formal/coq/math-comp/ssreflect/div.v

(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp Require Import ssreflect ssrfun ssrbool eqtype ssrnat seq.

(******************************************************************************)
(* This file deals with divisibility for natural numbers.                     *)
(* It contains the definitions of:                                            *)
(*      edivn m d   == the pair composed of the quotient and remainder        *)
(*                     of the Euclidean division of m by d.                   *)
(*          m %/ d  == quotient of the Euclidean division of m by d.          *)
(*          m %% d  == remainder of the Euclidean division of m by d.         *)
(*  m = n %[mod d]  <-> m equals n modulo d.                                  *)
(*  m == n %[mod d] <=> m equals n modulo d (boolean version).                *)
(*  m <> n %[mod d] <-> m differs from n modulo d.                            *)
(*  m != n %[mod d] <=> m differs from n modulo d (boolean version).          *)
(*           d %| m <=> d divides m.                                          *)
(*         gcdn m n == the GCD of m and n.                                    *)
(*        egcdn m n == the extended GCD (Bezout coefficient pair) of m and n. *)
(*                     If egcdn m n = (u, v), then gcdn m n = m * u - n * v.  *)
(*         lcmn m n == the LCM of m and n.                                    *)
(*      coprime m n <=> m and n are coprime (:= gcdn m n == 1).               *)
(*  chinese m n r s == witness of the chinese remainder theorem.              *)
(* We adjoin an m to operator suffixes to indicate a nested %% (modn), as in  *)
(*   modnDml : m %% d + n = m + n %[mod d].                                   *)
(******************************************************************************)

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

(** Euclidean division *)

Definition edivn_rec d :=
  fix loop m q := if m - d is m'.+1 then loop m' q.+1 else (q, m).

Definition edivn m d := if d > 0 then edivn_rec d.-1 m 0 else (0, m).

Variant edivn_spec m d : nat * nat -> Type :=
  EdivnSpec q r of m = q * d + r & (d > 0) ==> (r < d) : edivn_spec m d (q, r).

Lemma edivnP m d : edivn_spec m d (edivn m d).
Proof.
rewrite -[m in edivn_spec m]/(0 * d + m) /edivn; case: d => //= d.
elim/ltn_ind: m 0 => -[|m] IHm q //=; rewrite subn_if_gt.
case: ltnP => // le_dm; rewrite -[in m.+1](subnKC le_dm) -addSn.
by rewrite addnA -mulSnr; apply/IHm/leq_subr.
Qed.

Lemma edivn_eq d q r : r < d -> edivn (q * d + r) d = (q, r).
Proof.
move=> lt_rd; have d_gt0: 0 < d by apply: leq_trans lt_rd.
case: edivnP lt_rd => q' r'; rewrite d_gt0 /=.
wlog: q q' r r' / q <= q' by case/orP: (leq_total q q'); last symmetry; eauto.
have [||-> _ /addnI ->] //= := ltngtP q q'.
rewrite -(leq_pmul2r d_gt0) => /leq_add lt_qr _ eq_qr _ /lt_qr {lt_qr}.
by rewrite addnS ltnNge mulSn -addnA eq_qr addnCA addnA leq_addr.
Qed.

Definition divn m d := (edivn m d).1.

Notation "m %/ d" := (divn m d) : nat_scope.

(* We redefine modn so that it is structurally decreasing. *)

Definition modn_rec d := fix loop m := if m - d is m'.+1 then loop m' else m.

Definition modn m d := if d > 0 then modn_rec d.-1 m else m.

Notation "m %% d" := (modn m d) : nat_scope.
Notation "m = n %[mod d ]" := (m %% d = n %% d) : nat_scope.
Notation "m == n %[mod d ]" := (m %% d == n %% d) : nat_scope.
Notation "m <> n %[mod d ]" := (m %% d <> n %% d) : nat_scope.
Notation "m != n %[mod d ]" := (m %% d != n %% d) : nat_scope.

Lemma modn_def m d : m %% d = (edivn m d).2.
Proof.
case: d => //= d; rewrite /modn /edivn /=; elim/ltn_ind: m 0 => -[|m] IHm q //=.
by rewrite !subn_if_gt; case: (d <= m) => //; apply/IHm/leq_subr.
Qed.

Lemma edivn_def m d : edivn m d = (m %/ d, m %% d).
Proof. by rewrite /divn modn_def; case: (edivn m d). Qed.

Lemma divn_eq m d : m = m %/ d * d + m %% d.
Proof. by rewrite /divn modn_def; case: edivnP. Qed.

Lemma div0n d : 0 %/ d = 0. Proof. by case: d. Qed.
Lemma divn0 m : m %/ 0 = 0. Proof. by []. Qed.
Lemma mod0n d : 0 %% d = 0. Proof. by case: d. Qed.
Lemma modn0 m : m %% 0 = m. Proof. by []. Qed.

Lemma divn_small m d : m < d -> m %/ d = 0.
Proof. by move=> lt_md; rewrite /divn (edivn_eq 0). Qed.

Lemma divnMDl q m d : 0 < d -> (q * d + m) %/ d = q + m %/ d.
Proof.
move=> d_gt0; rewrite [in LHS](divn_eq m d) addnA -mulnDl.
by rewrite /divn edivn_eq // modn_def; case: edivnP; rewrite d_gt0.
Qed.

Lemma mulnK m d : 0 < d -> m * d %/ d = m.
Proof. by move=> d_gt0; rewrite -[m * d]addn0 divnMDl // div0n addn0. Qed.

Lemma mulKn m d : 0 < d -> d * m %/ d = m.
Proof. by move=> d_gt0; rewrite mulnC mulnK. Qed.

Lemma expnB p m n : p > 0 -> m >= n -> p ^ (m - n) = p ^ m %/ p ^ n.
Proof.
by move=> p_gt0 /subnK-Dm; rewrite -[in RHS]Dm expnD mulnK // expn_gt0 p_gt0.
Qed.

Lemma modn1 m : m %% 1 = 0.
Proof. by rewrite modn_def; case: edivnP => ? []. Qed.

Lemma divn1 m : m %/ 1 = m.
Proof. by rewrite [RHS](@divn_eq m 1) // modn1 addn0 muln1. Qed.

Lemma divnn d : d %/ d = (0 < d).
Proof. by case: d => // d; rewrite -[n in n %/ _]muln1 mulKn. Qed.

Lemma divnMl p m d : p > 0 -> p * m %/ (p * d) = m %/ d.
Proof.
move=> p_gt0; have [->|d_gt0] := posnP d; first by rewrite muln0.
rewrite [RHS]/divn; case: edivnP; rewrite d_gt0 /= => q r ->{m} lt_rd.
rewrite mulnDr mulnCA divnMDl; last by rewrite muln_gt0 p_gt0.
by rewrite addnC divn_small // ltn_pmul2l.
Qed.
Arguments divnMl [p m d].

Lemma divnMr p m d : p > 0 -> m * p %/ (d * p) = m %/ d.
Proof. by move=> p_gt0; rewrite -!(mulnC p) divnMl. Qed.
Arguments divnMr [p m d].

Lemma ltn_mod m d : (m %% d < d) = (0 < d).
Proof. by case: d => // d; rewrite modn_def; case: edivnP. Qed.

Lemma ltn_pmod m d : 0 < d -> m %% d < d.
Proof. by rewrite ltn_mod. Qed.

Lemma leq_trunc_div m d : m %/ d * d <= m.
Proof. by rewrite [leqRHS](divn_eq m d) leq_addr. Qed.

Lemma leq_mod m d : m %% d <= m.
Proof. by rewrite [leqRHS](divn_eq m d) leq_addl. Qed.

Lemma leq_div m d : m %/ d <= m.
Proof.
by case: d => // d; apply: leq_trans (leq_pmulr _ _) (leq_trunc_div _ _).
Qed.

Lemma ltn_ceil m d : 0 < d -> m < (m %/ d).+1 * d.
Proof.
by move=> d_gt0; rewrite [in m.+1](divn_eq m d) -addnS mulSnr leq_add2l ltn_mod.
Qed.

Lemma ltn_divLR m n d : d > 0 -> (m %/ d < n) = (m < n * d).
Proof.
move=> d_gt0; apply/idP/idP.
  by rewrite -(leq_pmul2r d_gt0); apply: leq_trans (ltn_ceil _ _).
rewrite !ltnNge -(@leq_pmul2r d n) //; apply: contra => le_nd_floor.
exact: leq_trans le_nd_floor (leq_trunc_div _ _).
Qed.

Lemma leq_divRL m n d : d > 0 -> (m <= n %/ d) = (m * d <= n).
Proof. by move=> d_gt0; rewrite leqNgt ltn_divLR // -leqNgt. Qed.

Lemma ltn_Pdiv m d : 1 < d -> 0 < m -> m %/ d < m.
Proof. by move=> d_gt1 m_gt0; rewrite ltn_divLR ?ltn_Pmulr // ltnW. Qed.

Lemma divn_gt0 d m : 0 < d -> (0 < m %/ d) = (d <= m).
Proof. by move=> d_gt0; rewrite leq_divRL ?mul1n. Qed.

Lemma leq_div2r d m n : m <= n -> m %/ d <= n %/ d.
Proof.
have [-> //| d_gt0 le_mn] := posnP d.
by rewrite leq_divRL // (leq_trans _ le_mn) -?leq_divRL.
Qed.

Lemma leq_div2l m d e : 0 < d -> d <= e -> m %/ e <= m %/ d.
Proof.
move/leq_divRL=> -> le_de.
by apply: leq_trans (leq_trunc_div m e); apply: leq_mul.
Qed.

Lemma edivnD m n d (offset := m %% d + n %% d >= d) : 0 < d ->
   edivn (m + n) d = (m %/ d + n %/ d + offset, m %% d + n %% d - offset * d).
Proof.
rewrite {}/offset; case: d => // d _; rewrite /divn !modn_def.
case: (edivnP m d.+1) (edivnP n d.+1) => [/= q r -> r_lt] [/= p s -> s_lt].
rewrite addnACA -mulnDl; have [r_le s_le] := (ltnW r_lt, ltnW s_lt).
have [d_ge|d_lt] := leqP; first by rewrite addn0 mul0n subn0 edivn_eq.
rewrite addn1 mul1n -[in LHS](subnKC d_lt) addnA -mulSnr edivn_eq//.
by rewrite ltn_subLR// -addnS leq_add.
Qed.

Lemma divnD m n d : 0 < d ->
  (m + n) %/ d = (m %/ d) + (n %/ d) + (m %% d + n %% d >= d).
Proof. by move=> /(@edivnD m n); rewrite edivn_def => -[]. Qed.

Lemma modnD m n d : 0 < d ->
  (m + n) %% d = m %% d + n %% d - (m %% d + n %% d >= d) * d.
Proof. by move=> /(@edivnD m n); rewrite edivn_def => -[]. Qed.

Lemma leqDmod m n d : 0 < d ->
  (d <= m %% d + n %% d) = ((m + n) %% d < n %% d).
Proof.
move=> d_gt0; rewrite modnD//.
have [d_le|_] := leqP d; last by rewrite subn0 ltnNge leq_addl.
by rewrite -(ltn_add2r d) mul1n (subnK d_le) addnC ltn_add2l ltn_pmod.
Qed.

Lemma divnB n m d : 0 < d ->
  (m - n) %/ d = (m %/ d) - (n %/ d) - (m %% d < n %% d).
Proof.
move=> d_gt0; have [mn|/ltnW nm] := leqP m n.
  by rewrite (eqP mn) (eqP (leq_div2r _ _)) ?div0n.
by rewrite -[in m %/ d](subnK nm) divnD// addnAC addnK leqDmod ?subnK ?addnK.
Qed.

Lemma modnB m n d : 0 < d -> n <= m ->
  (m - n) %% d = (m %% d < n %% d) * d + m %% d - n %% d.
Proof.
move=> d_gt0 nm; rewrite -[in m %% _](subnK nm) -leqDmod// modnD//.
have [d_le|_] := leqP d; last by rewrite mul0n add0n subn0 addnK.
by rewrite mul1n addnBA// addnC !addnK.
Qed.

Lemma edivnB m n d (offset := m %% d < n %% d) : 0 < d -> n <= m ->
   edivn (m - n) d = (m %/ d - n %/ d - offset, offset * d + m %% d - n %% d).
Proof. by move=> d_gt0 le_nm; rewrite edivn_def divnB// modnB. Qed.

Lemma leq_divDl p m n : (m + n) %/ p <= m %/ p + n %/ p + 1.
Proof. by have [->//|p_gt0] := posnP p; rewrite divnD// !leq_add// leq_b1. Qed.

Lemma geq_divBl k m p : k %/ p - m %/ p <= (k - m) %/ p + 1.
Proof.
rewrite leq_subLR addnA; apply: leq_trans (leq_divDl _ _ _).
by rewrite -maxnE leq_div2r ?leq_maxr.
Qed.

Lemma divnMA m n p : m %/ (n * p) = m %/ n %/ p.
Proof.
case: n p => [|n] [|p]; rewrite ?muln0 ?div0n //.
rewrite [in RHS](divn_eq m (n.+1 * p.+1)) mulnA mulnAC !divnMDl //.
by rewrite [_ %/ p.+1]divn_small ?addn0 // ltn_divLR // mulnC ltn_mod.
Qed.

Lemma divnAC m n p : m %/ n %/ p =  m %/ p %/ n.
Proof. by rewrite -!divnMA mulnC. Qed.

Lemma modn_small m d : m < d -> m %% d = m.
Proof. by move=> lt_md; rewrite [RHS](divn_eq m d) divn_small. Qed.

Lemma modn_mod m d : m %% d = m %[mod d].
Proof. by case: d => // d; apply: modn_small; rewrite ltn_mod. Qed.

Lemma modnMDl p m d : p * d + m = m %[mod d].
Proof.
have [->|d_gt0] := posnP d; first by rewrite muln0.
by rewrite [in LHS](divn_eq m d) addnA -mulnDl modn_def edivn_eq // ltn_mod.
Qed.

Lemma muln_modr p m d : p * (m %% d) = (p * m) %% (p * d).
Proof.
have [->//|p_gt0] := posnP p; apply: (@addnI (p * (m %/ d * d))).
by rewrite -mulnDr -divn_eq mulnCA -(divnMl p_gt0) -divn_eq.
Qed.

Lemma muln_modl p m d : (m %% d) * p = (m * p) %% (d * p).
Proof. by rewrite -!(mulnC p); apply: muln_modr. Qed.

Lemma modn_divl m n d : (m %/ d) %% n = m %% (n * d) %/ d.
Proof.
case: d n => [|d] [|n] //; rewrite [in LHS]/divn [in LHS]modn_def.
case: (edivnP m d.+1) edivnP => [/= _ r -> le_rd] [/= q s -> le_sn].
rewrite mulnDl -mulnA -addnA modnMDl modn_small ?divnMDl ?divn_small ?addn0//.
by rewrite mulSnr -addnS leq_add ?leq_mul2r.
Qed.

Lemma modnDl m d : d + m = m %[mod d].
Proof. by rewrite -[m %% _](modnMDl 1) mul1n. Qed.

Lemma modnDr m d : m + d = m %[mod d]. Proof. by rewrite addnC modnDl. Qed.

Lemma modnn d : d %% d = 0. Proof. by rewrite [d %% d](modnDr 0) mod0n. Qed.

Lemma modnMl p d : p * d %% d = 0.
Proof. by rewrite -[p * d]addn0 modnMDl mod0n. Qed.

Lemma modnMr p d : d * p %% d = 0. Proof. by rewrite mulnC modnMl. Qed.

Lemma modnDml m n d : m %% d + n = m + n %[mod d].
Proof. by rewrite [in RHS](divn_eq m d) -addnA modnMDl. Qed.

Lemma modnDmr m n d : m + n %% d = m + n %[mod d].
Proof. by rewrite !(addnC m) modnDml. Qed.

Lemma modnDm m n d : m %% d + n %% d = m + n %[mod d].
Proof. by rewrite modnDml modnDmr. Qed.

Lemma eqn_modDl p m n d : (p + m == p + n %[mod d]) = (m == n %[mod d]).
Proof.
case: d => [|d]; first by rewrite !modn0 eqn_add2l.
apply/eqP/eqP=> eq_mn; last by rewrite -modnDmr eq_mn modnDmr.
rewrite -(modnMDl p m) -(modnMDl p n) !mulnSr -!addnA.
by rewrite -modnDmr eq_mn modnDmr.
Qed.

Lemma eqn_modDr p m n d : (m + p == n + p %[mod d]) = (m == n %[mod d]).
Proof. by rewrite -!(addnC p) eqn_modDl. Qed.

Lemma modnMml m n d : m %% d * n = m * n %[mod d].
Proof. by rewrite [in RHS](divn_eq m d) mulnDl mulnAC modnMDl. Qed.

Lemma modnMmr m n d : m * (n %% d) = m * n %[mod d].
Proof. by rewrite !(mulnC m) modnMml. Qed.

Lemma modnMm m n d : m %% d * (n %% d) = m * n %[mod d].
Proof. by rewrite modnMml modnMmr. Qed.

Lemma modn2 m : m %% 2 = odd m.
Proof. by elim: m => //= m IHm; rewrite -addn1 -modnDml IHm; case odd. Qed.

Lemma divn2 m : m %/ 2 = m./2.
Proof. by rewrite [in RHS](divn_eq m 2) modn2 muln2 addnC half_bit_double. Qed.

Lemma odd_mod m d : odd d = false -> odd (m %% d) = odd m.
Proof.
by move=> d_even; rewrite [in RHS](divn_eq m d) oddD oddM d_even andbF.
Qed.

Lemma modnXm m n a : (a %% n) ^ m = a ^ m %[mod n].
Proof. by elim: m => // m IHm; rewrite !expnS -modnMmr IHm modnMml modnMmr. Qed.

(** Divisibility **)

Definition dvdn d m := m %% d == 0.

Notation "m %| d" := (dvdn m d) : nat_scope.

Lemma dvdnP d m : reflect (exists k, m = k * d) (d %| m).
Proof.
apply: (iffP eqP) => [md0 | [k ->]]; last by rewrite modnMl.
by exists (m %/ d); rewrite [LHS](divn_eq m d) md0 addn0.
Qed.
Arguments dvdnP {d m}.

Lemma dvdn0 d : d %| 0.
Proof. by case: d. Qed.

Lemma dvd0n n : (0 %| n) = (n == 0).
Proof. by case: n. Qed.

Lemma dvdn1 d : (d %| 1) = (d == 1).
Proof. by case: d => [|[|d]] //; rewrite /dvdn modn_small. Qed.

Lemma dvd1n m : 1 %| m.
Proof. by rewrite /dvdn modn1. Qed.

Lemma dvdn_gt0 d m : m > 0 -> d %| m -> d > 0.
Proof. by case: d => // /prednK <-. Qed.

Lemma dvdnn m : m %| m.
Proof. by rewrite /dvdn modnn. Qed.

Lemma dvdn_mull d m n : d %| n -> d %| m * n.
Proof. by case/dvdnP=> n' ->; rewrite /dvdn mulnA modnMl. Qed.

Lemma dvdn_mulr d m n : d %| m -> d %| m * n.
Proof. by move=> d_m; rewrite mulnC dvdn_mull. Qed.
#[global] Hint Resolve dvdn0 dvd1n dvdnn dvdn_mull dvdn_mulr : core.

Lemma dvdn_mul d1 d2 m1 m2 : d1 %| m1 -> d2 %| m2 -> d1 * d2 %| m1 * m2.
Proof.
by move=> /dvdnP[q1 ->] /dvdnP[q2 ->]; rewrite mulnCA -mulnA 2?dvdn_mull.
Qed.

Lemma dvdn_trans n d m : d %| n -> n %| m -> d %| m.
Proof. by move=> d_dv_n /dvdnP[n1 ->]; apply: dvdn_mull. Qed.

Lemma dvdn_eq d m : (d %| m) = (m %/ d * d == m).
Proof.
apply/eqP/eqP=> [modm0 | <-]; last exact: modnMl.
by rewrite [RHS](divn_eq m d) modm0 addn0.
Qed.

Lemma dvdn2 n : (2 %| n) = ~~ odd n.
Proof. by rewrite /dvdn modn2; case (odd n). Qed.

Lemma dvdn_odd m n : m %| n -> odd n -> odd m.
Proof. by move=> m_dv_n; apply: contraTT; rewrite -!dvdn2 => /dvdn_trans->. Qed.

Lemma divnK d m : d %| m -> m %/ d * d = m.
Proof. by rewrite dvdn_eq; move/eqP. Qed.

Lemma leq_divLR d m n : d %| m -> (m %/ d <= n) = (m <= n * d).
Proof. by case: d m => [|d] [|m] ///divnK=> {2}<-; rewrite leq_pmul2r. Qed.

Lemma ltn_divRL d m n : d %| m -> (n < m %/ d) = (n * d < m).
Proof. by move=> dv_d_m; rewrite !ltnNge leq_divLR. Qed.

Lemma eqn_div d m n : d > 0 -> d %| m -> (n == m %/ d) = (n * d == m).
Proof. by move=> d_gt0 dv_d_m; rewrite -(eqn_pmul2r d_gt0) divnK. Qed.

Lemma eqn_mul d m n : d > 0 -> d %| m -> (m == n * d) = (m %/ d == n).
Proof. by move=> d_gt0 dv_d_m; rewrite eq_sym -eqn_div // eq_sym. Qed.

Lemma divn_mulAC d m n : d %| m -> m %/ d * n = m * n %/ d.
Proof.
case: d m => [[] //| d m] dv_d_m; apply/eqP.
by rewrite eqn_div ?dvdn_mulr // mulnAC divnK.
Qed.

Lemma muln_divA d m n : d %| n -> m * (n %/ d) = m * n %/ d.
Proof. by move=> dv_d_m; rewrite !(mulnC m) divn_mulAC. Qed.

Lemma muln_divCA d m n : d %| m -> d %| n -> m * (n %/ d) = n * (m %/ d).
Proof. by move=> dv_d_m dv_d_n; rewrite mulnC divn_mulAC ?muln_divA. Qed.

Lemma divnA m n p : p %| n -> m %/ (n %/ p) = m * p %/ n.
Proof. by case: p => [|p] dv_n; rewrite -[in RHS](divnK dv_n) // divnMr. Qed.

Lemma modn_dvdm m n d : d %| m -> n %% m = n %[mod d].
Proof.
by case/dvdnP=> q def_m; rewrite [in RHS](divn_eq n m) def_m mulnA modnMDl.
Qed.

Lemma dvdn_leq d m : 0 < m -> d %| m -> d <= m.
Proof. by move=> m_gt0 /dvdnP[[|k] Dm]; rewrite Dm // leq_addr in m_gt0 *. Qed.

Lemma gtnNdvd n d : 0 < n -> n < d -> (d %| n) = false.
Proof. by move=> n_gt0 lt_nd; rewrite /dvdn eqn0Ngt modn_small ?n_gt0. Qed.

Lemma eqn_dvd m n : (m == n) = (m %| n) && (n %| m).
Proof.
case: m n => [|m] [|n] //; apply/idP/andP => [/eqP -> //| []].
by rewrite eqn_leq => Hmn Hnm; do 2 rewrite dvdn_leq //.
Qed.

Lemma dvdn_pmul2l p d m : 0 < p -> (p * d %| p * m) = (d %| m).
Proof. by case: p => // p _; rewrite /dvdn -muln_modr // muln_eq0. Qed.
Arguments dvdn_pmul2l [p d m].

Lemma dvdn_pmul2r p d m : 0 < p -> (d * p %| m * p) = (d %| m).
Proof. by move=> p_gt0; rewrite -!(mulnC p) dvdn_pmul2l. Qed.
Arguments dvdn_pmul2r [p d m].

Lemma dvdn_divLR p d m : 0 < p -> p %| d -> (d %/ p %| m) = (d %| m * p).
Proof. by move=> /(@dvdn_pmul2r p _ m) <- /divnK->. Qed.

Lemma dvdn_divRL p d m : p %| m -> (d %| m %/ p) = (d * p %| m).
Proof.
have [-> | /(@dvdn_pmul2r p d) <- /divnK-> //] := posnP p.
by rewrite divn0 muln0 dvdn0.
Qed.

Lemma dvdn_div d m : d %| m -> m %/ d %| m.
Proof. by move/divnK=> {2}<-; apply: dvdn_mulr. Qed.

Lemma dvdn_exp2l p m n : m <= n -> p ^ m %| p ^ n.
Proof. by move/subnK <-; rewrite expnD dvdn_mull. Qed.

Lemma dvdn_Pexp2l p m n : p > 1 -> (p ^ m %| p ^ n) = (m <= n).
Proof.
move=> p_gt1; case: leqP => [|gt_n_m]; first exact: dvdn_exp2l.
by rewrite gtnNdvd ?ltn_exp2l ?expn_gt0 // ltnW.
Qed.

Lemma dvdn_exp2r m n k : m %| n -> m ^ k %| n ^ k.
Proof. by case/dvdnP=> q ->; rewrite expnMn dvdn_mull. Qed.

Lemma divn_modl m n d : d %| n -> (m %% n) %/ d = (m %/ d) %% (n %/ d).
Proof. by move=> dvd_dn; rewrite modn_divl divnK. Qed.

Lemma dvdn_addr m d n : d %| m -> (d %| m + n) = (d %| n).
Proof. by case/dvdnP=> q ->; rewrite /dvdn modnMDl. Qed.

Lemma dvdn_addl n d m : d %| n -> (d %| m + n) = (d %| m).
Proof. by rewrite addnC; apply: dvdn_addr. Qed.

Lemma dvdn_add d m n : d %| m -> d %| n -> d %| m + n.
Proof. by move/dvdn_addr->. Qed.

Lemma dvdn_add_eq d m n : d %| m + n -> (d %| m) = (d %| n).
Proof. by move=> dv_d_mn; apply/idP/idP => [/dvdn_addr | /dvdn_addl] <-. Qed.

Lemma dvdn_subr d m n : n <= m -> d %| m -> (d %| m - n) = (d %| n).
Proof. by move=> le_n_m dv_d_m; apply: dvdn_add_eq; rewrite subnK. Qed.

Lemma dvdn_subl d m n : n <= m -> d %| n -> (d %| m - n) = (d %| m).
Proof. by move=> le_n_m dv_d_m; rewrite -(dvdn_addl _ dv_d_m) subnK. Qed.

Lemma dvdn_sub d m n : d %| m -> d %| n -> d %| m - n.
Proof.
by case: (leqP n m) => [le_nm /dvdn_subr <- // | /ltnW/eqnP ->]; rewrite dvdn0.
Qed.

Lemma dvdn_exp k d m : 0 < k -> d %| m -> d %| (m ^ k).
Proof. by case: k => // k _ d_dv_m; rewrite expnS dvdn_mulr. Qed.

Lemma dvdn_fact m n : 0 < m <= n -> m %| n`!.
Proof.
case: m => //= m; elim: n => //= n IHn; rewrite ltnS.
have [/IHn/dvdn_mull->||-> _] // := ltngtP m n; exact: dvdn_mulr.
Qed.

#[global] Hint Resolve dvdn_add dvdn_sub dvdn_exp : core.

Lemma eqn_mod_dvd d m n : n <= m -> (m == n %[mod d]) = (d %| m - n).
Proof.
by move/subnK=> Dm; rewrite -[n in LHS]add0n -[in LHS]Dm eqn_modDr mod0n.
Qed.

Lemma divnDMl q m d : 0 < d -> (m + q * d) %/ d = (m %/ d) + q.
Proof. by move=> d_gt0; rewrite addnC divnMDl// addnC. Qed.

Lemma divnMBl q m d : 0 < d -> (q * d - m) %/ d = q - (m %/ d) - (~~ (d %| m)).
Proof. by move=> d_gt0; rewrite divnB// mulnK// modnMl lt0n. Qed.

Lemma divnBMl q m d : (m - q * d) %/ d = (m %/ d) - q.
Proof. by case: d => [|d]//=; rewrite divnB// mulnK// modnMl ltn0 subn0. Qed.

Lemma divnDl m n d : d %| m -> (m + n) %/ d = m %/ d + n %/ d.
Proof. by case: d => // d /divnK-Dm; rewrite -[in LHS]Dm divnMDl. Qed.

Lemma divnDr m n d : d %| n -> (m + n) %/ d = m %/ d + n %/ d.
Proof. by move=> dv_n; rewrite addnC divnDl // addnC. Qed.

Lemma divnBl m n d : d %| m -> (m - n) %/ d = m %/ d - (n %/ d) - (~~ (d %| n)).
Proof. by case: d => [|d] // /divnK-Dm; rewrite -[in LHS]Dm divnMBl. Qed.

Lemma divnBr m n d : d %| n -> (m - n) %/ d = m %/ d - n %/ d.
Proof. by case: d => [|d]// /divnK-Dm; rewrite -[in LHS]Dm divnBMl. Qed.

Lemma edivnS m d : 0 < d -> edivn m.+1 d =
  if d %| m.+1 then ((m %/ d).+1, 0) else (m %/ d, (m %% d).+1).
Proof.
case: d => [|[|d]] //= _; first by rewrite edivn_def modn1 dvd1n !divn1.
rewrite -addn1 /dvdn modn_def edivnD//= (@modn_small 1)// (@divn_small 1)//.
rewrite addn1 addn0 ltnS; have [||<-] := ltngtP d.+1.
- by rewrite ltnNge -ltnS ltn_pmod.
- by rewrite addn0 mul0n subn0.
- by rewrite addn1 mul1n subnn.
Qed.

Lemma modnS m d : m.+1 %% d = if d %| m.+1 then 0 else (m %% d).+1.
Proof. by case: d => [|d]//; rewrite modn_def edivnS//; case: ifP. Qed.

Lemma divnS m d : 0 < d -> m.+1 %/ d = (d %| m.+1) + m %/ d.
Proof. by move=> d_gt0; rewrite /divn edivnS//; case: ifP. Qed.

Lemma divn_pred m d : m.-1 %/ d = (m %/ d) - (d %| m).
Proof.
by case: d m => [|d] [|m]; rewrite ?divn1 ?dvd1n ?subn1//= divnS// addnC addnK.
Qed.

Lemma modn_pred m d : d != 1 -> 0 < m ->
  m.-1 %% d = if d %| m then d.-1 else (m %% d).-1.
Proof.
rewrite -subn1; case: d m => [|[|d]] [|m]//= _ _.
  by rewrite ?modn1 ?dvd1n ?modn0 ?subn1.
rewrite modnB// (@modn_small 1)// [_ < _]leqn0 /dvdn mulnbl/= subn1.
by case: eqP => // ->; rewrite addn0.
Qed.

Lemma edivn_pred m d : d != 1 -> 0 < m ->
  edivn m.-1 d = if d %| m then ((m %/ d).-1, d.-1) else (m %/ d, (m %% d).-1).
Proof.
move=> d_neq1 m_gt0; rewrite edivn_def divn_pred modn_pred//.
by case: ifP; rewrite ?subn0 ?subn1.
Qed.

(***********************************************************************)
(*   A function that computes the gcd of 2 numbers                     *)
(***********************************************************************)

Fixpoint gcdn_rec m n :=
  let n' := n %% m in if n' is 0 then m else
  if m - n'.-1 is m'.+1 then gcdn_rec (m' %% n') n' else n'.

Definition gcdn := nosimpl gcdn_rec.

Lemma gcdnE m n : gcdn m n = if m == 0 then n else gcdn (n %% m) m.
Proof.
rewrite /gcdn; elim/ltn_ind: m n => -[|m] IHm [|n] //=.
case def_p: (_ %% _) => // [p].
have{def_p} lt_pm: p.+1 < m.+1 by rewrite -def_p ltn_pmod.
rewrite {}IHm // subn_if_gt ltnW //=; congr gcdn_rec.
by rewrite -(subnK (ltnW lt_pm)) modnDr.
Qed.

Lemma gcdnn : idempotent gcdn.
Proof. by case=> // n; rewrite gcdnE modnn. Qed.

Lemma gcdnC : commutative gcdn.
Proof.
move=> m n; wlog lt_nm: m n / n < m by have [? ->|? <-|-> //] := ltngtP n m.
by rewrite gcdnE -[in m == 0](ltn_predK lt_nm) modn_small.
Qed.

Lemma gcd0n : left_id 0 gcdn. Proof. by case. Qed.
Lemma gcdn0 : right_id 0 gcdn. Proof. by case. Qed.

Lemma gcd1n : left_zero 1 gcdn.
Proof. by move=> n; rewrite gcdnE modn1. Qed.

Lemma gcdn1 : right_zero 1 gcdn.
Proof. by move=> n; rewrite gcdnC gcd1n. Qed.

Lemma dvdn_gcdr m n : gcdn m n %| n.
Proof.
elim/ltn_ind: m n => -[|m] IHm [|n] //=.
rewrite gcdnE; case def_p: (_ %% _) => [|p]; first by rewrite /dvdn def_p.
have lt_pm: p < m by rewrite -ltnS -def_p ltn_pmod.
rewrite /= (divn_eq n.+1 m.+1) def_p dvdn_addr ?dvdn_mull //; last exact: IHm.
by rewrite gcdnE /= IHm // (ltn_trans (ltn_pmod _ _)).
Qed.

Lemma dvdn_gcdl m n : gcdn m n %| m.
Proof. by rewrite gcdnC dvdn_gcdr. Qed.

Lemma gcdn_gt0 m n : (0 < gcdn m n) = (0 < m) || (0 < n).
Proof.
by case: m n => [|m] [|n] //; apply: (@dvdn_gt0 _ m.+1) => //; apply: dvdn_gcdl.
Qed.

Lemma gcdnMDl k m n : gcdn m (k * m + n) = gcdn m n.
Proof. by rewrite !(gcdnE m) modnMDl mulnC; case: m. Qed.

Lemma gcdnDl m n : gcdn m (m + n) = gcdn m n.
Proof. by rewrite -[m in m + n]mul1n gcdnMDl. Qed.

Lemma gcdnDr m n : gcdn m (n + m) = gcdn m n.
Proof. by rewrite addnC gcdnDl. Qed.

Lemma gcdnMl n m : gcdn n (m * n) = n.
Proof. by case: n => [|n]; rewrite gcdnE modnMl // muln0. Qed.

Lemma gcdnMr n m : gcdn n (n * m) = n.
Proof. by rewrite mulnC gcdnMl. Qed.

Lemma gcdn_idPl {m n} : reflect (gcdn m n = m) (m %| n).
Proof.
by apply: (iffP idP) => [/dvdnP[q ->] | <-]; rewrite (gcdnMl, dvdn_gcdr).
Qed.

Lemma gcdn_idPr {m n} : reflect (gcdn m n = n) (n %| m).
Proof. by rewrite gcdnC; apply: gcdn_idPl. Qed.

Lemma expn_min e m n : e ^ minn m n = gcdn (e ^ m) (e ^ n).
Proof. by case: leqP => [|/ltnW] /(dvdn_exp2l e) /gcdn_idPl; rewrite gcdnC. Qed.

Lemma gcdn_modr m n : gcdn m (n %% m) = gcdn m n.
Proof. by rewrite [in RHS](divn_eq n m) gcdnMDl. Qed.

Lemma gcdn_modl m n : gcdn (m %% n) n = gcdn m n.
Proof. by rewrite !(gcdnC _ n) gcdn_modr. Qed.

(* Extended gcd, which computes Bezout coefficients. *)

Fixpoint Bezout_rec km kn qs :=
  if qs is q :: qs' then Bezout_rec kn (NatTrec.add_mul q kn km) qs'
  else (km, kn).

Fixpoint egcdn_rec m n s qs :=
  if s is s'.+1 then
    let: (q, r) := edivn m n in
    if r > 0 then egcdn_rec n r s' (q :: qs) else
    if odd (size qs) then qs else q.-1 :: qs
  else [::0].

Definition egcdn m n := Bezout_rec 0 1 (egcdn_rec m n n [::]).

Variant egcdn_spec m n : nat * nat -> Type :=
  EgcdnSpec km kn of km * m = kn * n + gcdn m n & kn * gcdn m n < m :
    egcdn_spec m n (km, kn).

Lemma egcd0n n : egcdn 0 n = (1, 0).
Proof. by case: n. Qed.

Lemma egcdnP m n : m > 0 -> egcdn_spec m n (egcdn m n).
Proof.
have [-> /= | n_gt0 m_gt0] := posnP n; first by split; rewrite // mul1n gcdn0.
rewrite /egcdn; set s := (s in egcdn_rec _ _ s); pose bz := Bezout_rec n m [::].
have: n < s.+1 by []; move defSpec: (egcdn_spec bz.2 bz.1) s => Spec s.
elim: s => [[]|s IHs] //= in n m (qs := [::]) bz defSpec n_gt0 m_gt0 *.
case: edivnP => q r def_m; rewrite n_gt0 ltnS /= => lt_rn le_ns1.
case: posnP => [r0 {s le_ns1 IHs lt_rn}|r_gt0]; last first.
  by apply: IHs => //=; [rewrite natTrecE -def_m | rewrite (leq_trans lt_rn)].
rewrite {r}r0 addn0 in def_m; set b := odd _; pose d := gcdn m n.
pose km := ~~ b : nat; pose kn := if b then 1 else q.-1.
rewrite [bz in Spec bz](_ : _ = Bezout_rec km kn qs); last first.
  by rewrite /kn /km; case: (b) => //=; rewrite natTrecE addn0 muln1.
have def_d: d = n by rewrite /d def_m gcdnC gcdnE modnMl gcd0n -[n]prednK.
have: km * m + 2 * b * d = kn * n + d.
  rewrite {}/kn {}/km def_m def_d -mulSnr; case: b; rewrite //= addn0 mul1n.
  by rewrite prednK //; apply: dvdn_gt0 m_gt0 _; rewrite def_m dvdn_mulr.
have{def_m}: kn * d <= m.
  have q_gt0 : 0 < q by rewrite def_m muln_gt0 n_gt0 ?andbT in m_gt0.
  by rewrite /kn; case b; rewrite def_d def_m leq_pmul2r // leq_pred.
have{def_d}: km * d <= n by rewrite -[n]mul1n def_d leq_pmul2r // leq_b1.
move: km {q}kn m_gt0 n_gt0 defSpec; rewrite {}/b {}/d {}/bz.
elim: qs m n => [|q qs IHq] n r kn kr n_gt0 r_gt0 /=.
  set d := gcdn n r; rewrite mul0n addn0 => <- le_kn_r _ def_d; split=> //.
  have d_gt0: 0 < d by rewrite gcdn_gt0 n_gt0.
  have /ltn_pmul2l<-: 0 < kn by rewrite -(ltn_pmul2r n_gt0) def_d ltn_addl.
  by rewrite def_d -addn1 leq_add // mulnCA leq_mul2l le_kn_r orbT.
rewrite !natTrecE; set m := _ + r; set km := _ + kn; pose d := gcdn m n.
have ->: gcdn n r = d by rewrite [d]gcdnC gcdnMDl.
have m_gt0: 0 < m by rewrite addn_gt0 r_gt0 orbT.
have d_gt0: 0 < d by rewrite gcdn_gt0 m_gt0.
move=> {}/IHq IHq le_kn_r le_kr_n def_d; apply: IHq => //; rewrite -/d.
  by rewrite mulnDl leq_add // -mulnA leq_mul2l le_kr_n orbT.
apply: (@addIn d); rewrite mulnDr -addnA addnACA -def_d addnACA mulnA.
rewrite -!mulnDl -mulnDr -addnA [kr * _]mulnC; congr addn.
by rewrite addnC addn_negb muln1 mul2n addnn.
Qed.

Lemma Bezoutl m n : m > 0 -> {a | a < m & m %| gcdn m n + a * n}.
Proof.
move=> m_gt0; case: (egcdnP n m_gt0) => km kn def_d lt_kn_m.
exists kn; last by rewrite addnC -def_d dvdn_mull.
apply: leq_ltn_trans lt_kn_m.
by rewrite -{1}[kn]muln1 leq_mul2l gcdn_gt0 m_gt0 orbT.
Qed.

Lemma Bezoutr m n : n > 0 -> {a | a < n & n %| gcdn m n + a * m}.
Proof. by rewrite gcdnC; apply: Bezoutl. Qed.

(* Back to the gcd. *)

Lemma dvdn_gcd p m n : p %| gcdn m n = (p %| m) && (p %| n).
Proof.
apply/idP/andP=> [dv_pmn | [dv_pm dv_pn]].
  by rewrite !(dvdn_trans dv_pmn) ?dvdn_gcdl ?dvdn_gcdr.
have [->|n_gt0] := posnP n; first by rewrite gcdn0.
case: (Bezoutr m n_gt0) => // km _ /(dvdn_trans dv_pn).
by rewrite dvdn_addl // dvdn_mull.
Qed.

Lemma gcdnAC : right_commutative gcdn.
Proof.
suffices dvd m n p: gcdn (gcdn m n) p %| gcdn (gcdn m p) n.
  by move=> m n p; apply/eqP; rewrite eqn_dvd !dvd.
rewrite !dvdn_gcd dvdn_gcdr.
by rewrite !(dvdn_trans (dvdn_gcdl _ p)) ?dvdn_gcdl ?dvdn_gcdr.
Qed.

Lemma gcdnA : associative gcdn.
Proof. by move=> m n p; rewrite !(gcdnC m) gcdnAC. Qed.

Lemma gcdnCA : left_commutative gcdn.
Proof. by move=> m n p; rewrite !gcdnA (gcdnC m). Qed.

Lemma gcdnACA : interchange gcdn gcdn.
Proof. by move=> m n p q; rewrite -!gcdnA (gcdnCA n). Qed.

Lemma muln_gcdr : right_distributive muln gcdn.
Proof.
move=> p m n; have [-> //|p_gt0] := posnP p.
elim/ltn_ind: m n => m IHm n; rewrite gcdnE [RHS]gcdnE muln_eq0 (gtn_eqF p_gt0).
by case: posnP => // m_gt0; rewrite -muln_modr //=; apply/IHm/ltn_pmod.
Qed.

Lemma muln_gcdl : left_distributive muln gcdn.
Proof. by move=> m n p; rewrite -!(mulnC p) muln_gcdr. Qed.

Lemma gcdn_def d m n :
    d %| m -> d %| n -> (forall d', d' %| m -> d' %| n -> d' %| d) ->
  gcdn m n = d.
Proof.
move=> dv_dm dv_dn gdv_d; apply/eqP.
by rewrite eqn_dvd dvdn_gcd dv_dm dv_dn gdv_d ?dvdn_gcdl ?dvdn_gcdr.
Qed.

Lemma muln_divCA_gcd n m : n * (m %/ gcdn n m)  = m * (n %/ gcdn n m).
Proof. by rewrite muln_divCA ?dvdn_gcdl ?dvdn_gcdr. Qed.

(* We derive the lcm directly. *)

Definition lcmn m n := m * n %/ gcdn m n.

Lemma lcmnC : commutative lcmn.
Proof. by move=> m n; rewrite /lcmn mulnC gcdnC. Qed.

Lemma lcm0n : left_zero 0 lcmn.  Proof. by move=> n; apply: div0n. Qed.
Lemma lcmn0 : right_zero 0 lcmn. Proof. by move=> n; rewrite lcmnC lcm0n. Qed.

Lemma lcm1n : left_id 1 lcmn.
Proof. by move=> n; rewrite /lcmn gcd1n mul1n divn1. Qed.

Lemma lcmn1 : right_id 1 lcmn.
Proof. by move=> n; rewrite lcmnC lcm1n. Qed.

Lemma muln_lcm_gcd m n : lcmn m n * gcdn m n = m * n.
Proof. by apply/eqP; rewrite divnK ?dvdn_mull ?dvdn_gcdr. Qed.

Lemma lcmn_gt0 m n : (0 < lcmn m n) = (0 < m) && (0 < n).
Proof. by rewrite -muln_gt0 ltn_divRL ?dvdn_mull ?dvdn_gcdr. Qed.

Lemma muln_lcmr : right_distributive muln lcmn.
Proof.
case=> // m n p; rewrite /lcmn -muln_gcdr -!mulnA divnMl // mulnCA.
by rewrite muln_divA ?dvdn_mull ?dvdn_gcdr.
Qed.

Lemma muln_lcml : left_distributive muln lcmn.
Proof. by move=> m n p; rewrite -!(mulnC p) muln_lcmr. Qed.

Lemma lcmnA : associative lcmn.
Proof.
move=> m n p; rewrite [LHS]/lcmn [RHS]/lcmn mulnC.
rewrite !divn_mulAC ?dvdn_mull ?dvdn_gcdr // -!divnMA ?dvdn_mulr ?dvdn_gcdl //.
rewrite mulnC mulnA !muln_gcdr; congr (_ %/ _).
by rewrite ![_ * lcmn _ _]mulnC !muln_lcm_gcd !muln_gcdl -!(mulnC m) gcdnA.
Qed.

Lemma lcmnCA : left_commutative lcmn.
Proof. by move=> m n p; rewrite !lcmnA (lcmnC m). Qed.

Lemma lcmnAC : right_commutative lcmn.
Proof. by move=> m n p; rewrite -!lcmnA (lcmnC n). Qed.

Lemma lcmnACA : interchange lcmn lcmn.
Proof. by move=> m n p q; rewrite -!lcmnA (lcmnCA n). Qed.

Lemma dvdn_lcml d1 d2 : d1 %| lcmn d1 d2.
Proof. by rewrite /lcmn -muln_divA ?dvdn_gcdr ?dvdn_mulr. Qed.

Lemma dvdn_lcmr d1 d2 : d2 %| lcmn d1 d2.
Proof. by rewrite lcmnC dvdn_lcml. Qed.

Lemma dvdn_lcm d1 d2 m : lcmn d1 d2 %| m = (d1 %| m) && (d2 %| m).
Proof.
case: d1 d2 => [|d1] [|d2]; try by case: m => [|m]; rewrite ?lcmn0 ?andbF.
rewrite -(@dvdn_pmul2r (gcdn d1.+1 d2.+1)) ?gcdn_gt0 // muln_lcm_gcd.
by rewrite muln_gcdr dvdn_gcd {1}mulnC andbC !dvdn_pmul2r.
Qed.

Lemma lcmnMl m n : lcmn m (m * n) = m * n.
Proof. by case: m => // m; rewrite /lcmn gcdnMr mulKn. Qed.

Lemma lcmnMr m n : lcmn n (m * n) = m * n.
Proof. by rewrite mulnC lcmnMl. Qed.

Lemma lcmn_idPr {m n} : reflect (lcmn m n = n) (m %| n).
Proof.
by apply: (iffP idP) => [/dvdnP[q ->] | <-]; rewrite (lcmnMr, dvdn_lcml).
Qed.

Lemma lcmn_idPl {m n} : reflect (lcmn m n = m) (n %| m).
Proof. by rewrite lcmnC; apply: lcmn_idPr. Qed.

Lemma expn_max e m n : e ^ maxn m n = lcmn (e ^ m) (e ^ n).
Proof. by case: leqP => [|/ltnW] /(dvdn_exp2l e) /lcmn_idPl; rewrite lcmnC. Qed.

(* Coprime factors *)

Definition coprime m n := gcdn m n == 1.

Lemma coprime1n n : coprime 1 n.
Proof. by rewrite /coprime gcd1n. Qed.

Lemma coprimen1 n : coprime n 1.
Proof. by rewrite /coprime gcdn1. Qed.

Lemma coprime_sym m n : coprime m n = coprime n m.
Proof. by rewrite /coprime gcdnC. Qed.

Lemma coprime_modl m n : coprime (m %% n) n = coprime m n.
Proof. by rewrite /coprime gcdn_modl. Qed.

Lemma coprime_modr m n : coprime m (n %% m) = coprime m n.
Proof. by rewrite /coprime gcdn_modr. Qed.

Lemma coprime2n n : coprime 2 n = odd n.
Proof. by rewrite -coprime_modr modn2; case: (odd n). Qed.

Lemma coprimen2 n : coprime n 2 = odd n.
Proof. by rewrite coprime_sym coprime2n. Qed.

Lemma coprimeSn n : coprime n.+1 n.
Proof. by rewrite -coprime_modl (modnDr 1) coprime_modl coprime1n. Qed.

Lemma coprimenS n : coprime n n.+1.
Proof. by rewrite coprime_sym coprimeSn. Qed.

Lemma coprimePn n : n > 0 -> coprime n.-1 n.
Proof. by case: n => // n _; rewrite coprimenS. Qed.

Lemma coprimenP n : n > 0 -> coprime n n.-1.
Proof. by case: n => // n _; rewrite coprimeSn. Qed.

Lemma coprimeP n m :
  n > 0 -> reflect (exists u, u.1 * n - u.2 * m = 1) (coprime n m).
Proof.
move=> n_gt0; apply: (iffP eqP) => [<-| [[kn km] /= kn_km_1]].
  by have [kn km kg _] := egcdnP m n_gt0; exists (kn, km); rewrite kg addKn.
apply gcdn_def; rewrite ?dvd1n // => d dv_d_n dv_d_m.
by rewrite -kn_km_1 dvdn_subr ?dvdn_mull // ltnW // -subn_gt0 kn_km_1.
Qed.

Lemma modn_coprime k n : 0 < k -> (exists u, (k * u) %% n = 1) -> coprime k n.
Proof.
move=> k_gt0 [u Hu]; apply/coprimeP=> //.
by exists (u, k * u %/ n); rewrite /= mulnC {1}(divn_eq (k * u) n) addKn.
Qed.

Lemma Gauss_dvd m n p : coprime m n -> (m * n %| p) = (m %| p) && (n %| p).
Proof. by move=> co_mn; rewrite -muln_lcm_gcd (eqnP co_mn) muln1 dvdn_lcm. Qed.

Lemma Gauss_dvdr m n p : coprime m n -> (m %| n * p) = (m %| p).
Proof.
case: n => [|n] co_mn; first by case: m co_mn => [|[]] // _; rewrite !dvd1n.
by symmetry; rewrite mulnC -(@dvdn_pmul2r n.+1) ?Gauss_dvd // andbC dvdn_mull.
Qed.

Lemma Gauss_dvdl m n p : coprime m p -> (m %| n * p) = (m %| n).
Proof. by rewrite mulnC; apply: Gauss_dvdr. Qed.

Lemma dvdn_double_leq m n : m %| n -> odd m -> ~~ odd n -> 0 < n -> m.*2 <= n.
Proof.
move=> m_dv_n odd_m even_n n_gt0.
by rewrite -muln2 dvdn_leq // Gauss_dvd ?coprimen2 ?m_dv_n ?dvdn2.
Qed.

Lemma dvdn_double_ltn m n : m %| n.-1 -> odd m -> odd n -> 1 < n -> m.*2 < n.
Proof. by case: n => //; apply: dvdn_double_leq. Qed.

Lemma Gauss_gcdr p m n : coprime p m -> gcdn p (m * n) = gcdn p n.
Proof.
move=> co_pm; apply/eqP; rewrite eqn_dvd !dvdn_gcd !dvdn_gcdl /=.
rewrite andbC dvdn_mull ?dvdn_gcdr //= -(@Gauss_dvdr _ m) ?dvdn_gcdr //.
by rewrite /coprime gcdnAC (eqnP co_pm) gcd1n.
Qed.

Lemma Gauss_gcdl p m n : coprime p n -> gcdn p (m * n) = gcdn p m.
Proof. by move=> co_pn; rewrite mulnC Gauss_gcdr. Qed.

Lemma coprimeMr p m n : coprime p (m * n) = coprime p m && coprime p n.
Proof.
case co_pm: (coprime p m) => /=; first by rewrite /coprime Gauss_gcdr.
apply/eqP=> co_p_mn; case/eqnP: co_pm; apply gcdn_def => // d dv_dp dv_dm.
by rewrite -co_p_mn dvdn_gcd dv_dp dvdn_mulr.
Qed.

Lemma coprimeMl p m n : coprime (m * n) p = coprime m p && coprime n p.
Proof. by rewrite -!(coprime_sym p) coprimeMr. Qed.

Lemma coprime_pexpl k m n : 0 < k -> coprime (m ^ k) n = coprime m n.
Proof.
case: k => // k _; elim: k => [|k IHk]; first by rewrite expn1.
by rewrite expnS coprimeMl -IHk; case coprime.
Qed.

Lemma coprime_pexpr k m n : 0 < k -> coprime m (n ^ k) = coprime m n.
Proof. by move=> k_gt0; rewrite !(coprime_sym m) coprime_pexpl. Qed.

Lemma coprimeXl k m n : coprime m n -> coprime (m ^ k) n.
Proof. by case: k => [|k] co_pm; rewrite ?coprime1n // coprime_pexpl. Qed.

Lemma coprimeXr k m n : coprime m n -> coprime m (n ^ k).
Proof. by rewrite !(coprime_sym m); apply: coprimeXl. Qed.

Lemma coprime_dvdl m n p : m %| n -> coprime n p -> coprime m p.
Proof. by case/dvdnP=> d ->; rewrite coprimeMl => /andP[]. Qed.

Lemma coprime_dvdr m n p : m %| n -> coprime p n -> coprime p m.
Proof. by rewrite !(coprime_sym p); apply: coprime_dvdl. Qed.

Lemma coprime_egcdn n m : n > 0 -> coprime (egcdn n m).1 (egcdn n m).2.
Proof.
move=> n_gt0; case: (egcdnP m n_gt0) => kn km /= /eqP.
have [/dvdnP[u defn] /dvdnP[v defm]] := (dvdn_gcdl n m, dvdn_gcdr n m).
rewrite -[gcdn n m]mul1n {1}defm {1}defn !mulnA -mulnDl addnC.
rewrite eqn_pmul2r ?gcdn_gt0 ?n_gt0 //; case: kn => // kn /eqP def_knu _.
by apply/coprimeP=> //; exists (u, v); rewrite mulnC def_knu mulnC addnK.
Qed.

Lemma dvdn_pexp2r m n k : k > 0 -> (m ^ k %| n ^ k) = (m %| n).
Proof.
move=> k_gt0; apply/idP/idP=> [dv_mn_k|]; last exact: dvdn_exp2r.
have [->|n_gt0] := posnP n; first by rewrite dvdn0.
have [n' def_n] := dvdnP (dvdn_gcdr m n); set d := gcdn m n in def_n.
have [m' def_m] := dvdnP (dvdn_gcdl m n); rewrite -/d in def_m.
have d_gt0: d > 0 by rewrite gcdn_gt0 n_gt0 orbT.
rewrite def_m def_n !expnMn dvdn_pmul2r ?expn_gt0 ?d_gt0 // in dv_mn_k.
have: coprime (m' ^ k) (n' ^ k).
  rewrite coprime_pexpl // coprime_pexpr // /coprime -(eqn_pmul2r d_gt0) mul1n.
  by rewrite muln_gcdl -def_m -def_n.
rewrite /coprime -gcdn_modr (eqnP dv_mn_k) gcdn0 -(exp1n k).
by rewrite (inj_eq (expIn k_gt0)) def_m; move/eqP->; rewrite mul1n dvdn_gcdr.
Qed.

Section Chinese.

(***********************************************************************)
(*   The chinese remainder theorem                                     *)
(***********************************************************************)

Variables m1 m2 : nat.
Hypothesis co_m12 : coprime m1 m2.

Lemma chinese_remainder x y :
  (x == y %[mod m1 * m2]) = (x == y %[mod m1]) && (x == y %[mod m2]).
Proof.
wlog le_yx : x y / y <= x; last by rewrite !eqn_mod_dvd // Gauss_dvd.
by have [?|/ltnW ?] := leqP y x; last rewrite !(eq_sym (x %% _)); apply.
Qed.

(***********************************************************************)
(*   A function that solves the chinese remainder problem              *)
(***********************************************************************)

Definition chinese r1 r2 :=
  r1 * m2 * (egcdn m2 m1).1 + r2 * m1 * (egcdn m1 m2).1.

Lemma chinese_modl r1 r2 : chinese r1 r2 = r1 %[mod m1].
Proof.
rewrite /chinese; case: (posnP m2) co_m12 => [-> /eqnP | m2_gt0 _].
  by rewrite gcdn0 => ->; rewrite !modn1.
case: egcdnP => // k2 k1 def_m1 _.
rewrite mulnAC -mulnA def_m1 gcdnC (eqnP co_m12) mulnDr mulnA muln1.
by rewrite addnAC (mulnAC _ m1) -mulnDl modnMDl.
Qed.

Lemma chinese_modr r1 r2 : chinese r1 r2 = r2 %[mod m2].
Proof.
rewrite /chinese; case: (posnP m1) co_m12 => [-> /eqnP | m1_gt0 _].
  by rewrite gcd0n => ->; rewrite !modn1.
case: (egcdnP m2) => // k1 k2 def_m2 _.
rewrite addnC mulnAC -mulnA def_m2 (eqnP co_m12) mulnDr mulnA muln1.
by rewrite addnAC (mulnAC _ m2) -mulnDl modnMDl.
Qed.

Lemma chinese_mod x : x = chinese (x %% m1) (x %% m2) %[mod m1 * m2].
Proof.
apply/eqP; rewrite chinese_remainder //.
by rewrite chinese_modl chinese_modr !modn_mod !eqxx.
Qed.

End Chinese.