eqP; rewrite eqEsubset mulG_subr andbT; case/andP: (valP G).
Qed.

Lemma mulGS A B : (G * A \subset G * B) = (A \subset G * B).
Proof.
apply/idP/idP; first exact: subset_trans (mulG_subr A).
by move/(mulgS G); rewrite mulgA mulGid.
Qed.

Lemma mulSG A B : (A * G \subset B * G) = (A \subset B * G).
Proof.
apply/idP/idP; first exact: subset_trans (mulG_subl A).
by move/(mulSg G); rewrite -mulgA mulGid.
Qed.

Lemma mul_subG A B : A \subset G -> B \subset G -> A * B \subset G.
Proof. by move=> sAG sBG; rewrite -mulGid mulgSS. Qed.

Lemma prod_subG (I : Type) (r : seq I) (P : {pred I}) (F : I -> {set gT}) :
  (forall i, P i -> F i \subset G) -> \prod_(i <- r | P i) F i \subset G.
Proof.
move=> subFG; elim/big_rec: _ => [|/= i A /subFG]; first by rewrite sub1set.
exact: mul_subG.
Qed.

(* Membership lemmas *)

Lemma groupM x y : x \in G -> y \in G -> x * y \in G.
Proof. by case/group_setP: (valP G) x y. Qed.

Lemma groupX x n : x \in G -> x ^+ n \in G.
Proof. by move=> Gx; elim: n => [|n IHn]; rewrite ?group1 // expgS groupM. Qed.

Lemma groupVr x : x \in G -> x^-1 \in G.
Proof.
move=> Gx; rewrite -(mul1g x^-1) -mem_rcoset ((G :* x =P G) _) //.
by rewrite eqEcard card_rcoset leqnn mul_subG ?sub1set.
Qed.

Lemma groupVl x : x^-1 \in G -> x \in G.
Proof. by move/groupVr; rewrite invgK. Qed.

Lemma groupV x : (x^-1 \in G) = (x \in G).
Proof. by apply/idP/idP; [apply: groupVl | apply: groupVr]. Qed.

Lemma groupMl x y : x \in G -> (x * y \in G) = (y \in G).
Proof.
move=> Gx; apply/idP/idP=> [Gxy|]; last exact: groupM.
by rewrite -(mulKg x y) groupM ?groupVr.
Qed.

Lemma groupMr x y : x \in G -> (y * x \in G) = (y \in G).
Proof. by move=> Gx; rewrite -[_ \in G]groupV invMg groupMl groupV. Qed.

Definition in_group := (group1, groupV, (groupMl, groupX)).

Lemma groupJ x y : x \in G -> y \in G -> x ^ y \in G.
Proof. by move=> Gx Gy; rewrite !in_group. Qed.

Lemma groupJr x y : y \in G -> (x ^ y \in G) = (x \in G).
Proof. by move=> Gy; rewrite groupMl (groupMr, groupV). Qed.

Lemma groupR x y : x \in G -> y \in G -> [~ x, y] \in G.
Proof. by move=> Gx Gy; rewrite !in_group. Qed.

Lemma group_prod I r (P : pred I) F :
  (forall i, P i -> F i \in G) -> \prod_(i <- r | P i) F i \in G.
Proof. by move=> G_P; elim/big_ind: _ => //; apply: groupM. Qed.

(* Inverse is an anti-morphism. *)

Lemma invGid : G^-1 = G. Proof. by apply/setP=> x; rewrite inE groupV. Qed.

Lemma inv_subG A : (A^-1 \subset G) = (A \subset G).
Proof. by rewrite -{1}invGid invSg. Qed.

Lemma invg_lcoset x : (x *: G)^-1 = G :* x^-1.
Proof. by rewrite invMg invGid invg_set1. Qed.

Lemma invg_rcoset x : (G :* x)^-1 = x^-1 *: G.
Proof. by rewrite invMg invGid invg_set1. Qed.

Lemma memV_lcosetV x y : (y^-1 \in x^-1 *: G) = (y \in G :* x).
Proof. by rewrite -invg_rcoset memV_invg. Qed.

Lemma memV_rcosetV x y : (y^-1 \in G :* x^-1) = (y \in x *: G).
Proof. by rewrite -invg_lcoset memV_invg. Qed.

(* Product idempotence *)

Lemma mulSgGid A x : x \in A -> A \subset G -> A * G = G.
Proof.
move=> Ax sAG; apply/eqP; rewrite eqEsubset -{2}mulGid mulSg //=.
apply/subsetP=> y Gy; rewrite -(mulKVg x y) mem_mulg // groupMr // groupV.
exact: (subsetP sAG).
Qed.

Lemma mulGSgid A x : x \in A -> A \subset G -> G * A = G.
Proof.
rewrite -memV_invg -invSg invGid => Ax sAG.
by apply: invg_inj; rewrite invMg invGid (mulSgGid Ax).
Qed.

(* Left cosets *)

Lemma lcoset_refl x : x \in x *: G.
Proof. by rewrite mem_lcoset mulVg group1. Qed.

Lemma lcoset_sym x y : (x \in y *: G) = (y \in x *: G).
Proof. by rewrite !mem_lcoset -groupV invMg invgK. Qed.

Lemma lcoset_eqP {x y} : reflect (x *: G = y *: G) (x \in y *: G).
Proof.
suffices <-: (x *: G == y *: G) = (x \in y *: G) by apply: eqP.
by rewrite eqEsubset !mulSG !sub1set lcoset_sym andbb.
Qed.

Lemma lcoset_transl x y z : x \in y *: G -> (x \in z *: G) = (y \in z *: G).
Proof. by move=> Gyx; rewrite -2!(lcoset_sym z) (lcoset_eqP Gyx). Qed.

Lemma lcoset_trans x y z : x \in y *: G -> y \in z *: G -> x \in z *: G.
Proof. by move/lcoset_transl->. Qed.

Lemma lcoset_id x : x \in G -> x *: G = G.
Proof. by move=> Gx; rewrite (lcoset_eqP (_ : x \in 1 *: G)) mul1g. Qed.

(* Right cosets, with an elimination form for repr. *)

Lemma rcoset_refl x : x \in G :* x.
Proof. by rewrite mem_rcoset mulgV group1. Qed.

Lemma rcoset_sym x y : (x \in G :* y) = (y \in G :* x).
Proof. by rewrite -!memV_lcosetV lcoset_sym. Qed.

Lemma rcoset_eqP {x y} : reflect (G :* x = G :* y) (x \in G :* y).
Proof.
suffices <-: (G :* x == G :* y) = (x \in G :* y) by apply: eqP.
by rewrite eqEsubset !mulGS !sub1set rcoset_sym andbb.
Qed.

Lemma rcoset_transl x y z : x \in G :* y -> (x \in G :* z) = (y \in G :* z).
Proof. by move=> Gyx; rewrite -2!(rcoset_sym z) (rcoset_eqP Gyx). Qed.

Lemma rcoset_trans x y z : x \in G :* y -> y \in G :* z -> x \in G :* z.
Proof. by move/rcoset_transl->. Qed.

Lemma rcoset_id x : x \in G -> G :* x = G.
Proof. by move=> Gx; rewrite (rcoset_eqP (_ : x \in G :* 1)) mulg1. Qed.

(* Elimination form. *)

Variant rcoset_repr_spec x : gT -> Type :=
  RcosetReprSpec g : g \in G -> rcoset_repr_spec x (g * x).

Lemma mem_repr_rcoset x : repr (G :* x) \in G :* x.
Proof. exact: mem_repr (rcoset_refl x). Qed.

(* This form sometimes fails because ssreflect 1.1 delegates matching to the *)
(* (weaker) primitive Coq algorithm for general (co)inductive type families. *)
Lemma repr_rcosetP x : rcoset_repr_spec x (repr (G :* x)).
Proof.
by rewrite -[repr _](mulgKV x); split; rewrite -mem_rcoset mem_repr_rcoset.
Qed.

Lemma rcoset_repr x : G :* (repr (G :* x)) = G :* x.
Proof. exact/rcoset_eqP/mem_repr_rcoset. Qed.

(* Coset spaces. *)

Lemma mem_rcosets A x : (G :* x \in rcosets G A) = (x \in G * A).
Proof.
apply/rcosetsP/mulsgP=> [[a Aa /rcoset_eqP/rcosetP[g]] | ]; first by exists g a.
by case=> g a Gg Aa ->{x}; exists a; rewrite // rcosetM rcoset_id.
Qed.

Lemma mem_lcosets A x : (x *: G \in lcosets G A) = (x \in A * G).
Proof.
rewrite -[LHS]memV_invg invg_lcoset invg_lcosets.
by rewrite -[RHS]memV_invg invMg invGid mem_rcosets.
Qed.

(* Conjugates. *)

Lemma group_setJ A x : group_set (A :^ x) = group_set A.
Proof. by rewrite /group_set mem_conjg conj1g -conjsMg conjSg. Qed.

Lemma group_set_conjG x : group_set (G :^ x).
Proof. by rewrite group_setJ groupP. Qed.

Canonical conjG_group x := group (group_set_conjG x).

Lemma conjGid : {in G, normalised G}.
Proof. by move=> x Gx; apply/setP=> y; rewrite mem_conjg groupJr ?groupV. Qed.

Lemma conj_subG x A : x \in G -> A \subset G -> A :^ x \subset G.
Proof. by move=> Gx sAG; rewrite -(conjGid Gx) conjSg. Qed.

(* Classes *)

Lemma class1G : 1 ^: G = 1. Proof. exact: class1g group1. Qed.

Lemma classes1 : [1] \in classes G. Proof. by rewrite -class1G mem_classes. Qed.

Lemma classGidl x y : y \in G -> (x ^ y) ^: G = x ^: G.
Proof. by move=> Gy; rewrite -class_lcoset lcoset_id. Qed.

Lemma classGidr x : {in G, normalised (x ^: G)}.
Proof. by move=> y Gy /=; rewrite -class_rcoset rcoset_id. Qed.

Lemma class_refl x : x \in x ^: G.
Proof. by apply/imsetP; exists 1; rewrite ?conjg1. Qed.
#[local] Hint Resolve class_refl : core.

Lemma class_eqP x y : reflect (x ^: G = y ^: G) (x \in y ^: G).
Proof.
by apply: (iffP idP) => [/imsetP[z Gz ->] | <-]; rewrite ?class_refl ?classGidl.
Qed.

Lemma class_sym x y : (x \in y ^: G) = (y \in x ^: G).
Proof. by apply/idP/idP=> /class_eqP->. Qed.

Lemma class_transl x y z : x \in y ^: G -> (x \in z ^: G) = (y \in z ^: G).
Proof. by rewrite -!(class_sym z) => /class_eqP->. Qed.

Lemma class_trans x y z : x \in y ^: G -> y \in z ^: G -> x \in z ^: G.
Proof. by move/class_transl->. Qed.

Lemma repr_class x : {y | y \in G & repr (x ^: G) = x ^ y}.
Proof.
set z := repr _; have: #|[set y in G | z == x ^ y]| > 0.
  have: z \in x ^: G by apply: (mem_repr x).
  by case/imsetP=> y Gy ->; rewrite (cardD1 y) inE Gy eqxx.
by move/card_mem_repr; move: (repr _) => y /setIdP[Gy /eqP]; exists y.
Qed.

Lemma classG_eq1 x : (x ^: G == 1) = (x == 1).
Proof.
apply/eqP/eqP=> [xG1 | ->]; last exact: class1G.
by have:= class_refl x; rewrite xG1 => /set1P.
Qed.

Lemma class_subG x A : x \in G -> A \subset G -> x ^: A \subset G.
Proof.
move=> Gx sAG; apply/subsetP=> _ /imsetP[y Ay ->].
by rewrite groupJ // (subsetP sAG).
Qed.

Lemma repr_classesP xG :
  reflect (repr xG \in G /\ xG = repr xG ^: G) (xG \in classes G).
Proof.
apply: (iffP imsetP) => [[x Gx ->] | []]; last by exists (repr xG).
by have [y Gy ->] := repr_class x; rewrite classGidl ?groupJ.
Qed.

Lemma mem_repr_classes xG : xG \in classes G -> repr xG \in xG.
Proof. by case/repr_classesP=> _ {2}->; apply: class_refl. Qed.

Lemma classes_gt0 : 0 < #|classes G|.
Proof. by rewrite (cardsD1 1) classes1. Qed.

Lemma classes_gt1 : (#|classes G| > 1) = (G :!=: 1).
Proof.
rewrite (cardsD1 1) classes1 ltnS lt0n cards_eq0.
apply/set0Pn/trivgPn=> [[xG /setD1P[nt_xG]] | [x Gx ntx]].
  by case/imsetP=> x Gx def_xG; rewrite def_xG classG_eq1 in nt_xG; exists x.
by exists (x ^: G); rewrite !inE classG_eq1 ntx; apply: imset_f.
Qed.

Lemma mem_class_support A x : x \in A -> x \in class_support A G.
Proof. by move=> Ax; rewrite -[x]conjg1 memJ_class_support. Qed.

Lemma class_supportGidl A x :
  x \in G -> class_support (A :^ x) G = class_support A G.
Proof.
by move=> Gx; rewrite -class_support_set1r -class_supportM lcoset_id.
Qed.

Lemma class_supportGidr A : {in G, normalised (class_support A G)}.
Proof.
by move=> x Gx /=; rewrite -class_support_set1r -class_supportM rcoset_id.
Qed.

Lemma class_support_subG A : A \subset G -> class_support A G \subset G.
Proof.
by move=> sAG; rewrite class_supportEr; apply/bigcupsP=> x Gx; apply: conj_subG.
Qed.

Lemma sub_class_support A : A \subset class_support A G.
Proof. by rewrite class_supportEr (bigcup_max 1) ?conjsg1. Qed.

Lemma class_support_id : class_support G G = G.
Proof.
by apply/eqP; rewrite eqEsubset sub_class_support class_support_subG.
Qed.

Lemma class_supportD1 A : (class_support A G)^# =  cover (A^# :^: G).
Proof.
rewrite cover_imset class_supportEr setDE big_distrl /=.
by apply: eq_bigr => x _; rewrite -setDE conjD1g.
Qed.

(* Subgroup Type construction. *)
(* We only expect to use this for abstract groups, so we don't project *)
(* the argument to a set.                                              *)

Inductive subg_of : predArgType := Subg x & x \in G.
Definition sgval u := let: Subg x _ := u in x.
Definition subg_of_Sub := Eval hnf in [isSub for sgval].
HB.instance Definition _ := subg_of_Sub.
#[hnf] HB.instance Definition _ := [Finite of subg_of by <:].

Lemma subgP u : sgval u \in G.
Proof. exact: valP. Qed.
Lemma subg_inj : injective sgval.
Proof. exact: val_inj. Qed.
Lemma congr_subg u v : u = v -> sgval u = sgval v.
Proof. exact: congr1. Qed.

Definition subg_one := Subg group1.
Definition subg_inv u := Subg (groupVr (subgP u)).
Definition subg_mul u v := Subg (groupM (subgP u) (subgP v)).
Lemma subg_oneP : left_id subg_one subg_mul.
Proof. by move=> u; apply: val_inj; apply: mul1g. Qed.

Lemma subg_invP : left_inverse subg_one subg_inv subg_mul.
Proof. by move=> u; apply: val_inj; apply: mulVg. Qed.
Lemma subg_mulP : associative subg_mul.
Proof. by move=> u v w; apply: val_inj; apply: mulgA. Qed.

HB.instance Definition _ := Finite_isGroup.Build subg_of
  subg_mulP subg_oneP subg_invP.

Lemma sgvalM : {in setT &, {morph sgval : x y / x * y}}. Proof. by []. Qed.
Lemma valgM : {in setT &, {morph val : x y / (x : subg_of) * y >-> x * y}}.
Proof. by []. Qed.

Definition subg : gT -> subg_of := insubd (1 : subg_of).
Lemma subgK x : x \in G -> val (subg x) = x.
Proof. by move=> Gx; rewrite insubdK. Qed.
Lemma sgvalK : cancel sgval subg.
Proof. by case=> x Gx; apply: val_inj; apply: subgK. Qed.
Lemma subg_default x : (x \in G) = false -> val (subg x) = 1.
Proof. by move=> Gx; rewrite val_insubd Gx. Qed.
Lemma subgM : {in G &, {morph subg : x y / x * y}}.
Proof. by move=> x y Gx Gy; apply: val_inj; rewrite /= !subgK ?groupM. Qed.

End OneGroup.

#[local] Hint Resolve group1 : core.

Lemma groupD1_inj G H : G^# = H^# -> G :=: H.
Proof. by move/(congr1 (setU 1)); rewrite !setD1K. Qed.

Lemma invMG G H : (G * H)^-1 = H * G.
Proof. by rewrite invMg !invGid. Qed.

Lemma mulSGid G H : H \subset G -> H * G = G.
Proof. exact: mulSgGid (group1 H). Qed.

Lemma mulGSid G H : H \subset G -> G * H = G.
Proof. exact: mulGSgid (group1 H). Qed.

Lemma mulGidPl G H : reflect (G * H = G) (H \subset G).
Proof. by apply: (iffP idP) => [|<-]; [apply: mulGSid | apply: mulG_subr]. Qed.

Lemma mulGidPr G H : reflect (G * H = H) (G \subset H).
Proof. by apply: (iffP idP) => [|<-]; [apply: mulSGid | apply: mulG_subl]. Qed.

Lemma comm_group_setP G H : reflect (commute G H) (group_set (G * H)).
Proof.
rewrite /group_set (subsetP (mulG_subl _ _)) ?group1 // andbC.
have <-: #|G * H| <= #|H * G| by rewrite -invMG card_invg.
by rewrite -mulgA mulGS mulgA mulSG -eqEcard eq_sym; apply: eqP.
Qed.

Lemma card_lcosets G H : #|lcosets H G| = #|G : H|.
Proof. by rewrite -card_invg invg_lcosets !invGid. Qed.

(* Group Modularity equations *)

Lemma group_modl A B G : A \subset G -> A * (B :&: G) = A * B :&: G.
Proof.
move=> sAG; apply/eqP; rewrite eqEsubset subsetI mulgS ?subsetIl //.
rewrite -{2}mulGid mulgSS ?subsetIr //.
apply/subsetP => _ /setIP[/mulsgP[a b Aa Bb ->] Gab].
by rewrite mem_mulg // inE Bb -(groupMl _ (subsetP sAG _ Aa)).
Qed.

Lemma group_modr A B G : B \subset G -> (G :&: A) * B = G :&: A * B.
Proof.
move=> sBG; apply: invg_inj; rewrite !(invMg, invIg) invGid !(setIC G).
by rewrite group_modl // -invGid invSg.
Qed.

End GroupProp.

#[global] Hint Extern 0 (is_true (1%g \in _)) => apply: group1 : core.
#[global] Hint Extern 0 (is_true (0 < #|_|)) => apply: cardG_gt0 : core.
#[global] Hint Extern 0 (is_true (0 < #|_ : _|)) => apply: indexg_gt0 : core.

Notation "G :^ x" := (conjG_group G x) : Group_scope.

Notation "[ 'subg' G ]" := (subg_of G) : type_scope.
Notation "[ 'subg' G ]" := [set: subg_of G] : group_scope.
Notation "[ 'subg' G ]" := [set: subg_of G]%G : Group_scope.

Prenex Implicits subg sgval subg_of.
Bind Scope group_scope with subg_of.
Arguments subgK {gT G}.
Arguments sgvalK {gT G}.
Arguments subg_inj {gT G} [u1 u2] eq_u12 : rename.

Arguments trivgP {gT G}.
Arguments trivGP {gT G}.
Arguments lcoset_eqP {gT G x y}.
Arguments rcoset_eqP {gT G x y}.
Arguments mulGidPl {gT G H}.
Arguments mulGidPr {gT G H}.
Arguments comm_group_setP {gT G H}.
Arguments class_eqP {gT G x y}.
Arguments repr_classesP {gT G xG}.

Section GroupInter.

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

Lemma group_setI G H : group_set (G :&: H).
Proof.
apply/group_setP; split=> [|x y]; rewrite !inE ?group1 //.
by case/andP=> Gx Hx; rewrite !groupMl.
Qed.

Canonical setI_group G H := group (group_setI G H).

Section Nary.

Variables (I : finType) (P : pred I) (F : I -> {group gT}).

Lemma group_set_bigcap : group_set (\bigcap_(i | P i) F i).
Proof.
by elim/big_rec: _ => [|i G _ gG]; rewrite -1?(insubdK 1%G gG) groupP.
Qed.

Canonical bigcap_group := group group_set_bigcap.

End Nary.

Lemma group_set_generated (A : {set gT}) : group_set <<A>>.
Proof. by rewrite unlock group_set_bigcap. Qed.

Canonical generated_group A := group (group_set_generated A).
Canonical gcore_group G A : {group _} := Eval hnf in [group of gcore G A].
Canonical commutator_group A B : {group _} := Eval hnf in [group of [~: A, B]].
Canonical joing_group A B : {group _} := Eval hnf in [group of A <*> B].
Canonical cycle_group x : {group _} := Eval hnf in [group of <[x]>].

Definition joinG G H := joing_group G H.

Definition subgroups A := [set G : {group gT} | G \subset A].

Lemma order_gt0 (x : gT) : 0 < #[x].
Proof. exact: cardG_gt0. Qed.

End GroupInter.

#[global] Hint Resolve order_gt0 : core.

Arguments generated_group _ _%_g.
Arguments joing_group _ _%_g _%_g.
Arguments subgroups _ _%_g.

Notation "G :&: H" := (setI_group G H) : Group_scope.
Notation "<< A >>"  := (generated_group A) : Group_scope.
Notation "<[ x ] >"  := (cycle_group x) : Group_scope.
Notation "[ ~: A1 , A2 , .. , An ]" :=
  (commutator_group .. (commutator_group A1 A2) .. An) : Group_scope.
Notation "A <*> B" := (joing_group A B) : Group_scope.
Notation "G * H" := (joinG G H) : Group_scope.
Prenex Implicits joinG subgroups.

Notation "\prod_ ( i <- r | P ) F" :=
  (\big[joinG/1%G]_(i <- r | P%B) F%G) : Group_scope.
Notation "\prod_ ( i <- r ) F" :=
  (\big[joinG/1%G]_(i <- r) F%G) : Group_scope.
Notation "\prod_ ( m <= i < n | P ) F" :=
  (\big[joinG/1%G]_(m <= i < n | P%B) F%G) : Group_scope.
Notation "\prod_ ( m <= i < n ) F" :=
  (\big[joinG/1%G]_(m <= i < n) F%G) : Group_scope.
Notation "\prod_ ( i | P ) F" :=
  (\big[joinG/1%G]_(i | P%B) F%G) : Group_scope.
Notation "\prod_ i F" :=
  (\big[joinG/1%G]_i F%G) : Group_scope.
Notation "\prod_ ( i : t | P ) F" :=
  (\big[joinG/1%G]_(i : t | P%B) F%G) (only parsing) : Group_scope.
Notation "\prod_ ( i : t ) F" :=
  (\big[joinG/1%G]_(i : t) F%G) (only parsing) : Group_scope.
Notation "\prod_ ( i < n | P ) F" :=
  (\big[joinG/1%G]_(i < n | P%B) F%G) : Group_scope.
Notation "\prod_ ( i < n ) F" :=
  (\big[joinG/1%G]_(i < n) F%G) : Group_scope.
Notation "\prod_ ( i 'in' A | P ) F" :=
  (\big[joinG/1%G]_(i in A | P%B) F%G) : Group_scope.
Notation "\prod_ ( i 'in' A ) F" :=
  (\big[joinG/1%G]_(i in A) F%G) : Group_scope.

Section Lagrange.

Variable gT : finGroupType.
Implicit Types G H K : {group gT}.

Lemma LagrangeI G H : (#|G :&: H| * #|G : H|)%N = #|G|.
Proof.
rewrite -[#|G|]sum1_card (partition_big_imset (rcoset H)) /=.
rewrite mulnC -sum_nat_const; apply: eq_bigr => _ /rcosetsP[x Gx ->].
rewrite -(card_rcoset _ x) -sum1_card; apply: eq_bigl => y.
by rewrite rcosetE (sameP eqP rcoset_eqP) group_modr ?sub1set // !inE.
Qed.

Lemma divgI G H : #|G| %/ #|G :&: H| = #|G : H|.
Proof. by rewrite -(LagrangeI G H) mulKn ?cardG_gt0. Qed.

Lemma divg_index G H : #|G| %/ #|G : H| = #|G :&: H|.
Proof. by rewrite -(LagrangeI G H) mulnK. Qed.

Lemma dvdn_indexg G H : #|G : H| %| #|G|.
Proof. by rewrite -(LagrangeI G H) dvdn_mull. Qed.

Theorem Lagrange G H : H \subset G -> (#|H| * #|G : H|)%N = #|G|.
Proof. by move/setIidPr=> sHG; rewrite -{1}sHG LagrangeI. Qed.

Lemma cardSg G H : H \subset G -> #|H| %| #|G|.
Proof. by move/Lagrange <-; rewrite dvdn_mulr. Qed.

Lemma lognSg p G H : G \subset H -> logn p #|G| <= logn p #|H|.
Proof. by move=> sGH; rewrite dvdn_leq_log ?cardSg. Qed.

Lemma piSg G H : G \subset H -> {subset \pi(gval G) <= \pi(gval H)}.
Proof.
move=> sGH p; rewrite !mem_primes !cardG_gt0 => /and3P[-> _ pG].
exact: dvdn_trans (cardSg sGH).
Qed.

Lemma divgS G H : H \subset G -> #|G| %/ #|H| = #|G : H|.
Proof. by move/Lagrange <-; rewrite mulKn. Qed.

Lemma divg_indexS G H : H \subset G -> #|G| %/ #|G : H| = #|H|.
Proof. by move/Lagrange <-; rewrite mulnK. Qed.

Lemma coprimeSg G H p : H \subset G -> coprime #|G| p -> coprime #|H| p.
Proof. by move=> sHG; apply: coprime_dvdl (cardSg sHG). Qed.

Lemma coprimegS G H p : H \subset G -> coprime p #|G| -> coprime p #|H|.
Proof. by move=> sHG; apply: coprime_dvdr (cardSg sHG). Qed.

Lemma indexJg G H x : #|G :^ x : H :^ x| = #|G : H|.
Proof. by rewrite -!divgI -conjIg !cardJg. Qed.

Lemma indexgg G : #|G : G| = 1%N.
Proof. by rewrite -divgS // divnn cardG_gt0. Qed.

Lemma rcosets_id G : rcosets G G = [set G : {set gT}].
Proof.
apply/esym/eqP; rewrite eqEcard sub1set [#|_|]indexgg cards1 andbT.
by apply/rcosetsP; exists 1; rewrite ?mulg1.
Qed.

Lemma Lagrange_index G H K :
  H \subset G -> K \subset H -> (#|G : H| * #|H : K|)%N = #|G : K|.
Proof.
move=> sHG sKH; apply/eqP; rewrite mulnC -(eqn_pmul2l (cardG_gt0 K)).
by rewrite mulnA !Lagrange // (subset_trans sKH).
Qed.

Lemma indexgI G H : #|G : G :&: H| = #|G : H|.
Proof. by rewrite -[RHS]divgI divgS ?subsetIl. Qed.

Lemma indexgS G H K : H \subset K -> #|G : K| %| #|G : H|.
Proof.
move=> sHK; rewrite -(@dvdn_pmul2l #|G :&: K|) ?cardG_gt0 // LagrangeI.
by rewrite -(Lagrange (setIS G sHK)) mulnAC LagrangeI dvdn_mulr.
Qed.

Lemma indexSg G H K : H \subset K -> K \subset G -> #|K : H| %| #|G : H|.
Proof.
move=> sHK sKG; rewrite -(@dvdn_pmul2l #|H|) ?cardG_gt0 //.
by rewrite !Lagrange ?(cardSg, subset_trans sHK).
Qed.

Lemma indexg_eq1 G H : (#|G : H| == 1%N) = (G \subset H).
Proof.
rewrite eqn_leq -(leq_pmul2l (cardG_gt0 (G :&: H))) LagrangeI muln1.
by rewrite indexg_gt0 andbT (sameP setIidPl eqP) eqEcard subsetIl.
Qed.

Lemma indexg_gt1 G H : (#|G : H| > 1) = ~~ (G \subset H).
Proof. by rewrite -indexg_eq1 eqn_leq indexg_gt0 andbT -ltnNge. Qed.

Lemma index1g G H : H \subset G -> #|G : H| = 1%N -> H :=: G.
Proof. by move=> sHG iHG; apply/eqP; rewrite eqEsubset sHG -indexg_eq1 iHG. Qed.

Lemma indexg1 G : #|G : 1| = #|G|.
Proof. by rewrite -divgS ?sub1G // cards1 divn1. Qed.

Lemma indexMg G A : #|G * A : G| = #|A : G|.
Proof.
apply/eq_card/setP/eqP; rewrite eqEsubset andbC imsetS ?mulG_subr //.
by apply/subsetP=> _ /rcosetsP[x GAx ->]; rewrite mem_rcosets.
Qed.

Lemma rcosets_partition_mul G H : partition (rcosets H G) (H * G).
Proof.
set HG := H * G; have sGHG: {subset G <= HG} by apply/subsetP/mulG_subr.
have defHx x: x \in HG -> [set y in HG | rcoset H x == rcoset H y] = H :* x.
  move=> HGx; apply/setP=> y; rewrite inE !rcosetE (sameP eqP rcoset_eqP).
  by rewrite rcoset_sym; apply/andb_idl/subsetP; rewrite mulGS sub1set.
have:= preim_partitionP (rcoset H) HG; congr (partition _ _); apply/setP=> Hx.
apply/imsetP/idP=> [[x HGx ->] | ]; first by rewrite defHx // mem_rcosets.
by case/rcosetsP=> x /sGHG-HGx ->; exists x; rewrite ?defHx.
Qed.

Lemma rcosets_partition G H : H \subset G -> partition (rcosets H G) G.
Proof. by move=> sHG; have:= rcosets_partition_mul G H; rewrite mulSGid. Qed.

Lemma LagrangeMl G H : (#|G| * #|H : G|)%N = #|G * H|.
Proof.
rewrite mulnC -(card_uniform_partition _ (rcosets_partition_mul H G)) //.
by move=> _ /rcosetsP[x Hx ->]; rewrite card_rcoset.
Qed.

Lemma LagrangeMr G H : (#|G : H| * #|H|)%N = #|G * H|.
Proof. by rewrite mulnC LagrangeMl -card_invg invMg !invGid. Qed.

Lemma mul_cardG G H : (#|G| * #|H| = #|G * H|%g * #|G :&: H|)%N.
Proof. by rewrite -LagrangeMr -(LagrangeI G H) -mulnA mulnC. Qed.

Lemma dvdn_cardMg G H : #|G * H| %| #|G| * #|H|.
Proof. by rewrite mul_cardG dvdn_mulr. Qed.

Lemma cardMg_divn G H : #|G * H| = (#|G| * #|H|) %/ #|G :&: H|.
Proof. by rewrite mul_cardG mulnK ?cardG_gt0. Qed.

Lemma cardIg_divn G H : #|G :&: H| = (#|G| * #|H|) %/ #|G * H|.
Proof. by rewrite mul_cardG mulKn // (cardD1 (1 * 1)) mem_mulg. Qed.

Lemma TI_cardMg G H : G :&: H = 1 -> #|G * H| = (#|G| * #|H|)%N.
Proof. by move=> tiGH; rewrite mul_cardG tiGH cards1 muln1. Qed.

Lemma cardMg_TI G H : #|G| * #|H| <= #|G * H| -> G :&: H = 1.
Proof.
move=> leGH; apply: card_le1_trivg.
rewrite -(@leq_pmul2l #|G * H|); last by rewrite -mul_cardG muln1.
by apply: leq_trans leGH; rewrite muln_gt0 !cardG_gt0.
Qed.

Lemma coprime_TIg G H : coprime #|G| #|H| -> G :&: H = 1.
Proof.
move=> coGH; apply/eqP; rewrite trivg_card1 -dvdn1 -{}(eqnP coGH).
by rewrite dvdn_gcd /= {2}setIC !cardSg ?subsetIl.
Qed.

Lemma prime_TIg G H : prime #|G| -> ~~ (G \subset H) -> G :&: H = 1.
Proof.
case/primeP=> _ /(_ _ (cardSg (subsetIl G H))).
rewrite (sameP setIidPl eqP) eqEcard subsetIl => /pred2P[/card1_trivg|] //= ->.
by case/negP.
Qed.

Lemma prime_meetG G H : prime #|G| -> G :&: H != 1 -> G \subset H.
Proof. by move=> prG; apply: contraR; move/prime_TIg->. Qed.

Lemma coprime_cardMg G H : coprime #|G| #|H| -> #|G * H| = (#|G| * #|H|)%N.
Proof. by move=> coGH; rewrite TI_cardMg ?coprime_TIg. Qed.

Lemma coprime_index_mulG G H K :
  H \subset G -> K \subset G -> coprime #|G : H| #|G : K| -> H * K = G.
Proof.
move=> sHG sKG co_iG_HK; apply/eqP; rewrite eqEcard mul_subG //=.
rewrite -(@leq_pmul2r #|H :&: K|) ?cardG_gt0 // -mul_cardG.
rewrite -(Lagrange sHG) -(LagrangeI K H) mulnAC setIC -mulnA.
rewrite !leq_pmul2l ?cardG_gt0 // dvdn_leq // -(Gauss_dvdr _ co_iG_HK).
by rewrite -(indexgI K) Lagrange_index ?indexgS ?subsetIl ?subsetIr.
Qed.

End Lagrange.

Section GeneratedGroup.

Variable gT : finGroupType.
Implicit Types x y z : gT.
Implicit Types A B C D : {set gT}.
Implicit Types G H K : {group gT}.

Lemma subset_gen A : A \subset <<A>>.
Proof. rewrite [@generated]unlock; exact/bigcapsP. Qed.

Lemma sub_gen A B : A \subset B -> A \subset <<B>>.
Proof. by move/subset_trans=> -> //; apply: subset_gen. Qed.

Lemma mem_gen x A : x \in A -> x \in <<A>>.
Proof. exact: subsetP (subset_gen A) x. Qed.

Lemma generatedP x A : reflect (forall G, A \subset G -> x \in G) (x \in <<A>>).
Proof. rewrite [@generated]unlock; exact: bigcapP. Qed.

Lemma gen_subG A G : (<<A>> \subset G) = (A \subset G).
Proof.
apply/idP/idP=> [|sAG]; first exact: subset_trans (subset_gen A).
by apply/subsetP=> x /generatedP; apply.
Qed.

Lemma genGid G : <<G>> = G.
Proof. by apply/eqP; rewrite eqEsubset gen_subG subset_gen andbT. Qed.

Lemma genGidG G : <<G>>%G = G.
Proof. by apply: val_inj; apply: genGid. Qed.

Lemma gen_set_id A : group_set A -> <<A>> = A.
Proof. by move=> gA; apply: (genGid (group gA)). Qed.

Lemma genS A B : A \subset B -> <<A>> \subset <<B>>.
Proof. by move=> sAB; rewrite gen_subG sub_gen. Qed.

Lemma gen0 : <<set0>> = 1 :> {set gT}.
Proof. by apply/eqP; rewrite eqEsubset sub1G gen_subG sub0set. Qed.

Lemma gen_expgs A : {n | <<A>> = (1 |: A) ^+ n}.
Proof.
set B := (1 |: A); pose N := #|gT|.
have BsubG n : B ^+ n \subset <<A>>.
  by elim: n => [|n IHn]; rewrite ?expgS ?mul_subG ?subUset ?sub1G ?subset_gen.
have B_1 n : 1 \in B ^+ n.
  by elim: n => [|n IHn]; rewrite ?set11 // expgS mulUg mul1g inE IHn.
case: (pickP (fun i : 'I_N => B ^+ i.+1 \subset B ^+ i)) => [n fixBn | no_fix].
  exists n; apply/eqP; rewrite eqEsubset BsubG andbT.
  rewrite -[B ^+ n]gen_set_id ?genS ?subsetUr //; last first.
    by apply: subset_trans fixBn; rewrite expgS mulUg subsetU ?mulg_subl ?orbT.
  rewrite /group_set B_1 /=.
  elim: {2}(n : nat) => [|m IHm]; first by rewrite mulg1.
  by apply: subset_trans fixBn; rewrite !expgSr mulgA mulSg.
suffices: N < #|B ^+ N| by rewrite ltnNge max_card.
have [] := ubnPgeq N; elim=> [|n IHn] lt_nN; first by rewrite cards1.
apply: leq_ltn_trans (IHn (ltnW lt_nN)) (proper_card _).
by rewrite /proper (no_fix (Ordinal lt_nN)) expgS mulUg mul1g subsetUl.
Qed.

Lemma gen_prodgP A x :
  reflect (exists n, exists2 c, forall i : 'I_n, c i \in A & x = \prod_i c i)
          (x \in <<A>>).
Proof.
apply: (iffP idP) => [|[n [c Ac ->]]]; last first.
  by apply: group_prod => i _; rewrite mem_gen ?Ac.
have [n ->] := gen_expgs A; rewrite /natexp Monoid.iteropE /=.
rewrite -[n]card_ord -big_const => /prodsgP[/= c Ac def_x]. 
have{Ac def_x} ->: x = \prod_(i | c i \in A) c i.
  rewrite big_mkcond {x}def_x; apply: eq_bigr => i _.
  by case/setU1P: (Ac i isT) => -> //; rewrite if_same.
have [e <- [_ /= mem_e] _] := big_enumP [preim c of A].
pose t := in_tuple e; rewrite -[e]/(val t) big_tuple.
by exists (size e), (c \o tnth t) => // i; rewrite -mem_e mem_tnth.
Qed.

Lemma genD A B : A \subset <<A :\: B>> -> <<A :\: B>> = <<A>>.
Proof.
by move=> sAB; apply/eqP; rewrite eqEsubset genS (subsetDl, gen_subG).
Qed.

Lemma genV A : <<A^-1>> = <<A>>.
Proof.
apply/eqP; rewrite eqEsubset !gen_subG -!(invSg _ <<_>>) invgK.
by rewrite !invGid !subset_gen.
Qed.

Lemma genJ A z : <<A :^z>> = <<A>> :^ z.
Proof.
by apply/eqP; rewrite eqEsubset sub_conjg !gen_subG conjSg -?sub_conjg !sub_gen.
Qed.

Lemma conjYg A B z : (A <*> B) :^z = A :^ z <*> B :^ z.
Proof. by rewrite -genJ conjUg. Qed.

Lemma genD1 A x : x \in <<A :\ x>> -> <<A :\ x>> = <<A>>.
Proof.
move=> gA'x; apply/eqP; rewrite eqEsubset genS; first by rewrite subsetDl.
rewrite gen_subG; apply/subsetP=> y Ay.
by case: (y =P x) => [-> //|]; move/eqP=> nyx; rewrite mem_gen // !inE nyx.
Qed.

Lemma genD1id A : <<A^#>> = <<A>>.
Proof. by rewrite genD1 ?group1. Qed.

Notation joingT := (@joing gT) (only parsing).
Notation joinGT := (@joinG gT) (only parsing).

Lemma joingE A B : A <*> B = <<A :|: B>>. Proof. by []. Qed.

Lemma joinGE G H : (G * H)%G = (G <*> H)%G. Proof. by []. Qed.

Lemma joingC : commutative joingT.
Proof. by move=> A B; rewrite /joing setUC. Qed.

Lemma joing_idr A B : A <*> <<B>> = A <*> B.
Proof.
apply/eqP; rewrite eqEsubset gen_subG subUset gen_subG /=.
by rewrite -subUset subset_gen genS // setUS // subset_gen.
Qed.

Lemma joing_idl A B : <<A>> <*> B = A <*> B.
Proof. by rewrite -!(joingC B) joing_idr. Qed.

Lemma joing_subl A B : A \subset A <*> B.
Proof. by rewrite sub_gen ?subsetUl. Qed.

Lemma joing_subr A B : B \subset A <*> B.
Proof. by rewrite sub_gen ?subsetUr. Qed.

Lemma join_subG A B G : (A <*> B \subset G) = (A \subset G) && (B \subset G).
Proof. by rewrite gen_subG subUset. Qed.

Lemma joing_idPl G A : reflect (G <*> A = G) (A \subset G).
Proof.
apply: (iffP idP) => [sHG | <-]; last by rewrite joing_subr.
by rewrite joingE (setUidPl sHG) genGid.
Qed.

Lemma joing_idPr A G : reflect (A <*> G = G) (A \subset G).
Proof. by rewrite joingC; apply: joing_idPl. Qed.

Lemma joing_subP A B G :
  reflect (A \subset G /\ B \subset G) (A <*> B \subset G).
Proof. by rewrite join_subG; apply: andP. Qed.

Lemma joing_sub A B C : A <*> B = C -> A \subset C /\ B \subset C.
Proof. by move <-; apply/joing_subP. Qed.

Lemma genDU A B C : A \subset C -> <<C :\: A>> = <<B>> -> <<A :|: B>> = <<C>>.
Proof.
move=> sAC; rewrite -joingE -joing_idr => <- {B}; rewrite joing_idr.
by congr <<_>>; rewrite setDE setUIr setUCr setIT; apply/setUidPr.
Qed.

Lemma joingA : associative joingT.
Proof. by move=> A B C; rewrite joing_idl joing_idr /joing setUA. Qed.

Lemma joing1G G : 1 <*> G = G.
Proof. by rewrite -gen0 joing_idl /joing set0U genGid. Qed.

Lemma joingG1 G : G <*> 1 = G.
Proof. by rewrite joingC joing1G. Qed.

Lemma genM_join G H : <<G * H>> = G <*> H.
Proof.
apply/eqP; rewrite eqEsubset gen_subG /= -{1}[G <*> H]mulGid.
rewrite genS; first by rewrite subUset mulG_subl mulG_subr.
by rewrite mulgSS ?(sub_gen, subsetUl, subsetUr).
Qed.

Lemma mulG_subG G H K : (G * H \subset K) = (G \subset K) && (H \subset K).
Proof. by rewrite -gen_subG genM_join join_subG. Qed.

Lemma mulGsubP K H G : reflect (K \subset G /\ H \subset G) (K * H \subset G).
Proof. by rewrite mulG_subG; apply: andP. Qed.

Lemma mulG_sub K H A : K * H = A -> K \subset A /\ H \subset A.
Proof. by move <-; rewrite mulG_subl mulG_subr. Qed.

Lemma trivMg G H : (G * H == 1) = (G :==: 1) && (H :==: 1).
Proof.
by rewrite !eqEsubset -{2}[1]mulGid mulgSS ?sub1G // !andbT mulG_subG.
Qed.

Lemma comm_joingE G H : commute G H -> G <*> H = G * H.
Proof.
by move/comm_group_setP=> gGH; rewrite -genM_join; apply: (genGid (group gGH)).
Qed.

Lemma joinGC : commutative joinGT.
Proof. by move=> G H; apply: val_inj; apply: joingC. Qed.

Lemma joinGA : associative joinGT.
Proof. by move=> G H K; apply: val_inj; apply: joingA. Qed.

Lemma join1G : left_id 1%G joinGT.
Proof. by move=> G; apply: val_inj; apply: joing1G. Qed.

Lemma joinG1 : right_id 1%G joinGT.
Proof. by move=> G; apply: val_inj; apply: joingG1. Qed.

HB.instance Definition _ := Monoid.isComLaw.Build {group gT} 1%G joinGT
  joinGA joinGC join1G.

Lemma bigprodGEgen I r (P : pred I) (F : I -> {set gT}) :
  (\prod_(i <- r | P i) <<F i>>)%G :=: << \bigcup_(i <- r | P i) F i >>.
Proof.
elim/big_rec2: _ => /= [|i A _ _ ->]; first by rewrite gen0.
by rewrite joing_idl joing_idr.
Qed.

Lemma bigprodGE I r (P : pred I) (F : I -> {group gT}) :
  (\prod_(i <- r | P i) F i)%G :=: << \bigcup_(i <- r | P i) F i >>.
Proof.
rewrite -bigprodGEgen /=; apply: congr_group.
by apply: eq_bigr => i _; rewrite genGidG.
Qed.

Lemma mem_commg A B x y : x \in A -> y \in B -> [~ x, y] \in [~: A, B].
Proof. by move=> Ax By; rewrite mem_gen ?imset2_f. Qed.

Lemma commSg A B C : A \subset B -> [~: A, C] \subset [~: B, C].
Proof. by move=> sAC; rewrite genS ?imset2S. Qed.

Lemma commgS A B C : B \subset C -> [~: A, B] \subset [~: A, C].
Proof. by move=> sBC; rewrite genS ?imset2S. Qed.

Lemma commgSS A B C D :
  A \subset B -> C \subset D -> [~: A, C] \subset [~: B, D].
Proof. by move=> sAB sCD; rewrite genS ?imset2S. Qed.

Lemma der1_subG G : [~: G, G] \subset G.
Proof.
by rewrite gen_subG; apply/subsetP=> _ /imset2P[x y Gx Gy ->]; apply: groupR.
Qed.

Lemma comm_subG A B G : A \subset G -> B \subset G -> [~: A, B] \subset G.
Proof.
by move=> sAG sBG; apply: subset_trans (der1_subG G); apply: commgSS.
Qed.

Lemma commGC A B : [~: A, B] = [~: B, A].
Proof.
rewrite -[[~: A, B]]genV; congr <<_>>; apply/setP=> z; rewrite inE.
by apply/imset2P/imset2P=> [] [x y Ax Ay]; last rewrite -{1}(invgK z);
  rewrite -invg_comm => /invg_inj->; exists y x.
Qed.

Lemma conjsRg A B x : [~: A, B] :^ x = [~: A :^ x, B :^ x].
Proof.
wlog suffices: A B x / [~: A, B] :^ x \subset [~: A :^ x, B :^ x].
  move=> subJ; apply/eqP; rewrite eqEsubset subJ /= -sub_conjgV.
  by rewrite -{2}(conjsgK x A) -{2}(conjsgK x B).
rewrite -genJ gen_subG; apply/subsetP=> _ /imsetP[_ /imset2P[y z Ay Bz ->] ->].
by rewrite conjRg mem_commg ?memJ_conjg.
Qed.

End GeneratedGroup.

Arguments gen_prodgP {gT A x}.
Arguments joing_idPl {gT G A}.
Arguments joing_idPr {gT A G}.
Arguments mulGsubP {gT K H G}.
Arguments joing_subP {gT A B G}.

Section Cycles.

(* Elementary properties of cycles and order, needed in perm.v.  *)
(* More advanced results on the structure of cyclic groups will  *)
(* be given in cyclic.v.                                         *)

Variable gT : finGroupType.
Implicit Types x y : gT.
Implicit Types G : {group gT}.

Import Monoid.Theory.

Lemma cycle1 : <[1]> = [1 gT].
Proof. exact: genGid. Qed.

Lemma order1 : #[1 : gT] = 1%N.
Proof. by rewrite /order cycle1 cards1. Qed.

Lemma cycle_id x : x \in <[x]>.
Proof. by rewrite mem_gen // set11. Qed.

Lemma mem_cycle x i : x ^+ i \in <[x]>.
Proof. by rewrite groupX // cycle_id. Qed.

Lemma cycle_subG x G : (<[x]> \subset G) = (x \in G).
Proof. by rewrite gen_subG sub1set. Qed.

Lemma cycle_eq1 x : (<[x]> == 1) = (x == 1).
Proof. by rewrite eqEsubset sub1G andbT cycle_subG inE. Qed.

Lemma orderE x : #[x] = #|<[x]>|. Proof. by []. Qed.

Lemma order_eq1 x : (#[x] == 1%N) = (x == 1).
Proof. by rewrite -trivg_card1 cycle_eq1. Qed.

Lemma order_gt1 x : (#[x] > 1) = (x != 1).
Proof. by rewrite ltnNge -trivg_card_le1 cycle_eq1. Qed.

Lemma cycle_traject x : <[x]> =i traject (mul x) 1 #[x].
Proof.
set t := _ 1; apply: fsym; apply/subset_cardP; last first.
  by apply/subsetP=> _ /trajectP[i _ ->]; rewrite -iteropE mem_cycle.
rewrite (card_uniqP _) ?size_traject //; case def_n: #[_] => // [n].
rewrite looping_uniq; apply: contraL (card_size (t n)) => /loopingP t_xi.
rewrite -ltnNge size_traject -def_n ?subset_leq_card //.
rewrite -(eq_subset_r (in_set _)) {}/t; set G := finset _.
rewrite -[x]mulg1 -[G]gen_set_id ?genS ?sub1set ?inE ?(t_xi 1%N)//.
apply/group_setP; split=> [|y z]; rewrite !inE ?(t_xi 0) //.
by do 2!case/trajectP=> ? _ ->; rewrite -!iteropE -expgD [x ^+ _]iteropE.
Qed.

Lemma cycle2g x : #[x] = 2 -> <[x]> = [set 1; x].
Proof. by move=> ox; apply/setP=> y; rewrite cycle_traject ox !inE mulg1. Qed.

Lemma cyclePmin x y : y \in <[x]> -> {i | i < #[x] & y = x ^+ i}.
Proof.
rewrite cycle_traject; set tx := traject _ _ #[x] => tx_y; pose i := index y tx.
have lt_i_x : i < #[x] by rewrite -index_mem size_traject in tx_y.
by exists i; rewrite // [x ^+ i]iteropE /= -(nth_traject _ lt_i_x) nth_index.
Qed.

Lemma cycleP x y : reflect (exists i, y = x ^+ i) (y \in <[x]>).
Proof.
by apply: (iffP idP) => [/cyclePmin[i _]|[i ->]]; [exists i | apply: mem_cycle].
Qed.

Lemma expg_order x : x ^+ #[x] = 1.
Proof.
have: uniq (traject (mul x) 1 #[x]).
  by apply/card_uniqP; rewrite size_traject -(eq_card (cycle_traject x)).
case/cyclePmin: (mem_cycle x #[x]) => [] [//|i] ltix.
rewrite -(subnKC ltix) addSnnS /= expgD; move: (_ - _) => j x_j1.
case/andP=> /trajectP[]; exists j; first exact: leq_addl.
by apply: (mulgI (x ^+ i.+1)); rewrite -iterSr iterS -iteropE -expgS mulg1.
Qed.

Lemma expg_mod p k x : x ^+ p = 1 -> x ^+ (k %% p) = x ^+ k.
Proof.
move=> xp.
by rewrite {2}(divn_eq k p) expgD mulnC expgM xp expg1n mul1g.
Qed.

Lemma expg_mod_order x i : x ^+ (i %% #[x]) = x ^+ i.
Proof. by rewrite expg_mod // expg_order. Qed.

Lemma invg_expg x : x^-1 = x ^+ #[x].-1.
Proof. by apply/eqP; rewrite eq_invg_mul -expgS prednK ?expg_order. Qed.

Lemma invg2id x : #[x] = 2 -> x^-1 = x.
Proof. by move=> ox; rewrite invg_expg ox. Qed.

Lemma cycleX x i : <[x ^+ i]> \subset <[x]>.
Proof. by rewrite cycle_subG; apply: mem_cycle. Qed.

Lemma cycleV x : <[x^-1]> = <[x]>.
Proof.
by apply/eqP; rewrite eq_sym eqEsubset !cycle_subG groupV -groupV !cycle_id.
Qed.

Lemma orderV x : #[x^-1] = #[x].
Proof. by rewrite /order cycleV. Qed.

Lemma cycleJ x y : <[x ^ y]> = <[x]> :^ y.
Proof. by rewrite -genJ conjg_set1. Qed.

Lemma orderJ x y : #[x ^ y] = #[x].
Proof. by rewrite /order cycleJ cardJg. Qed.

End Cycles.

Section Normaliser.

Variable gT : finGroupType.
Implicit Types x y z : gT.
Implicit Types A B C D : {set gT}.
Implicit Type G H K : {group gT}.

Lemma normP x A : reflect (A :^ x = A) (x \in 'N(A)).
Proof.
suffices ->: (x \in 'N(A)) = (A :^ x == A) by apply: eqP.
by rewrite eqEcard cardJg leqnn andbT inE.
Qed.
Arguments normP {x A}.

Lemma group_set_normaliser A : group_set 'N(A).
Proof.
apply/group_setP; split=> [|x y Nx Ny]; rewrite inE ?conjsg1 //.
by rewrite conjsgM !(normP _).
Qed.

Canonical normaliser_group A := group (group_set_normaliser A).

Lemma normsP A B : reflect {in A, normalised B} (A \subset 'N(B)).
Proof.
apply: (iffP subsetP) => nBA x Ax; last by rewrite inE nBA //.
by apply/normP; apply: nBA.
Qed.
Arguments normsP {A B}.

Lemma memJ_norm x y A : x \in 'N(A) -> (y ^ x \in A) = (y \in A).
Proof. by move=> Nx; rewrite -{1}(normP Nx) memJ_conjg. Qed.

Lemma norms_cycle x y : (<[y]> \subset 'N(<[x]>)) = (x ^ y \in <[x]>).
Proof. by rewrite cycle_subG inE -cycleJ cycle_subG. Qed.

Lemma norm1 : 'N(1) =  setT :> {set gT}.
Proof. by apply/setP=> x; rewrite !inE conjs1g subxx. Qed.

Lemma norms1 A : A \subset 'N(1).
Proof. by rewrite norm1 subsetT. Qed.

Lemma normCs A : 'N(~: A) = 'N(A).
Proof. by apply/setP=> x; rewrite -groupV !inE conjCg setCS sub_conjg. Qed.

Lemma normG G : G \subset 'N(G).
Proof. by apply/normsP; apply: conjGid. Qed.

Lemma normT : 'N([set: gT]) = [set: gT].
Proof. by apply/eqP; rewrite -subTset normG. Qed.

Lemma normsG A G : A \subset G -> A \subset 'N(G).
Proof. by move=> sAG; apply: subset_trans (normG G). Qed.

Lemma normC A B : A \subset 'N(B) -> commute A B.
Proof.
move/subsetP=> nBA; apply/setP=> u.
apply/mulsgP/mulsgP=> [[x y Ax By] | [y x By Ax]] -> {u}.
  by exists (y ^ x^-1) x; rewrite -?conjgCV // memJ_norm // groupV nBA.
by exists x (y ^ x); rewrite -?conjgC // memJ_norm // nBA.
Qed.

Lemma norm_joinEl G H : G \subset 'N(H) -> G <*> H = G * H.
Proof. by move/normC/comm_joingE. Qed.

Lemma norm_joinEr G H : H \subset 'N(G) -> G <*> H = G * H.
Proof. by move/normC=> cHG; apply: comm_joingE. Qed.

Lemma norm_rlcoset G x : x \in 'N(G) -> G :* x = x *: G.
Proof. by rewrite -sub1set => /normC. Qed.

Lemma rcoset_mul G x y : x \in 'N(G) -> (G :* x) * (G :* y) = G :* (x * y).
Proof.
move/norm_rlcoset=> GxxG.
by rewrite mulgA -(mulgA _ _ G) -GxxG mulgA mulGid -mulgA mulg_set1.
Qed.

Lemma normJ A x : 'N(A :^ x) = 'N(A) :^ x.
Proof.
by apply/setP=> y; rewrite mem_conjg !inE -conjsgM conjgCV conjsgM conjSg.
Qed.

Lemma norm_conj_norm x A B :
  x \in 'N(A) -> (A \subset 'N(B :^ x)) = (A \subset 'N(B)).
Proof. by move=> Nx; rewrite normJ -sub_conjgV (normP _) ?groupV. Qed.

Lemma norm_gen A : 'N(A) \subset 'N(<<A>>).
Proof. by apply/normsP=> x Nx; rewrite -genJ (normP Nx). Qed.

Lemma class_norm x G : G \subset 'N(x ^: G).
Proof. by apply/normsP=> y; apply: classGidr. Qed.

Lemma class_normal x G : x \in G -> x ^: G <| G.
Proof. by move=> Gx; rewrite /normal class_norm class_subG. Qed.

Lemma class_sub_norm G A x : G \subset 'N(A) -> (x ^: G \subset A) = (x \in A).
Proof.
move=> nAG; apply/subsetP/idP=> [-> // | Ax xy]; first exact: class_refl.
by case/imsetP=> y Gy ->; rewrite memJ_norm ?(subsetP nAG).
Qed.

Lemma class_support_norm A G : G \subset 'N(class_support A G).
Proof. by apply/normsP; apply: class_supportGidr. Qed.

Lemma class_support_sub_norm A B G :
  A \subset G -> B \subset 'N(G) -> class_support A B \subset G.
Proof.
move=> sAG nGB; rewrite class_supportEr.
by apply/bigcupsP=> x Bx; rewrite -(normsP nGB x Bx) conjSg.
Qed.

Section norm_trans.

Variables (A B C D : {set gT}).
Hypotheses (nBA : A \subset 'N(B)) (nCA : A \subset 'N(C)).

Lemma norms_gen : A \subset 'N(<<B>>).
Proof. exact: subset_trans nBA (norm_gen B). Qed.

Lemma norms_norm : A \subset 'N('N(B)).
Proof. by apply/normsP=> x Ax; rewrite -normJ (normsP nBA). Qed.

Lemma normsI : A \subset 'N(B :&: C).
Proof. by apply/normsP=> x Ax; rewrite conjIg !(normsP _ x Ax). Qed.

Lemma normsU : A \subset 'N(B :|: C).
Proof. by apply/normsP=> x Ax; rewrite conjUg !(normsP _ x Ax). Qed.

Lemma normsIs : B \subset 'N(D) -> A :&: B \subset 'N(C :&: D).
Proof.
move/normsP=> nDB; apply/normsP=> x; case/setIP=> Ax Bx.
by rewrite conjIg (normsP nCA) ?nDB.
Qed.

Lemma normsD : A \subset 'N(B :\: C).
Proof. by apply/normsP=> x Ax; rewrite conjDg !(normsP _ x Ax). Qed.

Lemma normsM : A \subset 'N(B * C).
Proof. by apply/normsP=> x Ax; rewrite conjsMg !(normsP _ x Ax). Qed.

Lemma normsY : A \subset 'N(B <*> C).
Proof. by apply/normsP=> x Ax; rewrite -genJ conjUg !(normsP _ x Ax). Qed.

Lemma normsR : A \subset 'N([~: B, C]).
Proof. by apply/normsP=> x Ax; rewrite conjsRg !(normsP _ x Ax). Qed.

Lemma norms_class_support : A \subset 'N(class_support B C).
Proof.
apply/subsetP=> x Ax; rewrite inE sub_conjg class_supportEr.
apply/bigcupsP=> y Cy; rewrite -sub_conjg -conjsgM conjgC conjsgM.
by rewrite (normsP nBA) // bigcup_sup ?memJ_norm ?(subsetP nCA).
Qed.

End norm_trans.

Lemma normsIG A B G : A \subset 'N(B) -> A :&: G \subset 'N(B :&: G).
Proof. by move/normsIs->; rewrite ?normG. Qed.

Lemma normsGI A B G : A \subset 'N(B) -> G :&: A \subset 'N(G :&: B).
Proof. by move=> nBA; rewrite !(setIC G) normsIG. Qed.

Lemma norms_bigcap I r (P : pred I) A (B_ : I -> {set gT}) :
    A \subset \bigcap_(i <- r | P i) 'N(B_ i) ->
  A \subset 'N(\bigcap_(i <- r | P i) B_ i).
Proof.
elim/big_rec2: _ => [|i B N _ IH /subsetIP[nBiA /IH]]; last exact: normsI.
by rewrite normT.
Qed.

Lemma norms_bigcup I r (P : pred I) A (B_ : I -> {set gT}) :
    A \subset \bigcap_(i <- r | P i) 'N(B_ i) ->
  A \subset 'N(\bigcup_(i <- r | P i) B_ i).
Proof.
move=> nBA; rewrite -normCs setC_bigcup norms_bigcap //.
by rewrite (eq_bigr _ (fun _ _ => normCs _)).
Qed.

Lemma normsD1 A B : A \subset 'N(B) -> A \subset 'N(B^#).
Proof. by move/normsD->; rewrite ?norms1. Qed.

Lemma normD1 A : 'N(A^#) = 'N(A).
Proof.
apply/eqP; rewrite eqEsubset normsD1 //.
rewrite -{2}(setID A 1) setIC normsU //; apply/normsP=> x _; apply/setP=> y.
by rewrite conjIg conjs1g !inE mem_conjg; case: eqP => // ->; rewrite conj1g.
Qed.

Lemma normalP A B : reflect (A \subset B /\ {in B, normalised A}) (A <| B).
Proof. by apply: (iffP andP)=> [] [sAB]; move/normsP. Qed.

Lemma normal_sub A B : A <| B -> A \subset B.
Proof. by case/andP. Qed.

Lemma normal_norm A B : A <| B -> B \subset 'N(A).
Proof. by case/andP. Qed.

Lemma normalS G H K : K \subset H -> H \subset G -> K <| G -> K <| H.
Proof.
by move=> sKH sHG /andP[_ nKG]; rewrite /(K <| _) sKH (subset_trans sHG).
Qed.

Lemma normal1 G : 1 <| G.
Proof. by rewrite /normal sub1set group1 norms1. Qed.

Lemma normal_refl G : G <| G.
Proof. by rewrite /(G <| _) normG subxx. Qed.

Lemma normalG G : G <| 'N(G).
Proof. by rewrite /(G <| _) normG subxx. Qed.

Lemma normalSG G H : H \subset G -> H <| 'N_G(H).
Proof. by move=> sHG; rewrite /normal subsetI sHG normG subsetIr. Qed.

Lemma normalJ A B x : (A :^ x <| B :^ x) = (A <| B).
Proof. by rewrite /normal normJ !conjSg. Qed.

Lemma normalM G A B : A <| G -> B <| G -> A * B <| G.
Proof.
by case/andP=> sAG nAG /andP[sBG nBG]; rewrite /normal mul_subG ?normsM.
Qed.

Lemma normalY G A B : A <| G -> B <| G -> A <*> B <| G.
Proof.
by case/andP=> sAG ? /andP[sBG ?]; rewrite /normal join_subG sAG sBG ?normsY.
Qed.

Lemma normalYl G H : (H <| H <*> G) = (G \subset 'N(H)).
Proof. by rewrite /normal joing_subl join_subG normG. Qed.

Lemma normalYr G H : (H <| G <*> H) = (G \subset 'N(H)).
Proof. by rewrite joingC normalYl. Qed.

Lemma normalI G A B : A <| G -> B <| G -> A :&: B <| G.
Proof.
by case/andP=> sAG nAG /andP[_ nBG]; rewrite /normal subIset ?sAG // normsI.
Qed.

Lemma norm_normalI G A : G \subset 'N(A) -> G :&: A <| G.
Proof. by move=> nAG; rewrite /normal subsetIl normsI ?normG. Qed.

Lemma normalGI G H A : H \subset G -> A <| G -> H :&: A <| H.
Proof.
by move=> sHG /andP[_ nAG]; apply: norm_normalI (subset_trans sHG nAG).
Qed.

Lemma normal_subnorm G H : (H <| 'N_G(H)) = (H \subset G).
Proof. by rewrite /normal subsetIr subsetI normG !andbT. Qed.

Lemma normalD1 A G : (A^# <| G) = (A <| G).
Proof. by rewrite /normal normD1 subDset (setUidPr (sub1G G)). Qed.

Lemma gcore_sub A G : gcore A G \subset A.
Proof. by rewrite (bigcap_min 1) ?conjsg1. Qed.

Lemma gcore_norm A G : G \subset 'N(gcore A G).
Proof.
apply/subsetP=> x Gx; rewrite inE; apply/bigcapsP=> y Gy.
by rewrite sub_conjg -conjsgM bigcap_inf ?groupM ?groupV.
Qed.

Lemma gcore_normal A G : A \subset G -> gcore A G <| G.
Proof.
by move=> sAG; rewrite /normal gcore_norm (subset_trans (gcore_sub A G)).
Qed.

Lemma gcore_max A B G : B \subset A -> G \subset 'N(B) -> B \subset gcore A G.
Proof.
move=> sBA nBG; apply/bigcapsP=> y Gy.
by rewrite -sub_conjgV (normsP nBG) ?groupV.
Qed.

Lemma sub_gcore A B G :
  G \subset 'N(B) -> (B \subset gcore A G) = (B \subset A).
Proof.
move=> nBG; apply/idP/idP=> [sBAG | sBA]; last exact: gcore_max.
exact: subset_trans (gcore_sub A G).
Qed.

(* An elementary proof that subgroups of index 2 are normal; it is almost as  *)
(* short as the "advanced" proof using group actions; besides, the fact that  *)
(* the coset is equal to the complement is used in extremal.v.                *)
Lemma rcoset_index2 G H x :
  H \subset G -> #|G : H| = 2 -> x \in G :\: H -> H :* x = G :\: H.
Proof.
move=> sHG indexHG => /setDP[Gx notHx]; apply/eqP.
rewrite eqEcard -(leq_add2l #|G :&: H|) cardsID -(LagrangeI G H) indexHG muln2.
rewrite (setIidPr sHG) card_rcoset addnn leqnn andbT.
apply/subsetP=> _ /rcosetP[y Hy ->]; apply/setDP.
by rewrite !groupMl // (subsetP sHG).
Qed.

Lemma index2_normal G H : H \subset G -> #|G : H| = 2 -> H <| G.
Proof.
move=> sHG indexHG; rewrite /normal sHG; apply/subsetP=> x Gx.
case Hx: (x \in H); first by rewrite inE conjGid.
rewrite inE conjsgE mulgA -sub_rcosetV -invg_rcoset.
by rewrite !(rcoset_index2 sHG) ?inE ?groupV ?Hx // invDg !invGid.
Qed.

Lemma cent1P x y : reflect (commute x y) (x \in 'C[y]).
Proof.
rewrite [x \in _]inE conjg_set1 sub1set !inE (sameP eqP conjg_fixP)commg1_sym.
exact: commgP.
Qed.

Lemma cent1id x : x \in 'C[x]. Proof. exact/cent1P. Qed.

Lemma cent1E x y : (x \in 'C[y]) = (x * y == y * x).
Proof. by rewrite (sameP (cent1P x y) eqP). Qed.

Lemma cent1C x y : (x \in 'C[y]) = (y \in 'C[x]).
Proof. by rewrite !cent1E eq_sym. Qed.

Canonical centraliser_group A : {group _} := Eval hnf in [group of 'C(A)].

Lemma cent_set1 x : 'C([set x]) = 'C[x].
Proof. by apply: big_pred1 => y /=; rewrite !inE. Qed.

Lemma cent1J x y : 'C[x ^ y] = 'C[x] :^ y.
Proof. by rewrite -conjg_set1 normJ. Qed.

Lemma centP A x : reflect (centralises x A) (x \in 'C(A)).
Proof. by apply: (iffP bigcapP) => cxA y /cxA/cent1P. Qed.

Lemma centsP A B : reflect {in A, centralised B} (A \subset 'C(B)).
Proof. by apply: (iffP subsetP) => cAB x /cAB/centP. Qed.

Lemma centsC A B : (A \subset 'C(B)) = (B \subset 'C(A)).
Proof. by apply/centsP/centsP=> cAB x ? y ?; rewrite /commute -cAB. Qed.

Lemma cents1 A : A \subset 'C(1).
Proof. by rewrite centsC sub1G. Qed.

Lemma cent1T : 'C(1) = setT :> {set gT}.
Proof. by apply/eqP; rewrite -subTset cents1. Qed.

Lemma cent11T : 'C[1] = setT :> {set gT}.
Proof. by rewrite -cent_set1 cent1T. Qed.

Lemma cent_sub A : 'C(A) \subset 'N(A).
Proof.
apply/subsetP=> x /centP cAx; rewrite inE.
by apply/subsetP=> _ /imsetP[y Ay ->]; rewrite /conjg -cAx ?mulKg.
Qed.

Lemma cents_norm A B : A \subset 'C(B) -> A \subset 'N(B).
Proof. by move=> cAB; apply: subset_trans (cent_sub B). Qed.

Lemma centC A B : A \subset 'C(B) -> commute A B.
Proof. by move=> cAB; apply: normC (cents_norm cAB). Qed.

Lemma cent_joinEl G H : G \subset 'C(H) -> G <*> H = G * H.
Proof. by move=> cGH; apply: norm_joinEl (cents_norm cGH). Qed.

Lemma cent_joinEr G H : H \subset 'C(G) -> G <*> H = G * H.
Proof. by move=> cGH; apply: norm_joinEr (cents_norm cGH). Qed.

Lemma centJ A x : 'C(A :^ x) = 'C(A) :^ x.
Proof.
apply/setP=> y; rewrite mem_conjg; apply/centP/centP=> cAy z Az.
  apply: (conjg_inj x).
  by rewrite conjMg [in RHS]conjMg conjgKV cAy ?memJ_conjg.
by apply: (conjg_inj x^-1); rewrite 2!conjMg cAy -?mem_conjg.
Qed.

Lemma cent_norm A : 'N(A) \subset 'N('C(A)).
Proof. by apply/normsP=> x nCx; rewrite -centJ (normP nCx). Qed.

Lemma norms_cent A B : A \subset 'N(B) -> A \subset 'N('C(B)).
Proof. by move=> nBA; apply: subset_trans nBA (cent_norm B). Qed.

Lemma cent_normal A : 'C(A) <| 'N(A).
Proof. by rewrite /(_ <| _) cent_sub cent_norm. Qed.

Lemma centS A B : B \subset A -> 'C(A) \subset 'C(B).
Proof. by move=> sAB; rewrite centsC (subset_trans sAB) 1?centsC. Qed.

Lemma centsS A B C : A \subset B -> C \subset 'C(B) -> C \subset 'C(A).
Proof. by move=> sAB cCB; apply: subset_trans cCB (centS sAB). Qed.

Lemma centSS A B C D :
  A \subset C -> B \subset D -> C \subset 'C(D) -> A \subset 'C(B).
Proof. by move=> sAC sBD cCD; apply: subset_trans (centsS sBD cCD). Qed.

Lemma centI A B : 'C(A) <*> 'C(B) \subset 'C(A :&: B).
Proof. by rewrite gen_subG subUset !centS ?(subsetIl, subsetIr). Qed.

Lemma centU A B : 'C(A :|: B) = 'C(A) :&: 'C(B).
Proof.
apply/eqP; rewrite eqEsubset subsetI 2?centS ?(subsetUl, subsetUr) //=.
by rewrite centsC subUset -centsC subsetIl -centsC subsetIr.
Qed.

Lemma cent_gen A : 'C(<<A>>) = 'C(A).
Proof. by apply/setP=> x; rewrite -!sub1set centsC gen_subG centsC. Qed.

Lemma cent_cycle x : 'C(<[x]>) = 'C[x].
Proof. by rewrite cent_gen cent_set1. Qed.

Lemma sub_cent1 A x : (A \subset 'C[x]) = (x \in 'C(A)).
Proof. by rewrite -cent_cycle centsC cycle_subG. Qed.

Lemma cents_cycle x y : commute x y -> <[x]> \subset 'C(<[y]>).
Proof. by move=> cxy; rewrite cent_cycle cycle_subG; apply/cent1P. Qed.

Lemma cycle_abelian x : abelian <[x]>.
Proof. exact: cents_cycle. Qed.

Lemma centY A B : 'C(A <*> B) = 'C(A) :&: 'C(B).
Proof. by rewrite cent_gen centU. Qed.

Lemma centM G H : 'C(G * H) = 'C(G) :&: 'C(H).
Proof. by rewrite -cent_gen genM_join centY. Qed.

Lemma cent_classP x G : reflect (x ^: G = [set x]) (x \in 'C(G)).
Proof.
apply: (iffP (centP _ _)) => [Cx | Cx1 y Gy].
  apply/eqP; rewrite eqEsubset sub1set class_refl andbT.
  by apply/subsetP=> _ /imsetP[y Gy ->]; rewrite !inE conjgE Cx ?mulKg.
by apply/commgP/conjg_fixP/set1P; rewrite -Cx1; apply/imsetP; exists y.
Qed.

Lemma commG1P A B : reflect ([~: A, B] = 1) (A \subset 'C(B)).
Proof.
apply: (iffP (centsP A B)) => [cAB | cAB1 x Ax y By].
  apply/trivgP; rewrite gen_subG; apply/subsetP=> _ /imset2P[x y Ax Ay ->].
  by rewrite inE; apply/commgP; apply: cAB.
by apply/commgP; rewrite -in_set1 -[[set 1]]cAB1 mem_commg.
Qed.

Lemma abelianE A : abelian A = (A \subset 'C(A)). Proof. by []. Qed.

Lemma abelian1 : abelian [1 gT]. Proof. exact: sub1G. Qed.

Lemma abelianS A B : A \subset B -> abelian B -> abelian A.
Proof. by move=> sAB; apply: centSS. Qed.

Lemma abelianJ A x : abelian (A :^ x) = abelian A.
Proof. by rewrite /abelian centJ conjSg. Qed.

Lemma abelian_gen A : abelian <<A>> = abelian A.
Proof. by rewrite /abelian cent_gen gen_subG. Qed.

Lemma abelianY A B :
  abelian (A <*> B) = [&& abelian A, abelian B & B \subset 'C(A)].
Proof.
rewrite /abelian join_subG /= centY !subsetI -!andbA; congr (_ && _).
by rewrite centsC andbA andbb andbC.
Qed.

Lemma abelianM G H :
  abelian (G * H) = [&& abelian G, abelian H & H \subset 'C(G)].
Proof. by rewrite -abelian_gen genM_join abelianY. Qed.

Section SubAbelian.

Variable A B C : {set gT}.
Hypothesis cAA : abelian A.

Lemma sub_abelian_cent : C \subset A -> A \subset 'C(C).
Proof. by move=> sCA; rewrite centsC (subset_trans sCA). Qed.

Lemma sub_abelian_cent2 : B \subset A -> C \subset A -> B \subset 'C(C).
Proof. by move=> sBA; move/sub_abelian_cent; apply: subset_trans. Qed.

Lemma sub_abelian_norm : C \subset A -> A \subset 'N(C).
Proof. by move=> sCA; rewrite cents_norm ?sub_abelian_cent. Qed.

Lemma sub_abelian_normal : (C \subset A) = (C <| A).
Proof.
by rewrite /normal; case sHG: (C \subset A); rewrite // sub_abelian_norm.
Qed.

End SubAbelian.

End Normaliser.

Arguments normP {gT x A}.
Arguments centP {gT A x}.
Arguments normsP {gT A B}.
Arguments cent1P {gT x y}.
Arguments normalP {gT A B}.
Arguments centsP {gT A B}.
Arguments commG1P {gT A B}.

Arguments normaliser_group _ _%_g.
Arguments centraliser_group _ _%_g.

Notation "''N' ( A )" := (normaliser_group A) : Group_scope.
Notation "''C' ( A )" := (centraliser_group A) : Group_scope.
Notation "''C' [ x ]" := (normaliser_group [set x%g]) : Group_scope.
Notation "''N_' G ( A )" := (setI_group G 'N(A)) : Group_scope.
Notation "''C_' G ( A )" := (setI_group G 'C(A)) : Group_scope.
Notation "''C_' ( G ) ( A )" := (setI_group G 'C(A))
  (only parsing) : Group_scope.
Notation "''C_' G [ x ]" := (setI_group G 'C[x]) : Group_scope.
Notation "''C_' ( G ) [ x ]" := (setI_group G 'C[x])
  (only parsing) : Group_scope.

#[global] Hint Extern 0 (is_true (_ \subset _)) => apply: normG : core.
#[global] Hint Extern 0 (is_true (_ <| _)) => apply: normal_refl : core.

Section MinMaxGroup.

Variable gT : finGroupType.
Implicit Types gP : pred {group gT}.

Definition maxgroup A gP := maxset (fun A => group_set A && gP <<A>>%G) A.
Definition mingroup A gP := minset (fun A => group_set A && gP <<A>>%G) A.

Variable gP : pred {group gT}.
Arguments gP _%_G.

Lemma ex_maxgroup : (exists G, gP G) -> {G : {group gT} | maxgroup G gP}.
Proof.
move=> exP; have [A maxA]: {A | maxgroup A gP}.
  apply: ex_maxset; case: exP => G gPG.
  by exists (G : {set gT}); rewrite groupP genGidG.
by exists <<A>>%G; rewrite /= gen_set_id; case/andP: (maxsetp maxA).
Qed.

Lemma ex_mingroup : (exists G, gP G) -> {G : {group gT} | mingroup G gP}.
Proof.
move=> exP; have [A minA]: {A | mingroup A gP}.
  apply: ex_minset; case: exP => G gPG.
  by exists (G : {set gT}); rewrite groupP genGidG.
by exists <<A>>%G; rewrite /= gen_set_id; case/andP: (minsetp minA).
Qed.

Variable G : {group gT}.

Lemma mingroupP :
  reflect (gP G /\ forall H, gP H -> H \subset G -> H :=: G) (mingroup G gP).
Proof.
apply: (iffP minsetP); rewrite /= groupP genGidG /= => [] [-> minG].
  by split=> // H gPH sGH; apply: minG; rewrite // groupP genGidG.
by split=> // A; case/andP=> gA gPA; rewrite -(gen_set_id gA); apply: minG.
Qed.

Lemma maxgroupP :
  reflect (gP G /\ forall H, gP H -> G \subset H -> H :=: G) (maxgroup G gP).
Proof.
apply: (iffP maxsetP); rewrite /= groupP genGidG /= => [] [-> maxG].
  by split=> // H gPH sGH; apply: maxG; rewrite // groupP genGidG.
by split=> // A; case/andP=> gA gPA; rewrite -(gen_set_id gA); apply: maxG.
Qed.

Lemma maxgroupp : maxgroup G gP -> gP G. Proof. by case/maxgroupP. Qed.

Lemma mingroupp : mingroup G gP -> gP G. Proof. by case/mingroupP. Qed.

Hypothesis gPG : gP G.

Lemma maxgroup_exists : {H : {group gT} | maxgroup H gP & G \subset H}.
Proof.
have [A maxA sGA]: {A | maxgroup A gP & G \subset A}.
  by apply: maxset_exists; rewrite groupP genGidG.
by exists <<A>>%G; rewrite /= gen_set_id; case/andP: (maxsetp maxA).
Qed.

Lemma mingroup_exists : {H : {group gT} | mingroup H gP & H \subset G}.
Proof.
have [A maxA sGA]: {A | mingroup A gP & A \subset G}.
  by apply: minset_exists; rewrite groupP genGidG.
by exists <<A>>%G; rewrite /= gen_set_id; case/andP: (minsetp maxA).
Qed.

End MinMaxGroup.

Arguments mingroup {gT} A%_g gP.
Arguments maxgroup {gT} A%_g gP.
Arguments mingroupP {gT gP G}.
Arguments maxgroupP {gT gP G}.

Notation "[ 'max' A 'of' G | gP ]" :=
  (maxgroup A (fun G : {group _} => gP)) : group_scope.
Notation "[ 'max' G | gP ]" := [max gval G of G | gP] : group_scope.
Notation "[ 'max' A 'of' G | gP & gQ ]" :=
  [max A of G | gP && gQ] : group_scope.
Notation "[ 'max' G | gP & gQ ]" := [max G | gP && gQ] : group_scope.
Notation "[ 'min' A 'of' G | gP ]" :=
  (mingroup A (fun G : {group _} => gP)) : group_scope.
Notation "[ 'min' G | gP ]" := [min gval G of G | gP] : group_scope.
Notation "[ 'min' A 'of' G | gP & gQ ]" :=
  [min A of G | gP && gQ] : group_scope.
Notation "[ 'min' G | gP & gQ ]" := [min G | gP && gQ] : group_scope.

HB.reexport.