(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
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(* This file contains the fine structure thorems for extraspecial p-groups.   *)
(* Together with the material in the maximal and extremal libraries, it       *)
(* completes the coverage of Aschbacher, section 23.                          *)
(* We define canonical representatives for the group classes that cover the   *)
(* extremal p-groups (non-abelian p-groups with a cyclic maximal subgroup):   *)
(* 'Mod_m == the modular group of order m, for m = p ^ n, p prime and n >= 3. *)
(*   'D_m == the dihedral group of order m, for m = 2n >= 4.                  *)
(*   'Q_m == the generalized quaternion group of order m, for q = 2 ^ n >= 8. *)
(*  'SD_m == the semi-dihedral group of order m, for m = 2 ^ n >= 16.         *)
(* In each case the notation is defined in the %type, %g and %G scopes, where *)
(* it denotes a finGroupType, a full gset and the full group for that type.   *)
(* However each notation is only meaningful under the given conditions, in    *)
(*   We construct and study the following extraspecial groups:                *)
(*    p^{1+2} == if p is prime, an extraspecial group of order p^3 that has   *)
(*               exponent p or 4, and p-rank 2: thus p^{1+2} is isomorphic to *)
(*               'D_8 if p - 2, and NOT isomorphic to 'Mod_(p^3) if p is odd. *)
(*  p^{1+2*n} == the central product of n copies of p^{1+2}, thus of order    *)
(*               p^(1+2*n) if p is a prime, and, when n > 0, a representative *)
(*               of the (unique) isomorphism class of extraspecial groups of  *)
(*               order p^(1+2*n), of exponent p or 4, and p-rank n+1.         *)
(*       'D^n == an alternative (and preferred) notation for 2^{1+2*n}, which *)
(*               is isomorphic to the central product of n copies od 'D_8.    *)
(*     'D^n*Q == the central product of 'D^n with 'Q_8, thus isomorphic to    *)
(*               all extraspecial groups of order 2 ^ (2 * n + 3) that are    *)
(*               not isomorphic to 'D^n.+1 (or, equivalently, have 2-rank n). *)
(* As in extremal.v, these notations are simultaneously defined in the %type, *)
(* %g and %G scopes -- depending on the syntactic context, they denote either *)
(* a finGroupType, the set, or the group of all its elements.                 *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Local Open Scope ring_scope.
Reserved Notation "p ^{1+2}" (at level 2, format "p ^{1+2}").
Reserved Notation "p ^{1+2* n }"
(at level 2, n at level 2, format "p ^{1+2* n }").
Reserved Notation "''D^' n" (at level 8, n at level 2, format "''D^' n").
Reserved Notation "''D^' n * 'Q'"
(at level 8, n at level 2, format "''D^' n * 'Q'").
Module Pextraspecial.
Section Construction.
Variable p : nat.
Definition act ij (k : 'Z_p) := let: (i, j) := ij in (i + k * j, j).

Lemma actP : is_action [set: 'Z_p] act.
Canonical action := Action actP.