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    Module mathcomp.finite_group.quotient

    From HB Require Import structures.
    From mathcomp Require Import ssreflect ssrfun ssrbool eqtype ssrnat seq div.
    From mathcomp Require Import choice fintype prime finset fingroup morphism.
    From mathcomp Require Import automorphism.

    This file contains the definitions of:
         coset_of H == the (sub)type of bilateral cosets of H (see below).
            coset H == the canonical projection into coset_of H.
              A / H == the quotient of A by H, that is, the morphic image
                       of A by coset H. We do not require H <| A, so in a
                       textbook A / H would be written 'N_A(H) * H / H.
  quotm f (nHG : H <| G) == the quotient morphism induced by f,
                            mapping G / H onto f @* G / f @* H.
  qisom f (eqHG : H = G) == the identity isomorphism between
                           [set: coset_of G] and [set: coset_of H].
We also prove the three isomorphism theorems, and counting lemmas for
morphisms.

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

    Local Open Scope group_scope.

    Section Cosets.

    Variables (gT : finGroupType) (Q A : {set gT}).

    Cosets are right cosets of elements in the normaliser.
We let cosets coerce to GroupSet.sort, so they inherit the group subset
base group structure. Later we will define a proper group structure on
cosets, which will then hide the inherited structure once coset_of unifies
with FinGroup.sort; the coercion to GroupSet.sort will no longer be used.
  Note that for Hx Hy : coset_of H, Hx * Hy : {set gT} can mean either
   set_of_coset (mulg Hx Hy) OR mulg (set_of_coset Hx) (set_of_coset Hy).
However, since the two terms are actually convertible, we can live with
this ambiguity.
  We take great care that neither the type coset_of H, nor its Canonical
finGroupType structure, nor the coset H morphism depend on the actual
group structure of H. Otherwise, rewriting would be extremely awkward
because all our equalities are stated at the set level.
  The trick we use is to interpret coset_of A, when A is any set, as the
type of cosets of the group <<A>> generated by A, in the group A <*> N(A)
generated by A and its normaliser. This coincides with the type of
bilateral cosets of A when A is a group. We restrict the domain of coset A
to 'N(A), so that we get almost all the same conversion equalities as if
we had forced A to be a group in the first place; the only exception, that
1 : coset_of A : {set gT} = <<A>> rather than A, can be handled by genGid.

    Notation H := <<A>>.
    Definition coset_range := [pred B in rcosets H 'N(A)].

    Record coset_of : Type :=
      Coset { set_of_coset :> GroupSet.sort gT; _ : coset_range set_of_coset }.

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


    Lemma coset_one_proof : coset_range H.
    Proof.
    by apply/rcosetsP; exists (1 : gT); rewrite (group1, mulg1). Qed.
    Definition coset_one := Coset coset_one_proof.

    Let nNH := subsetP (norm_gen A).

    Lemma coset_range_mul (B C : coset_of) : coset_range (B * C).
    Proof.
    case: B C => _ /= /rcosetsP[x Nx ->] [_ /= /rcosetsP[y Ny ->]].
    by apply/rcosetsP; exists (x * y); rewrite !(groupM, rcoset_mul, nNH).
    Qed.

    Definition coset_mul B C := Coset (coset_range_mul B C).

    Lemma coset_range_inv (B : coset_of) : coset_range B^-1.
    Proof.
    case: B => _ /= /rcosetsP[x Nx ->]; rewrite norm_rlcoset ?nNH // invg_lcoset.
    by apply/rcosetsP; exists x^-1; rewrite ?groupV.
    Qed.

    Definition coset_inv B := Coset (coset_range_inv B).

    Lemma coset_mulP : associative coset_mul.
    Proof.
    by move=> B C D; apply: val_inj; rewrite /= mulgA. Qed.

    Lemma coset_oneP : left_id coset_one coset_mul.
    Proof.
    case=> B coB; apply: val_inj => /=; case/rcosetsP: coB => x Hx ->{B}.
    by rewrite mulgA mulGid.
    Qed.

    Lemma coset_invP : left_inverse coset_one coset_inv coset_mul.
    Proof.
    case=> B coB; apply: val_inj => /=; case/rcosetsP: coB => x Hx ->{B}.
    rewrite invg_rcoset -mulgA (mulgA H) mulGid.
    by rewrite norm_rlcoset ?nNH // -lcosetM mulVg mul1g.
    Qed.

    HB.instance Definition _ :=
      Finite_isGroup.Build coset_of coset_mulP coset_oneP coset_invP.


    Definition coset x : coset_of := insubd (1 : coset_of) (H :* x).

    Lemma val_coset_prim x : x \in 'N(A) -> coset x :=: H :* x.
    Proof.
    by move=> Nx; rewrite val_insubd /= mem_rcosets -{1}(mul1g x) mem_mulg.
    Qed.

    Lemma coset_morphM : {in 'N(A) &, {morph coset : x y / x * y}}.
    Proof.
    move=> x y Nx Ny; apply: val_inj.
    by rewrite /= !val_coset_prim ?groupM //= rcoset_mul ?nNH.
    Qed.

    Canonical coset_morphism := Morphism coset_morphM.

    Lemma ker_coset_prim : 'ker coset = 'N_H(A).
    Proof.
    apply/setP=> z; rewrite !in_setI andbC 2!inE -val_eqE /=.
    case Nz: (z \in 'N(A)); rewrite ?andbF ?val_coset_prim // !andbT.
    by apply/eqP/idP=> [<-| Az]; rewrite (rcoset_refl, rcoset_id).
    Qed.

    Implicit Type xbar : coset_of.

    Lemma coset_mem y xbar : y \in xbar -> coset y = xbar.
    Proof.
    case: xbar => /= Hx NHx Hxy; apply: val_inj=> /=.
    case/rcosetsP: NHx (NHx) Hxy => x Nx -> NHx Hxy.
    by rewrite val_insubd /= (rcoset_eqP Hxy) NHx.
    Qed.


    Lemma mem_repr_coset xbar : repr xbar \in xbar.
    Proof.
    by case: xbar => /= _ /rcosetsP[x _ ->]; apply: mem_repr_rcoset. Qed.

    Lemma repr_coset1 : repr (1 : coset_of) = 1.
    Proof.
    exact: repr_group. Qed.

    Lemma coset_reprK : cancel (fun xbar => repr xbar) coset.
    Proof.
    by move=> xbar; apply: coset_mem (mem_repr_coset xbar). Qed.

    Lemma cosetP xbar : {x | x \in 'N(A) & xbar = coset x}.
    Proof.
    pose x := repr 'N_xbar(A).
    have [xbar_x Nx]: x \in xbar /\ x \in 'N(A).
      apply/setIP; rewrite {}/x; case: xbar => /= _ /rcosetsP[y Ny ->].
      by apply: (mem_repr y); rewrite inE rcoset_refl.
    by exists x; last rewrite (coset_mem xbar_x).
    Qed.

    Lemma coset_id x : x \in A -> coset x = 1.
    Proof.
    by move=> Ax; apply: coset_mem; apply: mem_gen. Qed.

    Lemma im_coset : coset @* 'N(A) = setT.
    Proof.
    by apply/setP=> xbar; case: (cosetP xbar) => x Nx ->; rewrite inE mem_morphim.
    Qed.

    Lemma sub_im_coset (C : {set coset_of}) : C \subset coset @* 'N(A).
    Proof.
    by rewrite im_coset subsetT. Qed.

    Lemma cosetpre_proper C D :
      (coset @*^-1 C \proper coset @*^-1 D) = (C \proper D).
    Proof.
    by rewrite morphpre_proper ?sub_im_coset. Qed.

    Definition quotient : {set coset_of} := coset @* Q.

    Lemma quotientE : quotient = coset @* Q
    Proof.
    by []. Qed.

    End Cosets.

    Arguments coset_of {gT} H%_g : rename.
    Arguments coset {gT} H%_g x%_g : rename.
    Arguments quotient {gT} A%_g H%_g : rename.
    Arguments coset_reprK {gT H%_g} xbar%_g : rename.

    Bind Scope group_scope with coset_of.

    Notation "A / H" := (quotient A H) : group_scope.

    Section CosetOfGroupTheory.

    Variables (gT : finGroupType) (H : {group gT}).
    Implicit Types (A B : {set gT}) (G K : {group gT}) (xbar yb : coset_of H).
    Implicit Types (C D : {set coset_of H}) (L M : {group coset_of H}).

    Canonical quotient_group G A : {group coset_of A} :=
      Eval hnf in [group of G / A].

    Infix "/" := quotient_group : Group_scope.

    Lemma val_coset x : x \in 'N(H) -> coset H x :=: H :* x.
    Proof.
    by move=> Nx; rewrite val_coset_prim // genGid. Qed.

    Lemma coset_default x : (x \in 'N(H)) = false -> coset H x = 1.
    Proof.
    move=> Nx; apply: val_inj.
    by rewrite val_insubd /= mem_rcosets /= genGid mulSGid ?normG ?Nx.
    Qed.

    Lemma coset_norm xbar : xbar \subset 'N(H).
    Proof.
    case: xbar => /= _ /rcosetsP[x Nx ->].
    by rewrite genGid mul_subG ?sub1set ?normG.
    Qed.

    Lemma ker_coset : 'ker (coset H) = H.
    Proof.
    by rewrite ker_coset_prim genGid (setIidPl _) ?normG. Qed.

    Lemma coset_idr x : x \in 'N(H) -> coset H x = 1 -> x \in H.
    Proof.
    by move=> Nx Hx1; rewrite -ker_coset mem_morphpre //= Hx1 set11. Qed.

    Lemma repr_coset_norm xbar : repr xbar \in 'N(H).
    Proof.
    exact: subsetP (coset_norm _) _ (mem_repr_coset _). Qed.

    Lemma imset_coset G : coset H @: G = G / H.
    Proof.
    apply/eqP; rewrite eqEsubset andbC imsetS ?subsetIr //=.
    apply/subsetP=> _ /imsetP[x Gx ->].
    by case Nx: (x \in 'N(H)); rewrite ?(coset_default Nx) ?mem_morphim ?group1.
    Qed.

    Lemma val_quotient A : val @: (A / H) = rcosets H 'N_A(H).
    Proof.
    apply/setP=> B; apply/imsetP/rcosetsP=> [[xbar Axbar]|[x /setIP[Ax Nx]]] ->{B}.
      case/morphimP: Axbar => x Nx Ax ->{xbar}.
      by exists x; [rewrite inE Ax | rewrite /= val_coset].
    by exists (coset H x); [apply/morphimP; exists x | rewrite /= val_coset].
    Qed.

    Lemma card_quotient_subnorm A : #|A / H| = #|'N_A(H) : H|.
    Proof.
    by rewrite -(card_imset _ val_inj) val_quotient. Qed.

    Lemma leq_quotient A : #|A / H| <= #|A|.
    Proof.
    exact: leq_morphim. Qed.

    Lemma ltn_quotient A : H :!=: 1 -> H \subset A -> #|A / H| < #|A|.
    Proof.
    by move=> ntH sHA; rewrite ltn_morphim // ker_coset (setIidPr sHA) proper1G.
    Qed.

    Lemma card_quotient A : A \subset 'N(H) -> #|A / H| = #|A : H|.
    Proof.
    by move=> nHA; rewrite card_quotient_subnorm (setIidPl nHA). Qed.

    Lemma divg_normal G : H <| G -> #|G| %/ #|H| = #|G / H|.
    Proof.
    by case/andP=> sHG nHG; rewrite divgS ?card_quotient. Qed.


    Lemma coset1 : coset H 1 :=: H.
    Proof.
    by rewrite morph1 /= genGid. Qed.

    Lemma cosetpre1 : coset H @*^-1 1 = H.
    Proof.
    by rewrite -kerE ker_coset. Qed.

    Lemma im_quotient : 'N(H) / H = setT.
    Proof.
    exact: im_coset. Qed.

    Lemma quotientT : setT / H = setT.
    Proof.
    by rewrite -im_quotient; apply: morphimT. Qed.

    Lemma quotientInorm A : 'N_A(H) / H = A / H.
    Proof.
    by rewrite /quotient setIC morphimIdom. Qed.

    Lemma quotient_setIpre A D : (A :&: coset H @*^-1 D) / H = A / H :&: D.
    Proof.
    exact: morphim_setIpre. Qed.

    Lemma mem_quotient x G : x \in G -> coset H x \in G / H.
    Proof.
    by move=> Gx; rewrite -imset_coset imset_f. Qed.

    Lemma quotientS A B : A \subset B -> A / H \subset B / H.
    Proof.
    exact: morphimS. Qed.

    Lemma quotient0 : set0 / H = set0.
    Proof.
    exact: morphim0. Qed.

    Lemma quotient_set1 x : x \in 'N(H) -> [set x] / H = [set coset H x].
    Proof.
    exact: morphim_set1. Qed.

    Lemma quotient1 : 1 / H = 1.
    Proof.
    exact: morphim1. Qed.

    Lemma quotientV A : A^-1 / H = (A / H)^-1.
    Proof.
    exact: morphimV. Qed.

    Lemma quotientMl A B : A \subset 'N(H) -> A * B / H = (A / H) * (B / H).
    Proof.
    exact: morphimMl. Qed.

    Lemma quotientMr A B : B \subset 'N(H) -> A * B / H = (A / H) * (B / H).
    Proof.
    exact: morphimMr. Qed.

    Lemma cosetpreM C D : coset H @*^-1 (C * D) = coset H @*^-1 C * coset H @*^-1 D.
    Proof.
    by rewrite morphpreMl ?sub_im_coset. Qed.

    Lemma quotientJ A x : x \in 'N(H) -> A :^ x / H = (A / H) :^ coset H x.
    Proof.
    exact: morphimJ. Qed.

    Lemma quotientU A B : (A :|: B) / H = A / H :|: B / H.
    Proof.
    exact: morphimU. Qed.

    Lemma quotientI A B : (A :&: B) / H \subset A / H :&: B / H.
    Proof.
    exact: morphimI. Qed.

    Lemma quotientY A B :
      A \subset 'N(H) -> B \subset 'N(H) -> (A <*> B) / H = (A / H) <*> (B / H).
    Proof.
    exact: morphimY. Qed.

    Lemma quotient_homg A : A \subset 'N(H) -> homg (A / H) A.
    Proof.
    exact: morphim_homg. Qed.

    Lemma coset_kerl x y : x \in H -> coset H (x * y) = coset H y.
    Proof.
    move=> Hx; case Ny: (y \in 'N(H)); first by rewrite mkerl ?ker_coset.
    by rewrite !coset_default ?groupMl // (subsetP (normG H)).
    Qed.

    Lemma coset_kerr x y : y \in H -> coset H (x * y) = coset H x.
    Proof.
    move=> Hy; case Nx: (x \in 'N(H)); first by rewrite mkerr ?ker_coset.
    by rewrite !coset_default ?groupMr // (subsetP (normG H)).
    Qed.

    Lemma rcoset_kercosetP x y :
      x \in 'N(H) -> y \in 'N(H) -> reflect (coset H x = coset H y) (x \in H :* y).
    Proof.
    by rewrite -{6}ker_coset; apply: rcoset_kerP. Qed.

    Lemma kercoset_rcoset x y :
        x \in 'N(H) -> y \in 'N(H) ->
      coset H x = coset H y -> exists2 z, z \in H & x = z * y.
    Proof.
    by move=> Nx Ny eqfxy; rewrite -ker_coset; apply: ker_rcoset. Qed.

    Lemma quotientGI G A : H \subset G -> (G :&: A) / H = G / H :&: A / H.
    Proof.
    by rewrite -{1}ker_coset; apply: morphimGI. Qed.

    Lemma quotientIG A G : H \subset G -> (A :&: G) / H = A / H :&: G / H.
    Proof.
    by rewrite -{1}ker_coset; apply: morphimIG. Qed.

    Lemma quotientD A B : A / H :\: B / H \subset (A :\: B) / H.
    Proof.
    exact: morphimD. Qed.

    Lemma quotientD1 A : (A / H)^# \subset A^# / H.
    Proof.
    exact: morphimD1. Qed.

    Lemma quotientDG A G : H \subset G -> (A :\: G) / H = A / H :\: G / H.
    Proof.
    by rewrite -{1}ker_coset; apply: morphimDG. Qed.

    Lemma quotientK A : A \subset 'N(H) -> coset H @*^-1 (A / H) = H * A.
    Proof.
    by rewrite -{8}ker_coset; apply: morphimK. Qed.

    Lemma quotientYK G : G \subset 'N(H) -> coset H @*^-1 (G / H) = H <*> G.
    Proof.
    by move=> nHG; rewrite quotientK ?norm_joinEr. Qed.

    Lemma quotientGK G : H <| G -> coset H @*^-1 (G / H) = G.
    Proof.
    by case/andP; rewrite -{1}ker_coset; apply: morphimGK. Qed.

    Lemma quotient_class x A :
      x \in 'N(H) -> A \subset 'N(H) -> x ^: A / H = coset H x ^: (A / H).
    Proof.
    exact: morphim_class. Qed.

    Lemma classes_quotient A :
      A \subset 'N(H) -> classes (A / H) = [set xA / H | xA in classes A].
    Proof.
    exact: classes_morphim. Qed.

    Lemma cosetpre_set1 x :
      x \in 'N(H) -> coset H @*^-1 [set coset H x] = H :* x.
    Proof.
    by rewrite -{9}ker_coset; apply: morphpre_set1. Qed.

    Lemma cosetpre_set1_coset xbar : coset H @*^-1 [set xbar] = xbar.
    Proof.
    by case: (cosetP xbar) => x Nx ->; rewrite cosetpre_set1 ?val_coset. Qed.

    Lemma cosetpreK C : coset H @*^-1 C / H = C.
    Proof.
    by rewrite /quotient morphpreK ?sub_im_coset. Qed.

    Lemma trivg_quotient : H / H = 1.
    Proof.
    by rewrite -[X in X / _]ker_coset /quotient morphim_ker. Qed.

    Lemma quotientS1 G : G \subset H -> G / H = 1.
    Proof.
    by move=> sGH; apply/trivgP; rewrite -trivg_quotient quotientS. Qed.

    Lemma sub_cosetpre M : H \subset coset H @*^-1 M.
    Proof.
    by rewrite -{1}ker_coset; apply: ker_sub_pre. Qed.

    Lemma quotient_proper G K :
      H <| G -> H <| K -> (G / H \proper K / H) = (G \proper K).
    Proof.
    by move=> nHG nHK; rewrite -cosetpre_proper ?quotientGK. Qed.

    Lemma normal_cosetpre M : H <| coset H @*^-1 M.
    Proof.
    by rewrite -{1}ker_coset; apply: ker_normal_pre. Qed.

    Lemma cosetpreSK C D :
      (coset H @*^-1 C \subset coset H @*^-1 D) = (C \subset D).
    Proof.
    by rewrite morphpreSK ?sub_im_coset. Qed.

    Lemma sub_quotient_pre A C :
      A \subset 'N(H) -> (A / H \subset C) = (A \subset coset H @*^-1 C).
    Proof.
    exact: sub_morphim_pre. Qed.

    Lemma sub_cosetpre_quo C G :
      H <| G -> (coset H @*^-1 C \subset G) = (C \subset G / H).
    Proof.
    by move=> nHG; rewrite -cosetpreSK quotientGK. Qed.

    Lemma quotient_sub1 A : A \subset 'N(H) -> (A / H \subset [1]) = (A \subset H).
    Proof.
    by move=> nHA /=; rewrite -[gval H in RHS]ker_coset ker_trivg_morphim nHA.
    Qed.

    Lemma quotientSK A B :
      A \subset 'N(H) -> (A / H \subset B / H) = (A \subset H * B).
    Proof.
    by move=> nHA; rewrite morphimSK ?ker_coset. Qed.

    Lemma quotientSGK A G :
      A \subset 'N(H) -> H \subset G -> (A / H \subset G / H) = (A \subset G).
    Proof.
    by rewrite -{2}ker_coset; apply: morphimSGK. Qed.

    Lemma quotient_injG :
      {in [pred G : {group gT} | H <| G] &, injective (fun G => G / H)}.
    Proof.
    by rewrite /normal -{1}ker_coset; apply: morphim_injG. Qed.

    Lemma quotient_inj G1 G2 :
       H <| G1 -> H <| G2 -> G1 / H = G2 / H -> G1 :=: G2.
    Proof.
    by rewrite /normal -[in mem H]ker_coset; apply: morphim_inj. Qed.

    Lemma quotient_neq1 A : H <| A -> (A / H != 1) = (H \proper A).
    Proof.
    case/andP=> sHA nHA; rewrite /proper sHA -trivg_quotient eqEsubset andbC.
    by rewrite quotientS //= quotientSGK.
    Qed.

    Lemma quotient_gen A : A \subset 'N(H) -> <<A>> / H = <<A / H>>.
    Proof.
    exact: morphim_gen. Qed.

    Lemma cosetpre_gen C :
      1 \in C -> coset H @*^-1 <<C>> = <<coset H @*^-1 C>>.
    Proof.
    by move=> C1; rewrite morphpre_gen ?sub_im_coset. Qed.

    Lemma quotientR A B :
      A \subset 'N(H) -> B \subset 'N(H) -> [~: A, B] / H = [~: A / H, B / H].
    Proof.
    exact: morphimR. Qed.

    Lemma quotient_norm A : 'N(A) / H \subset 'N(A / H).
    Proof.
    exact: morphim_norm. Qed.

    Lemma quotient_norms A B : A \subset 'N(B) -> A / H \subset 'N(B / H).
    Proof.
    exact: morphim_norms. Qed.

    Lemma quotient_subnorm A B : 'N_A(B) / H \subset 'N_(A / H)(B / H).
    Proof.
    exact: morphim_subnorm. Qed.

    Lemma quotient_normal A B : A <| B -> A / H <| B / H.
    Proof.
    exact: morphim_normal. Qed.

    Lemma quotient_cent1 x : 'C[x] / H \subset 'C[coset H x].
    Proof.
    case Nx: (x \in 'N(H)); first exact: morphim_cent1.
    by rewrite coset_default // cent11T subsetT.
    Qed.

    Lemma quotient_cent1s A x : A \subset 'C[x] -> A / H \subset 'C[coset H x].
    Proof.
    by move=> sAC; apply: subset_trans (quotientS sAC) (quotient_cent1 x).
    Qed.

    Lemma quotient_subcent1 A x : 'C_A[x] / H \subset 'C_(A / H)[coset H x].
    Proof.
    exact: subset_trans (quotientI _ _) (setIS _ (quotient_cent1 x)). Qed.

    Lemma quotient_cent A : 'C(A) / H \subset 'C(A / H).
    Proof.
    exact: morphim_cent. Qed.

    Lemma quotient_cents A B : A \subset 'C(B) -> A / H \subset 'C(B / H).
    Proof.
    exact: morphim_cents. Qed.

    Lemma quotient_abelian A : abelian A -> abelian (A / H).
    Proof.
    exact: morphim_abelian. Qed.

    Lemma quotient_subcent A B : 'C_A(B) / H \subset 'C_(A / H)(B / H).
    Proof.
    exact: morphim_subcent. Qed.

    Lemma norm_quotient_pre A C :
      A \subset 'N(H) -> A / H \subset 'N(C) -> A \subset 'N(coset H @*^-1 C).
    Proof.
    by move/sub_quotient_pre=> -> /subset_trans-> //; apply: morphpre_norm.
    Qed.

    Lemma cosetpre_normal C D : (coset H @*^-1 C <| coset H @*^-1 D) = (C <| D).
    Proof.
    by rewrite morphpre_normal ?sub_im_coset. Qed.

    Lemma quotient_normG G : H <| G -> 'N(G) / H = 'N(G / H).
    Proof.
    case/andP=> sHG nHG.
    by rewrite [_ / _]morphim_normG ?ker_coset // im_coset setTI.
    Qed.

    Lemma quotient_subnormG A G : H <| G -> 'N_A(G) / H = 'N_(A / H)(G / H).
    Proof.
    by case/andP=> sHG nHG; rewrite -morphim_subnormG ?ker_coset. Qed.

    Lemma cosetpre_cent1 x : 'C_('N(H))[x] \subset coset H @*^-1 'C[coset H x].
    Proof.
    case Nx: (x \in 'N(H)); first by rewrite morphpre_cent1.
    by rewrite coset_default // cent11T morphpreT subsetIl.
    Qed.

    Lemma cosetpre_cent1s C x :
      coset H @*^-1 C \subset 'C[x] -> C \subset 'C[coset H x].
    Proof.
    move=> sC; rewrite -cosetpreSK; apply: subset_trans (cosetpre_cent1 x).
    by rewrite subsetI subsetIl.
    Qed.

    Lemma cosetpre_subcent1 C x :
      'C_(coset H @*^-1 C)[x] \subset coset H @*^-1 'C_C[coset H x].
    Proof.
    by rewrite -morphpreIdom -setIA setICA morphpreI setIS // cosetpre_cent1.
    Qed.

    Lemma cosetpre_cent A : 'C_('N(H))(A) \subset coset H @*^-1 'C(A / H).
    Proof.
    exact: morphpre_cent. Qed.

    Lemma cosetpre_cents A C : coset H @*^-1 C \subset 'C(A) -> C \subset 'C(A / H).
    Proof.
    by apply: morphpre_cents; rewrite ?sub_im_coset. Qed.

    Lemma cosetpre_subcent C A :
      'C_(coset H @*^-1 C)(A) \subset coset H @*^-1 'C_C(A / H).
    Proof.
    exact: morphpre_subcent. Qed.

    Lemma restrm_quotientE G A (nHG : G \subset 'N(H)) :
      A \subset G -> restrm nHG (coset H) @* A = A / H.
    Proof.
    exact: restrmEsub. Qed.

    Section InverseImage.

    Variables (G : {group gT}) (Kbar : {group coset_of H}).

    Hypothesis nHG : H <| G.

    Variant inv_quotient_spec (P : pred {group gT}) : Prop :=
      InvQuotientSpec K of Kbar :=: K / H & H \subset K & P K.

    Lemma inv_quotientS :
      Kbar \subset G / H -> inv_quotient_spec (fun K => K \subset G).
    Proof.
    move=> sKH; exists (coset H @*^-1 Kbar); first by rewrite cosetpreK.
      by rewrite sub_cosetpre.
    by rewrite sub_cosetpre_quo.
    Qed.

    Lemma inv_quotientN : Kbar <| G / H -> inv_quotient_spec (fun K => K <| G).
    Proof.
    move=> nKbar; case/inv_quotientS: (normal_sub nKbar) => K defKbar sHK sKG.
    exists K => //; rewrite defKbar -cosetpre_normal !quotientGK // in nKbar.
    exact: normalS nHG.
    Qed.

    End InverseImage.

    Lemma quotientMidr A : A * H / H = A / H.
    Proof.
    by rewrite [_ /_]morphimMr ?normG //= -!quotientE trivg_quotient mulg1.
    Qed.

    Lemma quotientMidl A : H * A / H = A / H.
    Proof.
    by rewrite [_ /_]morphimMl ?normG //= -!quotientE trivg_quotient mul1g.
    Qed.

    Lemma quotientYidr G : G \subset 'N(H) -> G <*> H / H = G / H.
    Proof.
    move=> nHG; rewrite -genM_join quotient_gen ?mul_subG ?normG //.
    by rewrite quotientMidr genGid.
    Qed.

    Lemma quotientYidl G : G \subset 'N(H) -> H <*> G / H = G / H.
    Proof.
    by move=> nHG; rewrite joingC quotientYidr. Qed.

    Section Injective.

    Variables (G : {group gT}).
    Hypotheses (nHG : G \subset 'N(H)) (tiHG : H :&: G = 1).

    Lemma quotient_isom : isom G (G / H) (restrm nHG (coset H)).
    Proof.
    by apply/isomP; rewrite ker_restrm setIC ker_coset tiHG im_restrm. Qed.

    Lemma quotient_isog : isog G (G / H).
    Proof.
    exact: isom_isog quotient_isom. Qed.

    End Injective.

    End CosetOfGroupTheory.

    Notation "A / H" := (quotient_group A H) : Group_scope.

    Section Quotient1.

    Variables (gT : finGroupType) (A : {set gT}).

    Lemma coset1_injm : 'injm (@coset gT 1).
    Proof.
    by rewrite ker_coset /=. Qed.

    Lemma quotient1_isom : isom A (A / 1) (coset 1).
    Proof.
    by apply: sub_isom coset1_injm; rewrite ?norms1. Qed.

    Lemma quotient1_isog : isog A (A / 1).
    Proof.
    by apply: isom_isog quotient1_isom; apply: norms1. Qed.

    End Quotient1.

    Section QuotientMorphism.

    Variable (gT rT : finGroupType) (G H : {group gT}) (f : {morphism G >-> rT}).

    Implicit Types A : {set gT}.
    Implicit Types B : {set (coset_of H)}.
    Hypotheses (nsHG : H <| G).
    Let sHG : H \subset G := normal_sub nsHG.
    Let nHG : G \subset 'N(H) := normal_norm nsHG.
    Let nfHfG : f @* G \subset 'N(f @* H) := morphim_norms f nHG.

    Notation fH := (coset (f @* H) \o f).

    Lemma quotm_dom_proof : G \subset 'dom fH.
    Proof.
    by rewrite -sub_morphim_pre. Qed.

    Notation fH_G := (restrm quotm_dom_proof fH).

    Lemma quotm_ker_proof : 'ker (coset H) \subset 'ker fH_G.
    Proof.
    by rewrite ker_restrm ker_comp !ker_coset morphpreIdom morphimK ?mulG_subr.
    Qed.

    Definition quotm := factm quotm_ker_proof nHG.

    Canonical quotm_morphism := [morphism G / H of quotm].

    Lemma quotmE x : x \in G -> quotm (coset H x) = coset (f @* H) (f x).
    Proof.
    exact: factmE. Qed.

    Lemma morphim_quotm A : quotm @* (A / H) = f @* A / f @* H.
    Proof.
    by rewrite morphim_factm [LHS]morphim_restrm morphim_comp morphimIdom. Qed.

    Lemma morphpre_quotm Abar : quotm @*^-1 (Abar / f @* H) = f @*^-1 Abar / H.
    Proof.
    rewrite morphpre_factm morphpre_restrm morphpre_comp /=.
    rewrite morphpreIdom -[Abar / _]quotientInorm quotientK ?subsetIr //=.
    rewrite morphpreMl ?morphimS // morphimK // [_ * H]normC ?subIset ?nHG //.
    rewrite -quotientE -mulgA quotientMidl /= setIC -morphpreIim setIA.
    by rewrite (setIidPl nfHfG) morphpreIim -morphpreMl ?sub1G ?mul1g.
    Qed.

    Lemma ker_quotm : 'ker quotm = 'ker f / H.
    Proof.
    by rewrite -morphpre_quotm /quotient morphim1. Qed.

    Lemma injm_quotm : 'injm f -> 'injm quotm.
    Proof.
    by move/trivgP=> /= kf1; rewrite ker_quotm kf1 quotientE morphim1. Qed.

    End QuotientMorphism.

    Section EqIso.

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

    Hypothesis (eqGH : G :=: H).

    Lemma im_qisom_proof : 'N(H) \subset 'N(G)
    Proof.
    by rewrite eqGH. Qed.
    Lemma qisom_ker_proof : 'ker (coset G) \subset 'ker (coset H).
    Proof.
    by rewrite eqGH. Qed.
    Lemma qisom_restr_proof : setT \subset 'N(H) / G.
    Proof.
    by rewrite eqGH im_quotient. Qed.

    Definition qisom :=
      restrm qisom_restr_proof (factm qisom_ker_proof im_qisom_proof).

    Canonical qisom_morphism := Eval hnf in [morphism of qisom].

    Lemma qisomE x : qisom (coset G x) = coset H x.
    Proof.
    case Nx: (x \in 'N(H)); first exact: factmE.
    by rewrite !coset_default ?eqGH ?morph1.
    Qed.

    Lemma val_qisom Gx : val (qisom Gx) = val Gx.
    Proof.
    by case: (cosetP Gx) => x Nx ->{Gx}; rewrite qisomE /= !val_coset -?eqGH.
    Qed.

    Lemma morphim_qisom A : qisom @* (A / G) = A / H.
    Proof.
    by rewrite morphim_restrm setTI morphim_factm. Qed.

    Lemma morphpre_qisom A : qisom @*^-1 (A / H) = A / G.
    Proof.
    rewrite morphpre_restrm setTI morphpre_factm eqGH.
    by rewrite morphpreK // im_coset subsetT.
    Qed.

    Lemma injm_qisom : 'injm qisom.
    Proof.
    by rewrite -quotient1 -morphpre_qisom morphpreS ?sub1G. Qed.

    Lemma im_qisom : qisom @* setT = setT.
    Proof.
    by rewrite -{2}im_quotient morphim_qisom eqGH im_quotient. Qed.

    Lemma qisom_isom : isom setT setT qisom.
    Proof.
    by apply/isomP; rewrite injm_qisom im_qisom. Qed.

    Lemma qisom_isog : [set: coset_of G] \isog [set: coset_of H].
    Proof.
    exact: isom_isog qisom_isom. Qed.

    Lemma qisom_inj : injective qisom.
    Proof.
    by move=> x y; apply: (injmP injm_qisom); rewrite inE. Qed.

    Lemma morphim_qisom_inj : injective (fun Gx => qisom @* Gx).
    Proof.
    by move=> Gx Gy; apply: injm_morphim_inj; rewrite (injm_qisom, subsetT).
    Qed.

    End EqIso.

    Arguments qisom_inj {gT G H} eqGH [x1 x2].
    Arguments morphim_qisom_inj {gT G H} eqGH [x1 x2].

    Section FirstIsomorphism.

    Variables aT rT : finGroupType.

    Lemma first_isom (G : {group aT}) (f : {morphism G >-> rT}) :
      {g : {morphism G / 'ker f >-> rT} | 'injm g &
          forall A : {set aT}, g @* (A / 'ker f) = f @* A}.
    Proof.
    have nkG := ker_norm f.
    have skk: 'ker (coset ('ker f)) \subset 'ker f by rewrite ker_coset.
    exists (factm_morphism skk nkG) => /=; last exact: morphim_factm.
    by rewrite ker_factm -quotientE trivg_quotient.
    Qed.

    Variables (G H : {group aT}) (f : {morphism G >-> rT}).
    Hypothesis sHG : H \subset G.

    Lemma first_isog : (G / 'ker f) \isog (f @* G).
    Proof.
    by case: (first_isom f) => g injg im_g; apply/isogP; exists g; rewrite ?im_g.
    Qed.

    Lemma first_isom_loc : {g : {morphism H / 'ker_H f >-> rT} |
     'injm g & forall A : {set aT}, A \subset H -> g @* (A / 'ker_H f) = f @* A}.
    Proof.
    case: (first_isom (restrm_morphism sHG f)).
    rewrite ker_restrm => g injg im_g; exists g => // A sAH.
    by rewrite im_g morphim_restrm (setIidPr sAH).
    Qed.

    Lemma first_isog_loc : (H / 'ker_H f) \isog (f @* H).
    Proof.
    by case: first_isom_loc => g injg im_g; apply/isogP; exists g; rewrite ?im_g.
    Qed.

    End FirstIsomorphism.

    Section SecondIsomorphism.

    Variables (gT : finGroupType) (H K : {group gT}).

    Hypothesis nKH : H \subset 'N(K).

    Lemma second_isom : {f : {morphism H / (K :&: H) >-> coset_of K} |
      'injm f & forall A : {set gT}, A \subset H -> f @* (A / (K :&: H)) = A / K}.
    Proof.
    have ->: K :&: H = 'ker_H (coset K) by rewrite ker_coset setIC.
    exact: first_isom_loc.
    Qed.

    Lemma second_isog : H / (K :&: H) \isog H / K.
    Proof.
    by rewrite setIC -{1 3}(ker_coset K); apply: first_isog_loc. Qed.

    Lemma weak_second_isog : H / (K :&: H) \isog H * K / K.
    Proof.
    by rewrite quotientMidr; apply: second_isog. Qed.

    End SecondIsomorphism.

    Section ThirdIsomorphism.

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

    Lemma homg_quotientS (A : {set gT}) :
      A \subset 'N(H) -> A \subset 'N(K) -> H \subset K -> A / K \homg A / H.
    Proof.
    rewrite -!(gen_subG A) /=; set L := <<A>> => nHL nKL sKH.
    have sub_ker: 'ker (restrm nHL (coset H)) \subset 'ker (restrm nKL (coset K)).
      by rewrite !ker_restrm !ker_coset setIS.
    have sAL: A \subset L := subset_gen A; rewrite -(setIidPr sAL).
    rewrite -[_ / H](morphim_restrm nHL) -[_ / K](morphim_restrm nKL) /=.
    by rewrite -(morphim_factm sub_ker (subxx L)) morphim_homg ?morphimS.
    Qed.

    Hypothesis sHK : H \subset K.
    Hypothesis snHG : H <| G.
    Hypothesis snKG : K <| G.

    Theorem third_isom : {f : {morphism (G / H) / (K / H) >-> coset_of K} | 'injm f
       & forall A : {set gT}, A \subset G -> f @* (A / H / (K / H)) = A / K}.
    Proof.
    have [[sKG nKG] [sHG nHG]] := (andP snKG, andP snHG).
    have sHker: 'ker (coset H) \subset 'ker (restrm nKG (coset K)).
      by rewrite ker_restrm !ker_coset subsetI sHG.
    have:= first_isom_loc (factm_morphism sHker nHG) (subxx _) => /=.
    rewrite ker_factm_loc ker_restrm ker_coset !(setIidPr sKG) /= -!quotientE.
    case=> f injf im_f; exists f => // A sAG; rewrite im_f ?morphimS //.
    by rewrite morphim_factm morphim_restrm (setIidPr sAG).
    Qed.

    Theorem third_isog : (G / H / (K / H)) \isog (G / K).
    Proof.
    by case: third_isom => f inj_f im_f; apply/isogP; exists f; rewrite ?im_f.
    Qed.

    End ThirdIsomorphism.

    Lemma char_from_quotient (gT : finGroupType) (G H K : {group gT}) :
      H <| K -> H \char G -> K / H \char G / H -> K \char G.
    Proof.
    case/andP=> sHK nHK chHG.
    have nsHG := char_normal chHG; have [sHG nHG] := andP nsHG.
    case/charP; rewrite quotientSGK // => sKG /= chKG.
    apply/charP; split=> // f injf Gf; apply/morphim_fixP => //.
    rewrite -(quotientSGK _ sHK); first by rewrite -morphimIim Gf subIset ?nHG.
    have{chHG} Hf: f @* H = H by case/charP: chHG => _; apply.
    set q := quotm_morphism f nsHG; have{injf}: 'injm q by apply: injm_quotm.
    have: q @* _ = _ := morphim_quotm _ _ _; move: q; rewrite Hf => q im_q injq.
    by rewrite -im_q chKG // im_q Gf.
    Qed.


    Section CardMorphism.

    Variables (aT rT : finGroupType) (D : {group aT}) (f : {morphism D >-> rT}).
    Implicit Types G H : {group aT}.
    Implicit Types L M : {group rT}.

    Lemma card_morphim G : #|f @* G| = #|D :&: G : 'ker f|.
    Proof.
    rewrite -morphimIdom -indexgI -card_quotient.
      by rewrite normsI ?normG ?subIset ?ker_norm.
    by apply: esym (card_isog _); rewrite first_isog_loc ?subsetIl.
    Qed.

    Lemma dvdn_morphim G : #|f @* G| %| #|G|.
    Proof.
    rewrite card_morphim (dvdn_trans (dvdn_indexg _ _)) //.
    by rewrite cardSg ?subsetIr.
    Qed.

    Lemma logn_morphim p G : logn p #|f @* G| <= logn p #|G|.
    Proof.
    by rewrite dvdn_leq_log ?dvdn_morphim. Qed.

    Lemma coprime_morphl G p : coprime #|G| p -> coprime #|f @* G| p.
    Proof.
    exact: coprime_dvdl (dvdn_morphim G). Qed.

    Lemma coprime_morphr G p : coprime p #|G| -> coprime p #|f @* G|.
    Proof.
    exact: coprime_dvdr (dvdn_morphim G). Qed.

    Lemma coprime_morph G H : coprime #|G| #|H| -> coprime #|f @* G| #|f @* H|.
    Proof.
    by move=> coGH; rewrite coprime_morphl // coprime_morphr. Qed.

    Lemma index_morphim_ker G H :
        H \subset G -> G \subset D ->
      (#|f @* G : f @* H| * #|'ker_G f : H|)%N = #|G : H|.
    Proof.
    move=> sHG sGD; apply/eqP.
    rewrite -(eqn_pmul2l (cardG_gt0 (f @* H))) mulnA Lagrange ?morphimS //.
    rewrite !card_morphim (setIidPr sGD) (setIidPr (subset_trans sHG sGD)).
    rewrite -(eqn_pmul2l (cardG_gt0 ('ker_H f))) /=.
    by rewrite -{1}(setIidPr sHG) setIAC mulnCA mulnC mulnA !LagrangeI Lagrange.
    Qed.

    Lemma index_morphim G H : G :&: H \subset D -> #|f @* G : f @* H| %| #|G : H|.
    Proof.
    move=> dGH; rewrite -(indexgI G) -(setIidPr dGH) setIA.
    apply: dvdn_trans (indexSg (subsetIl _ H) (subsetIr D G)).
    rewrite -index_morphim_ker ?subsetIl ?subsetIr ?dvdn_mulr //= morphimIdom.
    by rewrite indexgS ?morphimS ?subsetIr.
    Qed.

    Lemma index_injm G H : 'injm f -> G \subset D -> #|f @* G : f @* H| = #|G : H|.
    Proof.
    move=> injf dG; rewrite -{2}(setIidPr dG) -(indexgI _ H) /=.
    rewrite -index_morphim_ker ?subsetIl ?subsetIr //= setIAC morphimIdom setIC.
    rewrite injmI ?subsetIr // indexgI /= morphimIdom setIC ker_injm //.
    by rewrite -(indexgI (1 :&: _)) /= -setIA !(setIidPl (sub1G _)) indexgg muln1.
    Qed.

    Lemma card_morphpre L : L \subset f @* D -> #|f @*^-1 L| = (#|'ker f| * #|L|)%N.
    Proof.
    move/morphpreK=> {2} <-; rewrite card_morphim morphpreIdom.
    by rewrite Lagrange // morphpreS ?sub1G.
    Qed.

    Lemma index_morphpre L M :
      L \subset f @* D -> #|f @*^-1 L : f @*^-1 M| = #|L : M|.
    Proof.
    move=> dL; rewrite -!divgI -morphpreI /= card_morphpre //.
    have: L :&: M \subset f @* D by rewrite subIset ?dL.
    by move/card_morphpre->; rewrite divnMl ?cardG_gt0.
    Qed.

    End CardMorphism.

    Lemma card_homg (aT rT : finGroupType) (G : {group aT}) (R : {group rT}) :
      G \homg R -> #|G| %| #|R|.
    Proof.
    by case/homgP=> f <-; rewrite card_morphim setIid dvdn_indexg. Qed.

    Section CardCosetpre.

    Variables (gT : finGroupType) (G H K : {group gT}) (L M : {group coset_of H}).

    Lemma dvdn_quotient : #|G / H| %| #|G|.
    Proof.
    exact: dvdn_morphim. Qed.

    Lemma index_quotient_ker :
         K \subset G -> G \subset 'N(H) ->
      (#|G / H : K / H| * #|G :&: H : K|)%N = #|G : K|.
    Proof.
    by rewrite -{5}(ker_coset H); apply: index_morphim_ker. Qed.

    Lemma index_quotient : G :&: K \subset 'N(H) -> #|G / H : K / H| %| #|G : K|.
    Proof.
    exact: index_morphim. Qed.

    Lemma index_quotient_eq :
        G :&: H \subset K -> K \subset G -> G \subset 'N(H) ->
      #|G / H : K / H| = #|G : K|.
    Proof.
    move=> sGH_K sKG sGN; rewrite -index_quotient_ker {sKG sGN}//.
    by rewrite -(indexgI _ K) (setIidPl sGH_K) indexgg muln1.
    Qed.

    Lemma card_cosetpre : #|coset H @*^-1 L| = (#|H| * #|L|)%N.
    Proof.
    by rewrite card_morphpre ?ker_coset ?sub_im_coset. Qed.

    Lemma index_cosetpre : #|coset H @*^-1 L : coset H @*^-1 M| = #|L : M|.
    Proof.
    by rewrite index_morphpre ?sub_im_coset. Qed.

    End CardCosetpre.
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