(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
(******************************************************************************)
(*   This file provides basic interfaces for the notion of "generic"          *)
(* characteristic subgroups; these amount to subfunctors of the identity      *)
(* functor in some category of groups.                                        *)
(* See "Generic Proof Tools And Finite Group Theory",                         *)
(* Francois Garillot, PhD, 2011, Chapter 3.                                   *)
(* The implementation proposed here is fairly basic, relying on first order   *)
(* function matching and on structure telescopes, both of which are somewhat  *)
(* limited and fragile. It should switch in the future to more general and    *)
(* more robust quotation matching.                                            *)
(*   The definitions in this file (types, properties and structures) are all  *)
(* packaged under the GFunctor submodule, i.e., client code should refer to   *)
(* GFunctor.continuous, GFunctor.map, etc. Notations, Coercions and Lemmas    *)
(* are exported and thus directly available, however.                         *)
(*   We provide the following:                                                *)
(*      object_map == the type of the (polymorphic) object map of a group     *)
(*                    functor; the %gF scope is bound to object_map.          *)
(*                 := forall gT : finGroupType, {set gT} -> {set gT}.         *)
(*  We define two operations on object_map (with notations in the %gF scope): *)
(*        F1 \o F2 == the composite map; (F1 \o F2) G expands to F1 (F2 G).   *)
(*        F1 %% F2 == F1 computed modulo F2; we have                          *)
(*                    (F1 %% F2) G / F2 G = F1 (G / F2 G)                     *)
(* We define the following (type-polymorphic) properties of an object_map F:  *)
(*     group_valued F <-> F G is a group when G is a group                    *)
(*           closed F <-> F G is a subgroup o fG when G is a group            *)
(*       continuous F <-> F is continuous with respect to morphism image:     *)
(*                        for any f : {morphism G >-> ..}, f @* (F G) is a    *)
(*                        a subgroup of F (f @* G); equivalently, F is        *)
(*                        functorial in the category Grp of groups.           *)
(*                        Most common "characteristic subgroup" are produced  *)
(*                        continuous object maps.                             *)
(*   iso_continuous F <-> F is continuous with respect to isomorphism image;  *)
(*                        equivalently, F is functorial in the Grp groupoid.  *)
(*                        The Puig and the Thompson J subgroups are examples  *)
(*                        of iso_continuous maps that are not continuous.     *)
(*      pcontinuous F <-> F is continuous with respect to partial morphism    *)
(*                        image, i.e., functorial in the category of groups   *)
(*                        and partial morphisms. The center and p-core are    *)
(*                        examples of pcontinuous maps.                       *)
(*       hereditary F <-> inclusion in the image of F is hereditary, i.e.,    *)
(*                        for any subgroup H of G, the intersection of H with *)
(*                        F G is included in H. Note that F is pcontinuous    *)
(*                        iff it is continuous and hereditary; indeed proofs  *)
(*                        of pcontinuous F coerce to proofs of hereditary F   *)
(*                        and continuous F.                                   *)
(*        monotonic F <-> F is monotonic with respect to inclusion: for any   *)
(*                        subgroup H of G, F H is a subgroup of F G. The      *)
(*                        derived and lower central series are examples of    *)
(*                        monotonic maps.                                     *)
(* Four structures provide interfaces to these properties:                    *)
(*    GFunctor.iso_map == structure for object maps that are group_valued,    *)
(*                        closed, and iso_continuous.                         *)
(*  [igFun by Fsub & !Fcont] == the iso_map structure for an object map F     *)
(*                        such that F G is canonically a group when G is, and *)
(*                        given Fsub : closed F and Fcont : iso_continuous F. *)
(*  [igFun by Fsub & Fcont] == as above, but expecting Fcont : continuous F.  *)
(*        [igFun of F] == clone an existing GFunctor.iso_map structure for F. *)
(*        GFunctor.map == structure for continuous object maps, inheriting    *)
(*                        from the GFunctor.iso_map structure.                *)
(*     [gFun by Fcont] == the map structure for an F with a canonical iso_map *)
(*                        structure, given Fcont : continuous F.              *)
(*         [gFun of F] == clone an existing GFunctor.map structure for F.     *)
(*       GFunctor.pmap == structure for pcontinuous object maps, inheriting   *)
(*                        from the GFunctor.map structure.                    *)
(*    [pgFun by Fher] == the pmap structure for an F with a canonical map     *)
(*                        structure, given Fher : hereditary F.               *)
(*        [pgFun of F] == clone an existing GFunctor.pmap structure for F.    *)
(*   GFunctor.mono_map == structure for monotonic, continuous object maps     *)
(*                        inheriting from the GFunctor.map structure.         *)
(*     [mgFun by Fmon] == the mono_map structure for an F with a canonical    *)
(*                        map structure, given Fmon : monotonic F.            *)
(*        [mgFun of F] == clone an existing GFunctor.mono_map structure for F *)
(* Lemmas for these group functors use either a 'gF' prefix or an 'F' suffix. *)
(* The (F1 \o F2) and (F1 %% F2) operations have canonical GFunctor.map       *)
(* structures when F1 is monotonic or hereditary, respectively.               *)
(******************************************************************************)
Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Delimit Scope gFun_scope with gF.
Module GFunctor.
Definition object_map := forall gT : finGroupType, {set gT} -> {set gT}.
Bind Scope gFun_scope with object_map.
Section Definitions.
Implicit Types gT hT : finGroupType.
Variable F : object_map.
(* Group closure. *)
Definition group_valued := forall gT (G : {group gT}), group_set (F G).
(* Subgroup closure. *)
Definition closed := forall gT (G : {group gT}), F G \subset G.
(* General functoriality, i.e., continuity of the object map *)
Definition continuous :=
forall gT hT (G : {group gT}) (phi : {morphism G >-> hT}),
phi @* F G \subset F (phi @* G).
(* Functoriality on the Grp groupoid (arrows are restricted to isos). *)
Definition iso_continuous :=
forall gT hT (G : {group gT}) (phi : {morphism G >-> hT}),
'injm phi -> phi @* F G \subset F (phi @* G).

Lemma continuous_is_iso_continuous : continuous -> iso_continuous.
Definition monotonic :=
forall gT (H G : {group gT}), H \subset G -> F H \subset F G.