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diff --git a/theories/Numbers/Integer/Abstract/ZDivFloor.v b/theories/Numbers/Integer/Abstract/ZDivFloor.v new file mode 100644 index 00000000..1e7624ba --- /dev/null +++ b/theories/Numbers/Integer/Abstract/ZDivFloor.v @@ -0,0 +1,632 @@ +(************************************************************************) +(* v * The Coq Proof Assistant / The Coq Development Team *) +(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *) +(* \VV/ **************************************************************) +(* // * This file is distributed under the terms of the *) +(* * GNU Lesser General Public License Version 2.1 *) +(************************************************************************) + +(** * Euclidean Division for integers (Floor convention) + + We use here the convention known as Floor, or Round-Toward-Bottom, + where [a/b] is the closest integer below the exact fraction. + It can be summarized by: + + [a = bq+r /\ 0 <= |r| < |b| /\ Sign(r) = Sign(b)] + + This is the convention followed historically by [Zdiv] in Coq, and + corresponds to convention "F" in the following paper: + + R. Boute, "The Euclidean definition of the functions div and mod", + ACM Transactions on Programming Languages and Systems, + Vol. 14, No.2, pp. 127-144, April 1992. + + See files [ZDivTrunc] and [ZDivEucl] for others conventions. +*) + +Require Import ZAxioms ZProperties NZDiv. + +Module Type ZDivSpecific (Import Z:ZAxiomsSig')(Import DM : DivMod' Z). + Axiom mod_pos_bound : forall a b, 0 < b -> 0 <= a mod b < b. + Axiom mod_neg_bound : forall a b, b < 0 -> b < a mod b <= 0. +End ZDivSpecific. + +Module Type ZDiv (Z:ZAxiomsSig) + := DivMod Z <+ NZDivCommon Z <+ ZDivSpecific Z. + +Module Type ZDivSig := ZAxiomsExtSig <+ ZDiv. +Module Type ZDivSig' := ZAxiomsExtSig' <+ ZDiv <+ DivModNotation. + +Module ZDivPropFunct (Import Z : ZDivSig')(Import ZP : ZPropSig Z). + +(** We benefit from what already exists for NZ *) + + Module ZD <: NZDiv Z. + Definition div := div. + Definition modulo := modulo. + Definition div_wd := div_wd. + Definition mod_wd := mod_wd. + Definition div_mod := div_mod. + Lemma mod_bound : forall a b, 0<=a -> 0<b -> 0 <= a mod b < b. + Proof. intros. now apply mod_pos_bound. Qed. + End ZD. + Module Import NZDivP := NZDivPropFunct Z ZP ZD. + +(** Another formulation of the main equation *) + +Lemma mod_eq : + forall a b, b~=0 -> a mod b == a - b*(a/b). +Proof. +intros. +rewrite <- add_move_l. +symmetry. now apply div_mod. +Qed. + +(** Uniqueness theorems *) + +Theorem div_mod_unique : forall b q1 q2 r1 r2 : t, + (0<=r1<b \/ b<r1<=0) -> (0<=r2<b \/ b<r2<=0) -> + b*q1+r1 == b*q2+r2 -> q1 == q2 /\ r1 == r2. +Proof. +intros b q1 q2 r1 r2 Hr1 Hr2 EQ. +destruct Hr1; destruct Hr2; try (intuition; order). +apply div_mod_unique with b; trivial. +rewrite <- (opp_inj_wd r1 r2). +apply div_mod_unique with (-b); trivial. +rewrite <- opp_lt_mono, opp_nonneg_nonpos; tauto. +rewrite <- opp_lt_mono, opp_nonneg_nonpos; tauto. +now rewrite 2 mul_opp_l, <- 2 opp_add_distr, opp_inj_wd. +Qed. + +Theorem div_unique: + forall a b q r, (0<=r<b \/ b<r<=0) -> a == b*q + r -> q == a/b. +Proof. +intros a b q r Hr EQ. +assert (Hb : b~=0) by (destruct Hr; intuition; order). +destruct (div_mod_unique b q (a/b) r (a mod b)); trivial. +destruct Hr; [left; apply mod_pos_bound|right; apply mod_neg_bound]; + intuition order. +now rewrite <- div_mod. +Qed. + +Theorem div_unique_pos: + forall a b q r, 0<=r<b -> a == b*q + r -> q == a/b. +Proof. intros; apply div_unique with r; auto. Qed. + +Theorem div_unique_neg: + forall a b q r, 0<=r<b -> a == b*q + r -> q == a/b. +Proof. intros; apply div_unique with r; auto. Qed. + +Theorem mod_unique: + forall a b q r, (0<=r<b \/ b<r<=0) -> a == b*q + r -> r == a mod b. +Proof. +intros a b q r Hr EQ. +assert (Hb : b~=0) by (destruct Hr; intuition; order). +destruct (div_mod_unique b q (a/b) r (a mod b)); trivial. +destruct Hr; [left; apply mod_pos_bound|right; apply mod_neg_bound]; + intuition order. +now rewrite <- div_mod. +Qed. + +Theorem mod_unique_pos: + forall a b q r, 0<=r<b -> a == b*q + r -> r == a mod b. +Proof. intros; apply mod_unique with q; auto. Qed. + +Theorem mod_unique_neg: + forall a b q r, b<r<=0 -> a == b*q + r -> r == a mod b. +Proof. intros; apply mod_unique with q; auto. Qed. + +(** Sign rules *) + +Ltac pos_or_neg a := + let LT := fresh "LT" in + let LE := fresh "LE" in + destruct (le_gt_cases 0 a) as [LE|LT]; [|rewrite <- opp_pos_neg in LT]. + +Fact mod_bound_or : forall a b, b~=0 -> 0<=a mod b<b \/ b<a mod b<=0. +Proof. +intros. +destruct (lt_ge_cases 0 b); [left|right]. + apply mod_pos_bound; trivial. apply mod_neg_bound; order. +Qed. + +Fact opp_mod_bound_or : forall a b, b~=0 -> + 0 <= -(a mod b) < -b \/ -b < -(a mod b) <= 0. +Proof. +intros. +destruct (lt_ge_cases 0 b); [right|left]. +rewrite <- opp_lt_mono, opp_nonpos_nonneg. + destruct (mod_pos_bound a b); intuition; order. +rewrite <- opp_lt_mono, opp_nonneg_nonpos. + destruct (mod_neg_bound a b); intuition; order. +Qed. + +Lemma div_opp_opp : forall a b, b~=0 -> -a/-b == a/b. +Proof. +intros. symmetry. apply div_unique with (- (a mod b)). +now apply opp_mod_bound_or. +rewrite mul_opp_l, <- opp_add_distr, <- div_mod; order. +Qed. + +Lemma mod_opp_opp : forall a b, b~=0 -> (-a) mod (-b) == - (a mod b). +Proof. +intros. symmetry. apply mod_unique with (a/b). +now apply opp_mod_bound_or. +rewrite mul_opp_l, <- opp_add_distr, <- div_mod; order. +Qed. + +(** With the current conventions, the other sign rules are rather complex. *) + +Lemma div_opp_l_z : + forall a b, b~=0 -> a mod b == 0 -> (-a)/b == -(a/b). +Proof. +intros a b Hb H. symmetry. apply div_unique with 0. +destruct (lt_ge_cases 0 b); [left|right]; intuition; order. +rewrite <- opp_0, <- H. +rewrite mul_opp_r, <- opp_add_distr, <- div_mod; order. +Qed. + +Lemma div_opp_l_nz : + forall a b, b~=0 -> a mod b ~= 0 -> (-a)/b == -(a/b)-1. +Proof. +intros a b Hb H. symmetry. apply div_unique with (b - a mod b). +destruct (lt_ge_cases 0 b); [left|right]. +rewrite le_0_sub. rewrite <- (sub_0_r b) at 5. rewrite <- sub_lt_mono_l. +destruct (mod_pos_bound a b); intuition; order. +rewrite le_sub_0. rewrite <- (sub_0_r b) at 1. rewrite <- sub_lt_mono_l. +destruct (mod_neg_bound a b); intuition; order. +rewrite <- (add_opp_r b), mul_sub_distr_l, mul_1_r, sub_add_simpl_r_l. +rewrite mul_opp_r, <-opp_add_distr, <-div_mod; order. +Qed. + +Lemma mod_opp_l_z : + forall a b, b~=0 -> a mod b == 0 -> (-a) mod b == 0. +Proof. +intros a b Hb H. symmetry. apply mod_unique with (-(a/b)). +destruct (lt_ge_cases 0 b); [left|right]; intuition; order. +rewrite <- opp_0, <- H. +rewrite mul_opp_r, <- opp_add_distr, <- div_mod; order. +Qed. + +Lemma mod_opp_l_nz : + forall a b, b~=0 -> a mod b ~= 0 -> (-a) mod b == b - a mod b. +Proof. +intros a b Hb H. symmetry. apply mod_unique with (-(a/b)-1). +destruct (lt_ge_cases 0 b); [left|right]. +rewrite le_0_sub. rewrite <- (sub_0_r b) at 5. rewrite <- sub_lt_mono_l. +destruct (mod_pos_bound a b); intuition; order. +rewrite le_sub_0. rewrite <- (sub_0_r b) at 1. rewrite <- sub_lt_mono_l. +destruct (mod_neg_bound a b); intuition; order. +rewrite <- (add_opp_r b), mul_sub_distr_l, mul_1_r, sub_add_simpl_r_l. +rewrite mul_opp_r, <-opp_add_distr, <-div_mod; order. +Qed. + +Lemma div_opp_r_z : + forall a b, b~=0 -> a mod b == 0 -> a/(-b) == -(a/b). +Proof. +intros. rewrite <- (opp_involutive a) at 1. +rewrite div_opp_opp; auto using div_opp_l_z. +Qed. + +Lemma div_opp_r_nz : + forall a b, b~=0 -> a mod b ~= 0 -> a/(-b) == -(a/b)-1. +Proof. +intros. rewrite <- (opp_involutive a) at 1. +rewrite div_opp_opp; auto using div_opp_l_nz. +Qed. + +Lemma mod_opp_r_z : + forall a b, b~=0 -> a mod b == 0 -> a mod (-b) == 0. +Proof. +intros. rewrite <- (opp_involutive a) at 1. +now rewrite mod_opp_opp, mod_opp_l_z, opp_0. +Qed. + +Lemma mod_opp_r_nz : + forall a b, b~=0 -> a mod b ~= 0 -> a mod (-b) == (a mod b) - b. +Proof. +intros. rewrite <- (opp_involutive a) at 1. +rewrite mod_opp_opp, mod_opp_l_nz by trivial. +now rewrite opp_sub_distr, add_comm, add_opp_r. +Qed. + +(** The sign of [a mod b] is the one of [b] *) + +(* TODO: a proper sgn function and theory *) + +Lemma mod_sign : forall a b, b~=0 -> (0 <= (a mod b) * b). +Proof. +intros. destruct (lt_ge_cases 0 b). +apply mul_nonneg_nonneg; destruct (mod_pos_bound a b); order. +apply mul_nonpos_nonpos; destruct (mod_neg_bound a b); order. +Qed. + +(** A division by itself returns 1 *) + +Lemma div_same : forall a, a~=0 -> a/a == 1. +Proof. +intros. pos_or_neg a. apply div_same; order. +rewrite <- div_opp_opp by trivial. now apply div_same. +Qed. + +Lemma mod_same : forall a, a~=0 -> a mod a == 0. +Proof. +intros. rewrite mod_eq, div_same by trivial. nzsimpl. apply sub_diag. +Qed. + +(** A division of a small number by a bigger one yields zero. *) + +Theorem div_small: forall a b, 0<=a<b -> a/b == 0. +Proof. exact div_small. Qed. + +(** Same situation, in term of modulo: *) + +Theorem mod_small: forall a b, 0<=a<b -> a mod b == a. +Proof. exact mod_small. Qed. + +(** * Basic values of divisions and modulo. *) + +Lemma div_0_l: forall a, a~=0 -> 0/a == 0. +Proof. +intros. pos_or_neg a. apply div_0_l; order. +rewrite <- div_opp_opp, opp_0 by trivial. now apply div_0_l. +Qed. + +Lemma mod_0_l: forall a, a~=0 -> 0 mod a == 0. +Proof. +intros; rewrite mod_eq, div_0_l; now nzsimpl. +Qed. + +Lemma div_1_r: forall a, a/1 == a. +Proof. +intros. symmetry. apply div_unique with 0. left. split; order || apply lt_0_1. +now nzsimpl. +Qed. + +Lemma mod_1_r: forall a, a mod 1 == 0. +Proof. +intros. rewrite mod_eq, div_1_r; nzsimpl; auto using sub_diag. +intro EQ; symmetry in EQ; revert EQ; apply lt_neq; apply lt_0_1. +Qed. + +Lemma div_1_l: forall a, 1<a -> 1/a == 0. +Proof. exact div_1_l. Qed. + +Lemma mod_1_l: forall a, 1<a -> 1 mod a == 1. +Proof. exact mod_1_l. Qed. + +Lemma div_mul : forall a b, b~=0 -> (a*b)/b == a. +Proof. +intros. symmetry. apply div_unique with 0. +destruct (lt_ge_cases 0 b); [left|right]; split; order. +nzsimpl; apply mul_comm. +Qed. + +Lemma mod_mul : forall a b, b~=0 -> (a*b) mod b == 0. +Proof. +intros. rewrite mod_eq, div_mul by trivial. rewrite mul_comm; apply sub_diag. +Qed. + +(** * Order results about mod and div *) + +(** A modulo cannot grow beyond its starting point. *) + +Theorem mod_le: forall a b, 0<=a -> 0<b -> a mod b <= a. +Proof. exact mod_le. Qed. + +Theorem div_pos : forall a b, 0<=a -> 0<b -> 0<= a/b. +Proof. exact div_pos. Qed. + +Lemma div_str_pos : forall a b, 0<b<=a -> 0 < a/b. +Proof. exact div_str_pos. Qed. + +Lemma div_small_iff : forall a b, b~=0 -> (a/b==0 <-> 0<=a<b \/ b<a<=0). +Proof. +intros a b Hb. +split. +intros EQ. +rewrite (div_mod a b Hb), EQ; nzsimpl. +now apply mod_bound_or. +destruct 1. now apply div_small. +rewrite <- div_opp_opp by trivial. apply div_small; trivial. +rewrite <- opp_lt_mono, opp_nonneg_nonpos; tauto. +Qed. + +Lemma mod_small_iff : forall a b, b~=0 -> (a mod b == a <-> 0<=a<b \/ b<a<=0). +Proof. +intros. +rewrite <- div_small_iff, mod_eq by trivial. +rewrite sub_move_r, <- (add_0_r a) at 1. rewrite add_cancel_l. +rewrite eq_sym_iff, eq_mul_0. tauto. +Qed. + +(** As soon as the divisor is strictly greater than 1, + the division is strictly decreasing. *) + +Lemma div_lt : forall a b, 0<a -> 1<b -> a/b < a. +Proof. exact div_lt. Qed. + +(** [le] is compatible with a positive division. *) + +Lemma div_le_mono : forall a b c, 0<c -> a<=b -> a/c <= b/c. +Proof. +intros a b c Hc Hab. +rewrite lt_eq_cases in Hab. destruct Hab as [LT|EQ]; + [|rewrite EQ; order]. +rewrite <- lt_succ_r. +rewrite (mul_lt_mono_pos_l c) by order. +nzsimpl. +rewrite (add_lt_mono_r _ _ (a mod c)). +rewrite <- div_mod by order. +apply lt_le_trans with b; trivial. +rewrite (div_mod b c) at 1 by order. +rewrite <- add_assoc, <- add_le_mono_l. +apply le_trans with (c+0). +nzsimpl; destruct (mod_pos_bound b c); order. +rewrite <- add_le_mono_l. destruct (mod_pos_bound a c); order. +Qed. + +(** In this convention, [div] performs Rounding-Toward-Bottom. + + Since we cannot speak of rational values here, we express this + fact by multiplying back by [b], and this leads to separates + statements according to the sign of [b]. + + First, [a/b] is below the exact fraction ... +*) + +Lemma mul_div_le : forall a b, 0<b -> b*(a/b) <= a. +Proof. +intros. +rewrite (div_mod a b) at 2; try order. +rewrite <- (add_0_r (b*(a/b))) at 1. +rewrite <- add_le_mono_l. +now destruct (mod_pos_bound a b). +Qed. + +Lemma mul_div_ge : forall a b, b<0 -> a <= b*(a/b). +Proof. +intros. rewrite <- div_opp_opp, opp_le_mono, <-mul_opp_l by order. +apply mul_div_le. now rewrite opp_pos_neg. +Qed. + +(** ... and moreover it is the larger such integer, since [S(a/b)] + is strictly above the exact fraction. +*) + +Lemma mul_succ_div_gt: forall a b, 0<b -> a < b*(S (a/b)). +Proof. +intros. +nzsimpl. +rewrite (div_mod a b) at 1; try order. +rewrite <- add_lt_mono_l. +destruct (mod_pos_bound a b); order. +Qed. + +Lemma mul_succ_div_lt: forall a b, b<0 -> b*(S (a/b)) < a. +Proof. +intros. rewrite <- div_opp_opp, opp_lt_mono, <-mul_opp_l by order. +apply mul_succ_div_gt. now rewrite opp_pos_neg. +Qed. + +(** NB: The four previous properties could be used as + specifications for [div]. *) + +(** Inequality [mul_div_le] is exact iff the modulo is zero. *) + +Lemma div_exact : forall a b, b~=0 -> (a == b*(a/b) <-> a mod b == 0). +Proof. +intros. +rewrite (div_mod a b) at 1; try order. +rewrite <- (add_0_r (b*(a/b))) at 2. +apply add_cancel_l. +Qed. + +(** Some additionnal inequalities about div. *) + +Theorem div_lt_upper_bound: + forall a b q, 0<b -> a < b*q -> a/b < q. +Proof. +intros. +rewrite (mul_lt_mono_pos_l b) by trivial. +apply le_lt_trans with a; trivial. +now apply mul_div_le. +Qed. + +Theorem div_le_upper_bound: + forall a b q, 0<b -> a <= b*q -> a/b <= q. +Proof. +intros. +rewrite <- (div_mul q b) by order. +apply div_le_mono; trivial. now rewrite mul_comm. +Qed. + +Theorem div_le_lower_bound: + forall a b q, 0<b -> b*q <= a -> q <= a/b. +Proof. +intros. +rewrite <- (div_mul q b) by order. +apply div_le_mono; trivial. now rewrite mul_comm. +Qed. + +(** A division respects opposite monotonicity for the divisor *) + +Lemma div_le_compat_l: forall p q r, 0<=p -> 0<q<=r -> p/r <= p/q. +Proof. exact div_le_compat_l. Qed. + +(** * Relations between usual operations and mod and div *) + +Lemma mod_add : forall a b c, c~=0 -> + (a + b * c) mod c == a mod c. +Proof. +intros. +symmetry. +apply mod_unique with (a/c+b); trivial. +now apply mod_bound_or. +rewrite mul_add_distr_l, add_shuffle0, <- div_mod by order. +now rewrite mul_comm. +Qed. + +Lemma div_add : forall a b c, c~=0 -> + (a + b * c) / c == a / c + b. +Proof. +intros. +apply (mul_cancel_l _ _ c); try order. +apply (add_cancel_r _ _ ((a+b*c) mod c)). +rewrite <- div_mod, mod_add by order. +rewrite mul_add_distr_l, add_shuffle0, <- div_mod by order. +now rewrite mul_comm. +Qed. + +Lemma div_add_l: forall a b c, b~=0 -> + (a * b + c) / b == a + c / b. +Proof. + intros a b c. rewrite (add_comm _ c), (add_comm a). + now apply div_add. +Qed. + +(** Cancellations. *) + +Lemma div_mul_cancel_r : forall a b c, b~=0 -> c~=0 -> + (a*c)/(b*c) == a/b. +Proof. +intros. +symmetry. +apply div_unique with ((a mod b)*c). +(* ineqs *) +destruct (lt_ge_cases 0 c). +rewrite <-(mul_0_l c), <-2mul_lt_mono_pos_r, <-2mul_le_mono_pos_r by trivial. +now apply mod_bound_or. +rewrite <-(mul_0_l c), <-2mul_lt_mono_neg_r, <-2mul_le_mono_neg_r by order. +destruct (mod_bound_or a b); tauto. +(* equation *) +rewrite (div_mod a b) at 1 by order. +rewrite mul_add_distr_r. +rewrite add_cancel_r. +rewrite <- 2 mul_assoc. now rewrite (mul_comm c). +Qed. + +Lemma div_mul_cancel_l : forall a b c, b~=0 -> c~=0 -> + (c*a)/(c*b) == a/b. +Proof. +intros. rewrite !(mul_comm c); now apply div_mul_cancel_r. +Qed. + +Lemma mul_mod_distr_l: forall a b c, b~=0 -> c~=0 -> + (c*a) mod (c*b) == c * (a mod b). +Proof. +intros. +rewrite <- (add_cancel_l _ _ ((c*b)* ((c*a)/(c*b)))). +rewrite <- div_mod. +rewrite div_mul_cancel_l by trivial. +rewrite <- mul_assoc, <- mul_add_distr_l, mul_cancel_l by order. +apply div_mod; order. +rewrite <- neq_mul_0; auto. +Qed. + +Lemma mul_mod_distr_r: forall a b c, b~=0 -> c~=0 -> + (a*c) mod (b*c) == (a mod b) * c. +Proof. + intros. rewrite !(mul_comm _ c); now rewrite mul_mod_distr_l. +Qed. + + +(** Operations modulo. *) + +Theorem mod_mod: forall a n, n~=0 -> + (a mod n) mod n == a mod n. +Proof. +intros. rewrite mod_small_iff by trivial. +now apply mod_bound_or. +Qed. + +Lemma mul_mod_idemp_l : forall a b n, n~=0 -> + ((a mod n)*b) mod n == (a*b) mod n. +Proof. + intros a b n Hn. symmetry. + rewrite (div_mod a n) at 1 by order. + rewrite add_comm, (mul_comm n), (mul_comm _ b). + rewrite mul_add_distr_l, mul_assoc. + intros. rewrite mod_add by trivial. + now rewrite mul_comm. +Qed. + +Lemma mul_mod_idemp_r : forall a b n, n~=0 -> + (a*(b mod n)) mod n == (a*b) mod n. +Proof. + intros. rewrite !(mul_comm a). now apply mul_mod_idemp_l. +Qed. + +Theorem mul_mod: forall a b n, n~=0 -> + (a * b) mod n == ((a mod n) * (b mod n)) mod n. +Proof. + intros. now rewrite mul_mod_idemp_l, mul_mod_idemp_r. +Qed. + +Lemma add_mod_idemp_l : forall a b n, n~=0 -> + ((a mod n)+b) mod n == (a+b) mod n. +Proof. + intros a b n Hn. symmetry. + rewrite (div_mod a n) at 1 by order. + rewrite <- add_assoc, add_comm, mul_comm. + intros. now rewrite mod_add. +Qed. + +Lemma add_mod_idemp_r : forall a b n, n~=0 -> + (a+(b mod n)) mod n == (a+b) mod n. +Proof. + intros. rewrite !(add_comm a). now apply add_mod_idemp_l. +Qed. + +Theorem add_mod: forall a b n, n~=0 -> + (a+b) mod n == (a mod n + b mod n) mod n. +Proof. + intros. now rewrite add_mod_idemp_l, add_mod_idemp_r. +Qed. + +(** With the current convention, the following result isn't always + true for negative divisors. For instance + [ 3/(-2)/(-2) = 1 <> 0 = 3 / (-2*-2) ]. *) + +Lemma div_div : forall a b c, 0<b -> 0<c -> + (a/b)/c == a/(b*c). +Proof. + intros a b c Hb Hc. + apply div_unique with (b*((a/b) mod c) + a mod b). + (* begin 0<= ... <b*c \/ ... *) + left. + destruct (mod_pos_bound (a/b) c), (mod_pos_bound a b); trivial. + split. + apply add_nonneg_nonneg; trivial. + apply mul_nonneg_nonneg; order. + apply lt_le_trans with (b*((a/b) mod c) + b). + now rewrite <- add_lt_mono_l. + now rewrite <- mul_succ_r, <- mul_le_mono_pos_l, le_succ_l. + (* end 0<= ... < b*c \/ ... *) + rewrite (div_mod a b) at 1 by order. + rewrite add_assoc, add_cancel_r. + rewrite <- mul_assoc, <- mul_add_distr_l, mul_cancel_l by order. + apply div_mod; order. +Qed. + +(** A last inequality: *) + +Theorem div_mul_le: + forall a b c, 0<=a -> 0<b -> 0<=c -> c*(a/b) <= (c*a)/b. +Proof. exact div_mul_le. Qed. + +(** mod is related to divisibility *) + +Lemma mod_divides : forall a b, b~=0 -> + (a mod b == 0 <-> exists c, a == b*c). +Proof. +intros a b Hb. split. +intros Hab. exists (a/b). rewrite (div_mod a b Hb) at 1. + rewrite Hab. now nzsimpl. +intros (c,Hc). +rewrite Hc, mul_comm. +now apply mod_mul. +Qed. + +End ZDivPropFunct. + |