(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
From mathcomp
(******************************************************************************)
(*  Properties of cyclic groups.                                              *)
(* Definitions:                                                               *)
(* Defined in fingroup.v:                                                     *)
(*           <[x]> == the cycle (cyclic group) generated by x.                *)
(*            #[x] == the order of x, i.e., the cardinal of <[x]>.            *)
(* Defined in prime.v:                                                        *)
(*       totient n == Euler's totient function                                *)
(* Definitions in this file:                                                  *)
(*        cyclic G <=> G is a cyclic group.                                   *)
(*    metacyclic G <=> G is a metacyclic group (i.e., a cyclic extension of a *)
(*                     cyclic group).                                         *)
(*   generator G x <=> x is a generator of the (cyclic) group G.              *)
(*           Zpm x == the isomorphism mapping the additive group of integers  *)
(*                    mod #[x] to the cyclic group <[x]>.                     *)
(*      cyclem x n == the endomorphism y |-> y ^+ n of <[x]>.                 *)
(*      Zp_unitm x == the isomorphism mapping the multiplicative group of the *)
(*                    units of the ring of integers mod #[x] to the group of  *)
(*                    automorphisms of <[x]> (i.e., Aut <[x]>).               *)
(*                    Zp_unitm x maps u to cyclem x u.                        *)
(*    eltm dvd_y_x == the smallest morphism (with domain <[x]>) mapping x to  *)
(*                    y, given a proof dvd_y_x : #[y] %| #[x].                *)
(*   expg_invn G k == if coprime #|G| k, the inverse of exponent k in G.      *)
(* Basic results for these notions, plus the classical result that any finite *)
(* group isomorphic to a subgroup of a field is cyclic, hence that Aut G is   *)
(* cyclic when G is of prime order.                                           *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
(***********************************************************************)
(*  Cyclic groups.                                                     *)
(***********************************************************************)
Section Cyclic.
Variable gT : finGroupType.
Implicit Types (a x y : gT) (A B : {set gT}) (G K H : {group gT}).
Definition cyclic A := [exists x, A == <[x]>].

Lemma cyclic_dprod K H G :
K \x H = G -> cyclic K -> cyclic H -> cyclic G = coprime #|K| #|H| .