(* (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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(*           H <|<| G   <=> H is subnormal in G, i.e., H <| ... <| G.         *)
(* invariant_factor A H G <=> A normalises both H and G, and H <| G.          *)
(*         A.-invariant <=> the (invariant_factor A) relation, in the context *)
(*                          of the g_rel.-series notation.                    *)
(*    g_rel.-series H s <=> H :: s is a sequence of groups whose projection   *)
(*                          to sets satisfies relation g_rel pairwise; for    *)
(*                          example H <|<| G iff G = last H s for some s such *)
(*                          that normal.-series H s.                          *)
(*   stable_factor A H G == H <| G and A centralises G / H.                   *)
(*             A.-stable == the stable_factor relation, in the scope of the   *)
(*                          r.-series notation.                               *)
(*            G.-central == the central_factor relation, in the scope of the  *)
(*                          r.-series notation.                               *)
(*           maximal M G == M is a maximal proper subgroup of G.              *)
(*        maximal_eq M G == (M == G) or (maximal M G).                        *)
(*       maxnormal M G N == M is a maximal subgroup of G normalized by N.     *)
(*         minnormal M N == M is a minimal nontrivial group normalized by N.  *)
(*              simple G == G is a (nontrivial) simple group.                 *)
(*                       := minnormal G G                                     *)
(*              G.-chief == the chief_factor relation, in the scope of the    *)
(*                          r.-series notation.                               *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Section GroupDefs.
Variable gT : finGroupType.
Implicit Types A B U V : {set gT}.
Local Notation groupT := (group_of (Phant gT)).
Definition subnormal A B :=
(A \subset B) && (iter #|B| (fun N => generated (class_support A N)) B == A).
Definition invariant_factor A B C :=
[&& A \subset 'N(B), A \subset 'N(C) & B <| C].
Definition group_rel_of (r : rel {set gT}) := [rel H G : groupT | r H G].
Definition stable_factor A V U :=
([~: U, A] \subset V) && (V <| U). (* this orders allows and3P to be used *)
Definition central_factor A V U :=
[&& [~: U, A] \subset V, V \subset U & U \subset A].
Definition maximal A B := [max A of G | G \proper B].
Definition maximal_eq A B := (A == B) || maximal A B.
Definition maxnormal A B U := [max A of G | G \proper B & U \subset 'N(G)].
Definition minnormal A B := [min A of G | G :!=: 1 & B \subset 'N(G)].
Definition simple A := minnormal A A.
Definition chief_factor A V U := maxnormal V U A && (U <| A).
End GroupDefs.
Arguments subnormal {gT} A%g B%g.
Arguments invariant_factor {gT} A%g B%g C%g.
Arguments stable_factor {gT} A%g V%g U%g.
Arguments central_factor {gT} A%g V%g U%g.
Arguments maximal {gT} A%g B%g.
Arguments maximal_eq {gT} A%g B%g.
Arguments maxnormal {gT} A%g B%g U%g.
Arguments minnormal {gT} A%g B%g.
Arguments simple {gT} A%g.
Arguments chief_factor {gT} A%g V%g U%g.
Notation "H <|<| G" := (subnormal H G)
(at level 70, no associativity) : group_scope.
Notation "A .-invariant" := (invariant_factor A)
(at level 2, format "A .-invariant") : group_rel_scope.
Notation "A .-stable" := (stable_factor A)
(at level 2, format "A .-stable") : group_rel_scope.
Notation "A .-central" := (central_factor A)
(at level 2, format "A .-central") : group_rel_scope.
Notation "G .-chief" := (chief_factor G)
(at level 2, format "G .-chief") : group_rel_scope.
Arguments group_rel_of {gT} r%group_rel_scope _%G _%G : extra scopes.
Notation "r .-series" := (path (rel_of_simpl_rel (group_rel_of r)))
(at level 2, format "r .-series") : group_scope.
Section Subnormal.
Variable gT : finGroupType.
Implicit Types (A B C D : {set gT}) (G H K : {group gT}).
Let setIgr H G := (G :&: H)%G.
Let sub_setIgr G H : G \subset H -> G = setIgr H G.

Lemma subnormalP H G :
reflect (exists2 s, normal.-series H s & last H s = G) (H <|<| G).