(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
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(*  Definition of the additive group and ring Zp, represented as 'I_p         *)
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(* Definitions:                                                               *)
(* From fintype.v:                                                            *)
(*     'I_p == the subtype of integers less than p, taken here as the type of *)
(*             the integers mod p.                                            *)
(* This file:                                                                 *)
(*     inZp == the natural projection from nat into the integers mod p,       *)
(*             represented as 'I_p. Here p is implicit, but MUST be of the    *)
(*             form n.+1.                                                     *)
(* The operations:                                                            *)
(*      Zp0 == the identity element for addition                              *)
(*      Zp1 == the identity element for multiplication, and a generator of    *)
(*             additive group                                                 *)
(*   Zp_opp == inverse function for addition                                  *)
(*   Zp_add == addition                                                       *)
(*   Zp_mul == multiplication                                                 *)
(*   Zp_inv == inverse function for multiplication                            *)
(* Note that while 'I_n.+1 has canonical finZmodType and finGroupType         *)
(* structures, only 'I_n.+2 has a canonical ring structure (it has, in fact,  *)
(* a canonical finComUnitRing structure), and hence an associated             *)
(* multiplicative unit finGroupType. To mitigate the issues caused by the     *)
(* trivial "ring" (which is, indeed is NOT a ring in the ssralg/finalg        *)
(* formalization), we define additional notation:                             *)
(*       'Z_p == the type of integers mod (max p 2); this is always a proper  *)
(*               ring, by constructions. Note that 'Z_p is provably equal to  *)
(*               'I_p if p > 1, and convertible to 'I_p if p is of the form   *)
(*               n.+2.                                                        *)
(*       Zp p == the subgroup of integers mod (max p 1) in 'Z_p; this is thus *)
(*               is thus all of 'Z_p if p > 1, and else the trivial group.    *)
(* units_Zp p == the group of all units of 'Z_p -- i.e., the group of         *)
(*               (multiplicative) automorphisms of Zp p.                      *)
(* We show that Zp and units_Zp are abelian, and compute their orders.        *)
(* We use a similar technique to represent the prime fields:                  *)
(*        'F_p == the finite field of integers mod the first prime divisor of *)
(*                maxn p 2. This is provably equal to 'Z_p and 'I_p if p is   *)
(*                provably prime, and indeed convertible to the above if p is *)
(*                a concrete prime such as 2, 5 or 23.                        *)
(* Note finally that due to the canonical structures it is possible to use    *)
(* 0%R instead of Zp0, and 1%R instead of Zp1 (for the latter, p must be of   *)
(* the form n.+2, and 1%R : nat will simplify to 1%N).                        *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Local Open Scope ring_scope.
Section ZpDef.
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(*                                                                     *)
(*  Mod p arithmetic on the finite set {0, 1, 2, ..., p - 1}           *)
(*                                                                     *)
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Variable p' : nat.
Local Notation p := p'.+1.
Implicit Types x y z : 'I_p.
(* Standard injection; val (inZp i) = i %% p *)
Definition inZp i := Ordinal (ltn_pmod i (ltn0Sn p')).

Lemma big_ord1 R idx (op : @Monoid.law R idx) F :
\big[op/idx]_(i < 1) F i = F 0.