(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
From mathcomp
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(* This files establishes Jordan-Holder theorems for finite groups. These     *)
(* theorems state the uniqueness up to permutation and isomorphism for the    *)
(* series of quotient built from the successive elements of any composition   *)
(* series of the same group. These quotients are also called factors of the   *)
(* composition series. To avoid the heavy use of highly polymorphic lists     *)
(* describing these quotient series, we introduce sections.                   *)
(* This library defines:                                                      *)
(*         (G1 / G2)%sec == alias for the pair (G1, G2) of groups in the same *)
(*                          finGroupType, coerced to the actual quotient group*)
(*                          group G1 / G2. We call this pseudo-quotient a     *)
(*                          section of G1 and G2.                             *)
(*    section_isog s1 s2 == s1 and s2 respectively coerce to isomorphic       *)
(*                          quotient groups.                                  *)
(*        section_repr s == canonical representative of the isomorphism class *)
(*                          of the section s.                                 *)
(*         mksrepr G1 G2 == canonical representative of the isomorphism class *)
(*                          of (G1 / G2)%sec.                                 *)
(*         mkfactors G s == if s is [:: s1, s2, ..., sn], constructs the list *)
(*                     [:: mksrepr G s1, mksrepr s1 s2, ..., mksrepr sn-1 sn] *)
(*             comps G s == s is a composition series for G i.e. s is a       *)
(*                          decreasing sequence of subgroups of G             *)
(*                          in which two adjacent elements are maxnormal one  *)
(*                          in the other and the last element of s is 1.      *)
(* Given aT and rT two finGroupTypes, (D : {group rT}), (A : {group aT}) and  *)
(* (to : groupAction A D) an external action.                                 *)
(*        maxainv to B C == C is a maximal proper normal subgroup of B        *)
(*                          invariant by (the external action of A via) to.   *)
(*          asimple to B == the maximal proper normal subgroup of B invariant *)
(*                          by the external action to is trivial.             *)
(*         acomps to G s == s is a composition series for G invariant by to,  *)
(*                          i.e. s is a decreasing sequence of subgroups of G *)
(*                          in which two adjacent elements are maximally      *)
(*                          invariant by to one in the other and the          *)
(*                          last element of s is 1.                           *)
(* We prove two versions of the result:                                       *)
(*    - JordanHolderUniqueness establishes the uniqueness up to permutation   *)
(*       and isomorphism of the lists of factors in composition series of a   *)
(*       given group.                                                         *)
(*    - StrongJordanHolderUniqueness extends the result to composition series *)
(*       invariant by an external group action.                               *)
(* See also "The Rooster and the Butterflies", proceedings of Calculemus 2013,*)
(* by Assia Mahboubi.                                                         *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Inductive section (gT : finGroupType) := GSection of {group gT} * {group gT}.
Delimit Scope section_scope with sec.
Bind Scope section_scope with section.
Definition mkSec (gT : finGroupType) (G1 G2 : {group gT}) := GSection (G1, G2).
Infix "/" := mkSec : section_scope.
Coercion pair_of_section gT (s : section gT) := let: GSection u := s in u.
Coercion quotient_of_section gT (s : section gT) : GroupSet.sort _ := s.1 / s.2.
Coercion section_group gT (s : section gT) : {group (coset_of s.2)} :=
Eval hnf in [group of s].
Section Sections.
Variables (gT : finGroupType).
Implicit Types (G : {group gT}) (s : section gT).
Canonical section_subType := Eval hnf in [newType for @pair_of_section gT].
Definition section_eqMixin := Eval hnf in [eqMixin of section gT by <:].
Canonical section_eqType := Eval hnf in EqType (section gT) section_eqMixin.
Definition section_choiceMixin := [choiceMixin of section gT by <:].
Canonical section_choiceType :=
Eval hnf in ChoiceType (section gT) section_choiceMixin.
Definition section_countMixin := [countMixin of section gT by <:].
Canonical section_countType :=
Eval hnf in CountType (section gT) section_countMixin.
Canonical section_subCountType := Eval hnf in [subCountType of section gT].
Definition section_finMixin := [finMixin of section gT by <:].
Canonical section_finType := Eval hnf in FinType (section gT) section_finMixin.
Canonical section_subFinType := Eval hnf in [subFinType of section gT].
Canonical section_group.
(* Isomorphic sections *)
Definition section_isog := [rel x y : section gT | x \isog y].
(* A witness of the isomorphism class of a section *)
Definition section_repr s := odflt (1 / 1)%sec (pick (section_isog ^~ s)).
Definition mksrepr G1 G2 := section_repr (mkSec G1 G2).

Lemma section_repr_isog s1 s2 :
s1 \isog s2 -> section_repr s1 = section_repr s2.