(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
(******************************************************************************)
(* Finite dimensional free algebras, usually known as F-algebras.             *)
(*       FalgType K   == the interface type for F-algebras over K; it simply  *)
(*                       joins the unitAlgType K and vectType K interfaces.   *)
(* [FalgType K of aT] == an FalgType K structure for a type aT that has both  *)
(*                       unitAlgType K and vectType K canonical structures.   *)
(* [FalgType K of aT for vT] == an FalgType K structure for a type aT with a  *)
(*                       unitAlgType K canonical structure, given a structure *)
(*                       vT : vectType K whose lmodType K projection matches  *)
(*                       the canonical lmodType for aT.                       *)
(* FalgUnitRingType T == a default unitRingType structure for a type T with   *)
(*                       both algType and vectType structures.                *)
(*   Any aT with an FalgType structure inherits all the Vector, Ring and      *)
(* Algebra operations, and supports the following additional operations:      *)
(*           \dim_A M == (\dim M %/ dim A)%N -- free module dimension.        *)
(*            amull u == the linear function v |-> u * v, for u, v : aT.      *)
(*            amulr u == the linear function v |-> v * u, for u, v : aT.      *)
(*   1, f * g, f ^+ n == the identity function, the composite g \o f, the nth *)
(*                       iterate of f, for 1, f, g in 'End(aT). This is just  *)
(*                       the usual F-algebra structure on 'End(aT). It is NOT *)
(*                       canonical by default, but can be activated by the    *)
(*                       line Import FalgLfun. Beware also that (f^-1)%VF is  *)
(*                       the linear function inverse, not the ring inverse of *)
(*                       f (though they do coincide when f is injective).     *)
(*               1%VS == the line generated by 1 : aT.                        *)
(*         (U * V)%VS == the smallest subspace of aT that contains all        *)
(*                       products u * v for u in U, v in V.                   *)
(*        (U ^+ n)%VS == (U * U * ... * U), n-times.  U ^+ 0 = 1%VS           *)
(*           'C[u]%VS == the centraliser subspace of the vector u.            *)
(*         'C_U[v]%VS := (U :&: 'C[v])%VS.                                    *)
(*           'C(V)%VS == the centraliser subspace of the subspace V.          *)
(*         'C_U(V)%VS := (U :&: 'C(V))%VS.                                    *)
(*           'Z(V)%VS == the center subspace of the subspace V.               *)
(*            agenv U == the smallest subalgebra containing U ^+ n for all n. *)
(*        <<U; v>>%VS == agenv (U + <[v]>) (adjoin v to U).                   *)
(*      <<U & vs>>%VS == agenv (U + <<vs>>) (adjoin vs to U).                 *)
(*        {aspace aT} == a subType of {vspace aT} consisting of sub-algebras  *)
(*                       of aT (see below); for A : {aspace aT}, subvs_of A   *)
(*                       has a canonical FalgType K structure.                *)
(*        is_aspace U <=> the characteristic predicate of {aspace aT} stating *)
(*                       that U is closed under product and contains an       *)
(*                       identity element, := has_algid U && (U * U <= U)%VS. *)
(*            algid A == the identity element of A : {aspace aT}, which need  *)
(*                       not be equal to 1 (indeed, in a Wedderburn           *)
(*                       decomposition it is not even a unit in aT).          *)
(*       is_algid U e <-> e : aT is an identity element for the subspace U:   *)
(*                       e in U, e != 0 & e * u = u * e = u for all u in U.   *)
(*        has_algid U <=> there is an e such that is_algid U e.               *)
(*      [aspace of U] == a clone of an existing {aspace aT} structure on      *)
(*                       U : {vspace aT} (more instances of {aspace aT} will  *)
(*                       be defined in extFieldType).                         *)
(* [aspace of U for A] == a clone of A : {aspace aT} for U : {vspace aT}.     *)
(*               1%AS == the canonical sub-algebra 1%VS.                      *)
(*           {:aT}%AS == the canonical full algebra.                          *)
(*           <<U>>%AS == the canonical algebra for agenv U; note that this is *)
(*                       unrelated to <<vs>>%VS, the subspace spanned by vs.  *)
(*        <<U; v>>%AS == the canonical algebra for <<U; v>>%VS.               *)
(*      <<U & vs>>%AS == the canonical algebra for <<U & vs>>%VS.             *)
(*        ahom_in U f <=> f : 'Hom(aT, rT) is a multiplicative homomorphism   *)
(*                       inside U, and in addition f 1 = 1 (even if U doesn't *)
(*                       contain 1). Note that f @: U need not be a           *)
(*                       subalgebra when U is, as f could annilate U.         *)
(*      'AHom(aT, rT) == the type of algebra homomorphisms from aT to rT,     *)
(*                       where aT and rT ARE FalgType structures. Elements of *)
(*                       'AHom(aT, rT) coerce to 'End(aT, rT) and aT -> rT.   *)
(* --> Caveat: aT and rT must denote actual FalgType structures, not their    *)
(*     projections on Type.                                                   *)
(*          'AEnd(aT) == algebra endomorphisms of aT (:= 'AHom(aT, aT)).      *)
(******************************************************************************)
Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Local Open Scope ring_scope.
Reserved Notation "{ 'aspace' T }" (at level 0, format "{ 'aspace'  T }").
Reserved Notation "<< U & vs >>" (at level 0, format "<< U  &  vs >>").
Reserved Notation "<< U ; x >>" (at level 0, format "<< U ;  x >>").
Reserved Notation "''AHom' ( T , rT )"
(at level 8, format "''AHom' ( T ,  rT )").
Reserved Notation "''AEnd' ( T )" (at level 8, format "''AEnd' ( T )").
Notation "\dim_ E V" := (divn (\dim V) (\dim E))
(at level 10, E at level 2, V at level 8, format "\dim_ E  V") : nat_scope.
(* Finite dimensional algebra *)
Module Falgebra.
(* Supply a default unitRing mixin for the default unitAlgType base type. *)
Section DefaultBase.
Variables (K : fieldType) (A : algType K).

Lemma BaseMixin : Vector.mixin_of A -> GRing.UnitRing.mixin_of A.
Definition BaseType T :=
fun c vAm & phant_id c (GRing.UnitRing.Class (BaseMixin vAm)) =>
fun (vT : vectType K) & phant vT
& phant_id (Vector.mixin (Vector.class vT)) vAm =>
@GRing.UnitRing.Pack T c.