lean2/library/data/nat/div.lean
2015-09-30 17:36:32 -07:00

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/-
Copyright (c) 2014 Jeremy Avigad. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jeremy Avigad, Leonardo de Moura
Definitions and properties of div and mod. Much of the development follows Isabelle's library.
-/
import data.nat.sub
open eq.ops well_founded decidable prod
namespace nat
/- div -/
-- auxiliary lemma used to justify div
private definition div_rec_lemma {x y : nat} : 0 < y ∧ y ≤ x → x - y < x :=
and.rec (λ ypos ylex, sub_lt (lt_of_lt_of_le ypos ylex) ypos)
private definition div.F (x : nat) (f : Π x₁, x₁ < x → nat → nat) (y : nat) : nat :=
if H : 0 < y ∧ y ≤ x then f (x - y) (div_rec_lemma H) y + 1 else zero
definition divide := fix div.F
notation a div b := divide a b
theorem divide_def (x y : nat) : divide x y = if 0 < y ∧ y ≤ x then divide (x - y) y + 1 else 0 :=
congr_fun (fix_eq div.F x) y
theorem div_zero (a : ) : a div 0 = 0 :=
divide_def a 0 ⬝ if_neg (!not_and_of_not_left (lt.irrefl 0))
theorem div_eq_zero_of_lt {a b : } (h : a < b) : a div b = 0 :=
divide_def a b ⬝ if_neg (!not_and_of_not_right (not_le_of_gt h))
theorem zero_div (b : ) : 0 div b = 0 :=
divide_def 0 b ⬝ if_neg (and.rec not_le_of_gt)
theorem div_eq_succ_sub_div {a b : } (h₁ : b > 0) (h₂ : a ≥ b) : a div b = succ ((a - b) div b) :=
divide_def a b ⬝ if_pos (and.intro h₁ h₂)
theorem add_div_self (x : ) {z : } (H : z > 0) : (x + z) div z = succ (x div z) :=
calc
(x + z) div z = if 0 < z ∧ z ≤ x + z then (x + z - z) div z + 1 else 0 : !divide_def
... = (x + z - z) div z + 1 : if_pos (and.intro H (le_add_left z x))
... = succ (x div z) : {!add_sub_cancel}
theorem add_div_self_left {x : } (z : ) (H : x > 0) : (x + z) div x = succ (z div x) :=
!add.comm ▸ !add_div_self H
theorem add_mul_div_self {x y z : } (H : z > 0) : (x + y * z) div z = x div z + y :=
nat.induction_on y
(calc (x + zero * z) div z = (x + zero) div z : zero_mul
... = x div z : add_zero
... = x div z + zero : add_zero)
(take y,
assume IH : (x + y * z) div z = x div z + y, calc
(x + succ y * z) div z = (x + (y * z + z)) div z : succ_mul
... = (x + y * z + z) div z : add.assoc
... = succ ((x + y * z) div z) : !add_div_self H
... = succ (x div z + y) : IH)
theorem add_mul_div_self_left (x z : ) {y : } (H : y > 0) : (x + y * z) div y = x div y + z :=
!mul.comm ▸ add_mul_div_self H
theorem mul_div_cancel (m : ) {n : } (H : n > 0) : m * n div n = m :=
calc
m * n div n = (0 + m * n) div n : zero_add
... = 0 div n + m : add_mul_div_self H
... = 0 + m : zero_div
... = m : zero_add
theorem mul_div_cancel_left {m : } (n : ) (H : m > 0) : m * n div m = n :=
!mul.comm ▸ !mul_div_cancel H
/- mod -/
private definition mod.F (x : nat) (f : Π x₁, x₁ < x → nat → nat) (y : nat) : nat :=
if H : 0 < y ∧ y ≤ x then f (x - y) (div_rec_lemma H) y else x
definition modulo := fix mod.F
notation a mod b := modulo a b
notation a ≡ b `[mod `:100 c `]`:0 := a mod c = b mod c
theorem modulo_def (x y : nat) : modulo x y = if 0 < y ∧ y ≤ x then modulo (x - y) y else x :=
congr_fun (fix_eq mod.F x) y
theorem mod_zero (a : ) : a mod 0 = a :=
modulo_def a 0 ⬝ if_neg (!not_and_of_not_left (lt.irrefl 0))
theorem mod_eq_of_lt {a b : } (h : a < b) : a mod b = a :=
modulo_def a b ⬝ if_neg (!not_and_of_not_right (not_le_of_gt h))
theorem zero_mod (b : ) : 0 mod b = 0 :=
modulo_def 0 b ⬝ if_neg (λ h, and.rec_on h (λ l r, absurd (lt_of_lt_of_le l r) (lt.irrefl 0)))
theorem mod_eq_sub_mod {a b : } (h₁ : b > 0) (h₂ : a ≥ b) : a mod b = (a - b) mod b :=
modulo_def a b ⬝ if_pos (and.intro h₁ h₂)
theorem add_mod_self (x z : ) : (x + z) mod z = x mod z :=
by_cases_zero_pos z
(by rewrite add_zero)
(take z, assume H : z > 0,
calc
(x + z) mod z = if 0 < z ∧ z ≤ x + z then (x + z - z) mod z else _ : modulo_def
... = (x + z - z) mod z : if_pos (and.intro H (le_add_left z x))
... = x mod z : add_sub_cancel)
theorem add_mod_self_left (x z : ) : (x + z) mod x = z mod x :=
!add.comm ▸ !add_mod_self
theorem add_mul_mod_self (x y z : ) : (x + y * z) mod z = x mod z :=
nat.induction_on y
(calc (x + zero * z) mod z = (x + zero) mod z : zero_mul
... = x mod z : add_zero)
(take y,
assume IH : (x + y * z) mod z = x mod z,
calc
(x + succ y * z) mod z = (x + (y * z + z)) mod z : succ_mul
... = (x + y * z + z) mod z : add.assoc
... = (x + y * z) mod z : !add_mod_self
... = x mod z : IH)
theorem add_mul_mod_self_left (x y z : ) : (x + y * z) mod y = x mod y :=
!mul.comm ▸ !add_mul_mod_self
theorem mul_mod_left (m n : ) : (m * n) mod n = 0 :=
by rewrite [-zero_add (m * n), add_mul_mod_self, zero_mod]
theorem mul_mod_right (m n : ) : (m * n) mod m = 0 :=
!mul.comm ▸ !mul_mod_left
theorem mod_lt (x : ) {y : } (H : y > 0) : x mod y < y :=
nat.case_strong_induction_on x
(show 0 mod y < y, from !zero_mod⁻¹ ▸ H)
(take x,
assume IH : ∀x', x' ≤ x → x' mod y < y,
show succ x mod y < y, from
by_cases -- (succ x < y)
(assume H1 : succ x < y,
have succ x mod y = succ x, from mod_eq_of_lt H1,
show succ x mod y < y, from this⁻¹ ▸ H1)
(assume H1 : ¬ succ x < y,
have y ≤ succ x, from le_of_not_gt H1,
have h : succ x mod y = (succ x - y) mod y, from mod_eq_sub_mod H this,
have succ x - y < succ x, from sub_lt !succ_pos H,
have succ x - y ≤ x, from le_of_lt_succ this,
show succ x mod y < y, from h⁻¹ ▸ IH _ this))
theorem mod_one (n : ) : n mod 1 = 0 :=
have H1 : n mod 1 < 1, from !mod_lt !succ_pos,
eq_zero_of_le_zero (le_of_lt_succ H1)
/- properties of div and mod -/
-- the quotient / remainder theorem
theorem eq_div_mul_add_mod (x y : ) : x = x div y * y + x mod y :=
by_cases_zero_pos y
(show x = x div 0 * 0 + x mod 0, from
(calc
x div 0 * 0 + x mod 0 = 0 + x mod 0 : mul_zero
... = x mod 0 : zero_add
... = x : mod_zero)⁻¹)
(take y,
assume H : y > 0,
show x = x div y * y + x mod y, from
nat.case_strong_induction_on x
(show 0 = (0 div y) * y + 0 mod y, by rewrite [zero_mod, add_zero, zero_div, zero_mul])
(take x,
assume IH : ∀x', x' ≤ x → x' = x' div y * y + x' mod y,
show succ x = succ x div y * y + succ x mod y, from
if H1 : succ x < y then
have H2 : succ x div y = 0, from div_eq_zero_of_lt H1,
have H3 : succ x mod y = succ x, from mod_eq_of_lt H1,
H2⁻¹ ▸ H3⁻¹ ▸ !zero_mul⁻¹ ▸ !zero_add⁻¹
else
have H2 : y ≤ succ x, from le_of_not_gt H1,
have H3 : succ x div y = succ ((succ x - y) div y), from div_eq_succ_sub_div H H2,
have H4 : succ x mod y = (succ x - y) mod y, from mod_eq_sub_mod H H2,
have H5 : succ x - y < succ x, from sub_lt !succ_pos H,
have H6 : succ x - y ≤ x, from le_of_lt_succ H5,
(calc
succ x div y * y + succ x mod y =
succ ((succ x - y) div y) * y + succ x mod y : H3
... = ((succ x - y) div y) * y + y + succ x mod y : succ_mul
... = ((succ x - y) div y) * y + y + (succ x - y) mod y : H4
... = ((succ x - y) div y) * y + (succ x - y) mod y + y : add.right_comm
... = succ x - y + y : {!(IH _ H6)⁻¹}
... = succ x : sub_add_cancel H2)⁻¹))
theorem mod_eq_sub_div_mul (x y : ) : x mod y = x - x div y * y :=
eq_sub_of_add_eq (!add.comm ▸ !eq_div_mul_add_mod)⁻¹
theorem mod_add_mod (m n k : ) : (m mod n + k) mod n = (m + k) mod n :=
by rewrite [eq_div_mul_add_mod m n at {2}, add.assoc, add.comm (m div n * n), add_mul_mod_self]
theorem add_mod_mod (m n k : ) : (m + n mod k) mod k = (m + n) mod k :=
by rewrite [add.comm, mod_add_mod, add.comm]
theorem add_mod_eq_add_mod_right {m n k : } (i : ) (H : m mod n = k mod n) :
(m + i) mod n = (k + i) mod n :=
by rewrite [-mod_add_mod, -mod_add_mod k, H]
theorem add_mod_eq_add_mod_left {m n k : } (i : ) (H : m mod n = k mod n) :
(i + m) mod n = (i + k) mod n :=
by rewrite [add.comm, add_mod_eq_add_mod_right _ H, add.comm]
theorem mod_eq_mod_of_add_mod_eq_add_mod_right {m n k i : } :
(m + i) mod n = (k + i) mod n → m mod n = k mod n :=
by_cases_zero_pos n
(by rewrite [*mod_zero]; apply eq_of_add_eq_add_right)
(take n,
assume npos : n > 0,
assume H1 : (m + i) mod n = (k + i) mod n,
have H2 : (m + i mod n) mod n = (k + i mod n) mod n, by rewrite [*add_mod_mod, H1],
assert H3 : (m + i mod n + (n - i mod n)) mod n = (k + i mod n + (n - i mod n)) mod n,
from add_mod_eq_add_mod_right _ H2,
begin
revert H3,
rewrite [*add.assoc, add_sub_of_le (le_of_lt (!mod_lt npos)), *add_mod_self],
intros, assumption
end)
theorem mod_eq_mod_of_add_mod_eq_add_mod_left {m n k i : } :
(i + m) mod n = (i + k) mod n → m mod n = k mod n :=
by rewrite [add.comm i m, add.comm i k]; apply mod_eq_mod_of_add_mod_eq_add_mod_right
theorem mod_le {x y : } : x mod y ≤ x :=
!eq_div_mul_add_mod⁻¹ ▸ !le_add_left
theorem eq_remainder {q1 r1 q2 r2 y : } (H1 : r1 < y) (H2 : r2 < y)
(H3 : q1 * y + r1 = q2 * y + r2) : r1 = r2 :=
calc
r1 = r1 mod y : mod_eq_of_lt H1
... = (r1 + q1 * y) mod y : !add_mul_mod_self⁻¹
... = (q1 * y + r1) mod y : add.comm
... = (r2 + q2 * y) mod y : by rewrite [H3, add.comm]
... = r2 mod y : !add_mul_mod_self
... = r2 : mod_eq_of_lt H2
theorem eq_quotient {q1 r1 q2 r2 y : } (H1 : r1 < y) (H2 : r2 < y)
(H3 : q1 * y + r1 = q2 * y + r2) : q1 = q2 :=
have H4 : q1 * y + r2 = q2 * y + r2, from (eq_remainder H1 H2 H3) ▸ H3,
have H5 : q1 * y = q2 * y, from add.cancel_right H4,
have H6 : y > 0, from lt_of_le_of_lt !zero_le H1,
show q1 = q2, from eq_of_mul_eq_mul_right H6 H5
theorem mul_div_mul_left {z : } (x y : ) (zpos : z > 0) : (z * x) div (z * y) = x div y :=
if H : y = 0 then H⁻¹ ▸ !mul_zero⁻¹ ▸ !div_zero⁻¹ ▸ !div_zero
else
have ypos : y > 0, from pos_of_ne_zero H,
have zypos : z * y > 0, from mul_pos zpos ypos,
have H1 : (z * x) mod (z * y) < z * y, from !mod_lt zypos,
have H2 : z * (x mod y) < z * y, from mul_lt_mul_of_pos_left (!mod_lt ypos) zpos,
eq_quotient H1 H2
(calc
((z * x) div (z * y)) * (z * y) + (z * x) mod (z * y) = z * x : eq_div_mul_add_mod
... = z * (x div y * y + x mod y) : eq_div_mul_add_mod
... = z * (x div y * y) + z * (x mod y) : mul.left_distrib
... = (x div y) * (z * y) + z * (x mod y) : mul.left_comm)
theorem mul_div_mul_right {x z y : } (zpos : z > 0) : (x * z) div (y * z) = x div y :=
!mul.comm ▸ !mul.comm ▸ !mul_div_mul_left zpos
theorem mul_mod_mul_left (z x y : ) : (z * x) mod (z * y) = z * (x mod y) :=
or.elim (eq_zero_or_pos z)
(assume H : z = 0, H⁻¹ ▸ calc
(0 * x) mod (z * y) = 0 mod (z * y) : zero_mul
... = 0 : zero_mod
... = 0 * (x mod y) : zero_mul)
(assume zpos : z > 0,
or.elim (eq_zero_or_pos y)
(assume H : y = 0, by rewrite [H, mul_zero, *mod_zero])
(assume ypos : y > 0,
have zypos : z * y > 0, from mul_pos zpos ypos,
have H1 : (z * x) mod (z * y) < z * y, from !mod_lt zypos,
have H2 : z * (x mod y) < z * y, from mul_lt_mul_of_pos_left (!mod_lt ypos) zpos,
eq_remainder H1 H2
(calc
((z * x) div (z * y)) * (z * y) + (z * x) mod (z * y) = z * x : eq_div_mul_add_mod
... = z * (x div y * y + x mod y) : eq_div_mul_add_mod
... = z * (x div y * y) + z * (x mod y) : mul.left_distrib
... = (x div y) * (z * y) + z * (x mod y) : mul.left_comm)))
theorem mul_mod_mul_right (x z y : ) : (x * z) mod (y * z) = (x mod y) * z :=
mul.comm z x ▸ mul.comm z y ▸ !mul.comm ▸ !mul_mod_mul_left
theorem mod_self (n : ) : n mod n = 0 :=
nat.cases_on n (by rewrite zero_mod)
(take n, by rewrite [-zero_add (succ n) at {1}, add_mod_self])
theorem mul_mod_eq_mod_mul_mod (m n k : nat) : (m * n) mod k = ((m mod k) * n) mod k :=
calc
(m * n) mod k = (((m div k) * k + m mod k) * n) mod k : eq_div_mul_add_mod
... = ((m mod k) * n) mod k :
by rewrite [mul.right_distrib, mul.right_comm, add.comm, add_mul_mod_self]
theorem mul_mod_eq_mul_mod_mod (m n k : nat) : (m * n) mod k = (m * (n mod k)) mod k :=
!mul.comm ▸ !mul.comm ▸ !mul_mod_eq_mod_mul_mod
theorem div_one (n : ) : n div 1 = n :=
assert n div 1 * 1 + n mod 1 = n, from !eq_div_mul_add_mod⁻¹,
begin rewrite [-this at {2}, mul_one, mod_one] end
theorem div_self {n : } (H : n > 0) : n div n = 1 :=
assert (n * 1) div (n * 1) = 1 div 1, from !mul_div_mul_left H,
by rewrite [div_one at this, -this, *mul_one]
theorem div_mul_cancel_of_mod_eq_zero {m n : } (H : m mod n = 0) : m div n * n = m :=
by rewrite [eq_div_mul_add_mod m n at {2}, H, add_zero]
theorem mul_div_cancel_of_mod_eq_zero {m n : } (H : m mod n = 0) : n * (m div n) = m :=
!mul.comm ▸ div_mul_cancel_of_mod_eq_zero H
/- dvd -/
theorem dvd_of_mod_eq_zero {m n : } (H : n mod m = 0) : m n :=
dvd.intro (!mul.comm ▸ div_mul_cancel_of_mod_eq_zero H)
theorem mod_eq_zero_of_dvd {m n : } (H : m n) : n mod m = 0 :=
dvd.elim H (take z, assume H1 : n = m * z, H1⁻¹ ▸ !mul_mod_right)
theorem dvd_iff_mod_eq_zero (m n : ) : m n ↔ n mod m = 0 :=
iff.intro mod_eq_zero_of_dvd dvd_of_mod_eq_zero
definition dvd.decidable_rel [instance] : decidable_rel dvd :=
take m n, decidable_of_decidable_of_iff _ (iff.symm !dvd_iff_mod_eq_zero)
theorem div_mul_cancel {m n : } (H : n m) : m div n * n = m :=
div_mul_cancel_of_mod_eq_zero (mod_eq_zero_of_dvd H)
theorem mul_div_cancel' {m n : } (H : n m) : n * (m div n) = m :=
!mul.comm ▸ div_mul_cancel H
theorem dvd_of_dvd_add_left {m n₁ n₂ : } (H₁ : m n₁ + n₂) (H₂ : m n₁) : m n₂ :=
obtain (c₁ : nat) (Hc₁ : n₁ + n₂ = m * c₁), from H₁,
obtain (c₂ : nat) (Hc₂ : n₁ = m * c₂), from H₂,
have aux : m * (c₁ - c₂) = n₂, from calc
m * (c₁ - c₂) = m * c₁ - m * c₂ : mul_sub_left_distrib
... = n₁ + n₂ - m * c₂ : Hc₁
... = n₁ + n₂ - n₁ : Hc₂
... = n₂ : add_sub_cancel_left,
dvd.intro aux
theorem dvd_of_dvd_add_right {m n₁ n₂ : } (H : m n₁ + n₂) : m n₂ → m n₁ :=
dvd_of_dvd_add_left (!add.comm ▸ H)
theorem dvd_sub {m n₁ n₂ : } (H1 : m n₁) (H2 : m n₂) : m n₁ - n₂ :=
by_cases
(assume H3 : n₁ ≥ n₂,
have H4 : n₁ = n₁ - n₂ + n₂, from (sub_add_cancel H3)⁻¹,
show m n₁ - n₂, from dvd_of_dvd_add_right (H4 ▸ H1) H2)
(assume H3 : ¬ (n₁ ≥ n₂),
have H4 : n₁ - n₂ = 0, from sub_eq_zero_of_le (le_of_lt (lt_of_not_ge H3)),
show m n₁ - n₂, from H4⁻¹ ▸ dvd_zero _)
theorem dvd.antisymm {m n : } : m n → n m → m = n :=
by_cases_zero_pos n
(assume H1, assume H2 : 0 m, eq_zero_of_zero_dvd H2)
(take n,
assume Hpos : n > 0,
assume H1 : m n,
assume H2 : n m,
obtain k (Hk : n = m * k), from exists_eq_mul_right_of_dvd H1,
obtain l (Hl : m = n * l), from exists_eq_mul_right_of_dvd H2,
have n * (l * k) = n, from !mul.assoc ▸ Hl ▸ Hk⁻¹,
have l * k = 1, from eq_one_of_mul_eq_self_right Hpos this,
have k = 1, from eq_one_of_mul_eq_one_left this,
show m = n, from (mul_one m)⁻¹ ⬝ (this ▸ Hk⁻¹))
theorem mul_div_assoc (m : ) {n k : } (H : k n) : m * n div k = m * (n div k) :=
or.elim (eq_zero_or_pos k)
(assume H1 : k = 0,
calc
m * n div k = m * n div 0 : H1
... = 0 : div_zero
... = m * 0 : mul_zero m
... = m * (n div 0) : div_zero
... = m * (n div k) : H1)
(assume H1 : k > 0,
have H2 : n = n div k * k, from (div_mul_cancel H)⁻¹,
calc
m * n div k = m * (n div k * k) div k : H2
... = m * (n div k) * k div k : mul.assoc
... = m * (n div k) : mul_div_cancel _ H1)
theorem dvd_of_mul_dvd_mul_left {m n k : } (kpos : k > 0) (H : k * m k * n) : m n :=
dvd.elim H
(take l,
assume H1 : k * n = k * m * l,
have H2 : n = m * l, from eq_of_mul_eq_mul_left kpos (H1 ⬝ !mul.assoc),
dvd.intro H2⁻¹)
theorem dvd_of_mul_dvd_mul_right {m n k : } (kpos : k > 0) (H : m * k n * k) : m n :=
dvd_of_mul_dvd_mul_left kpos (!mul.comm ▸ !mul.comm ▸ H)
lemma dvd_of_eq_mul (i j n : nat) : n = j*i → j n :=
begin intros, subst n, apply dvd_mul_right end
theorem div_dvd_div {k m n : } (H1 : k m) (H2 : m n) : m div k n div k :=
have H3 : m = m div k * k, from (div_mul_cancel H1)⁻¹,
have H4 : n = n div k * k, from (div_mul_cancel (dvd.trans H1 H2))⁻¹,
or.elim (eq_zero_or_pos k)
(assume H5 : k = 0,
have H6: n div k = 0, from (congr_arg _ H5 ⬝ !div_zero),
H6⁻¹ ▸ !dvd_zero)
(assume H5 : k > 0,
dvd_of_mul_dvd_mul_right H5 (H3 ▸ H4 ▸ H2))
theorem div_eq_iff_eq_mul_right {m n : } (k : ) (H : n > 0) (H' : n m) :
m div n = k ↔ m = n * k :=
iff.intro
(assume H1, by rewrite [-H1, mul_div_cancel' H'])
(assume H1, by rewrite [H1, !mul_div_cancel_left H])
theorem div_eq_iff_eq_mul_left {m n : } (k : ) (H : n > 0) (H' : n m) :
m div n = k ↔ m = k * n :=
!mul.comm ▸ !div_eq_iff_eq_mul_right H H'
theorem eq_mul_of_div_eq_right {m n k : } (H1 : n m) (H2 : m div n = k) :
m = n * k :=
calc
m = n * (m div n) : mul_div_cancel' H1
... = n * k : H2
theorem div_eq_of_eq_mul_right {m n k : } (H1 : n > 0) (H2 : m = n * k) :
m div n = k :=
calc
m div n = n * k div n : H2
... = k : !mul_div_cancel_left H1
theorem eq_mul_of_div_eq_left {m n k : } (H1 : n m) (H2 : m div n = k) :
m = k * n :=
!mul.comm ▸ !eq_mul_of_div_eq_right H1 H2
theorem div_eq_of_eq_mul_left {m n k : } (H1 : n > 0) (H2 : m = k * n) :
m div n = k :=
!div_eq_of_eq_mul_right H1 (!mul.comm ▸ H2)
lemma add_mod_eq_of_dvd (i j n : nat) : n j → (i + j) mod n = i mod n :=
assume h,
obtain k (hk : j = n * k), from exists_eq_mul_right_of_dvd h,
begin
subst j, rewrite mul.comm,
apply add_mul_mod_self
end
/- div and ordering -/
lemma le_of_dvd {m n} : n > 0 → m n → m ≤ n :=
assume (h₁ : n > 0) (h₂ : m n),
assert h₃ : n mod m = 0, from mod_eq_zero_of_dvd h₂,
by_contradiction
(λ nle : ¬ m ≤ n,
have h₄ : m > n, from lt_of_not_ge nle,
assert h₅ : n mod m = n, from mod_eq_of_lt h₄,
begin
rewrite h₃ at h₅, subst n,
exact absurd h₁ (lt.irrefl 0)
end)
theorem div_mul_le (m n : ) : m div n * n ≤ m :=
calc
m = m div n * n + m mod n : eq_div_mul_add_mod
... ≥ m div n * n : le_add_right
theorem div_le_of_le_mul {m n k : } (H : m ≤ n * k) : m div k ≤ n :=
or.elim (eq_zero_or_pos k)
(assume H1 : k = 0,
calc
m div k = m div 0 : H1
... = 0 : div_zero
... ≤ n : zero_le)
(assume H1 : k > 0,
le_of_mul_le_mul_right (calc
m div k * k ≤ m div k * k + m mod k : le_add_right
... = m : eq_div_mul_add_mod
... ≤ n * k : H) H1)
theorem div_le_self (m n : ) : m div n ≤ m :=
nat.cases_on n (!div_zero⁻¹ ▸ !zero_le)
take n,
have H : m ≤ m * succ n, from calc
m = m * 1 : mul_one
... ≤ m * succ n : !mul_le_mul_left (succ_le_succ !zero_le),
div_le_of_le_mul H
theorem mul_le_of_le_div {m n k : } (H : m ≤ n div k) : m * k ≤ n :=
calc
m * k ≤ n div k * k : !mul_le_mul_right H
... ≤ n : div_mul_le
theorem le_div_of_mul_le {m n k : } (H1 : k > 0) (H2 : m * k ≤ n) : m ≤ n div k :=
have H3 : m * k < (succ (n div k)) * k, from
calc
m * k ≤ n : H2
... = n div k * k + n mod k : eq_div_mul_add_mod
... < n div k * k + k : add_lt_add_left (!mod_lt H1)
... = (succ (n div k)) * k : succ_mul,
le_of_lt_succ (lt_of_mul_lt_mul_right H3)
theorem le_div_iff_mul_le {m n k : } (H : k > 0) : m ≤ n div k ↔ m * k ≤ n :=
iff.intro !mul_le_of_le_div (!le_div_of_mul_le H)
theorem div_le_div {m n : } (k : ) (H : m ≤ n) : m div k ≤ n div k :=
by_cases_zero_pos k
(by rewrite [*div_zero])
(take k, assume H1 : k > 0, le_div_of_mul_le H1 (le.trans !div_mul_le H))
theorem div_lt_of_lt_mul {m n k : } (H : m < n * k) : m div k < n :=
lt_of_mul_lt_mul_right (calc
m div k * k ≤ m div k * k + m mod k : le_add_right
... = m : eq_div_mul_add_mod
... < n * k : H)
theorem lt_mul_of_div_lt {m n k : } (H1 : k > 0) (H2 : m div k < n) : m < n * k :=
assert H3 : succ (m div k) * k ≤ n * k, from !mul_le_mul_right (succ_le_of_lt H2),
have H4 : m div k * k + k ≤ n * k, by rewrite [succ_mul at H3]; apply H3,
calc
m = m div k * k + m mod k : eq_div_mul_add_mod
... < m div k * k + k : add_lt_add_left (!mod_lt H1)
... ≤ n * k : H4
theorem div_lt_iff_lt_mul {m n k : } (H : k > 0) : m div k < n ↔ m < n * k :=
iff.intro (!lt_mul_of_div_lt H) !div_lt_of_lt_mul
theorem div_le_iff_le_mul_of_div {m n : } (k : ) (H : n > 0) (H' : n m) :
m div n ≤ k ↔ m ≤ k * n :=
by rewrite [propext (!le_iff_mul_le_mul_right H), !div_mul_cancel H']
theorem le_mul_of_div_le_of_div {m n k : } (H1 : n > 0) (H2 : n m) (H3 : m div n ≤ k) :
m ≤ k * n :=
iff.mp (!div_le_iff_le_mul_of_div H1 H2) H3
-- needed for integer division
theorem mul_sub_div_of_lt {m n k : } (H : k < m * n) :
(m * n - (k + 1)) div m = n - k div m - 1 :=
have H1 : k div m < n, from div_lt_of_lt_mul (!mul.comm ▸ H),
have H2 : n - k div m ≥ 1, from
le_sub_of_add_le (calc
1 + k div m = succ (k div m) : add.comm
... ≤ n : succ_le_of_lt H1),
assert H3 : n - k div m = n - k div m - 1 + 1, from (sub_add_cancel H2)⁻¹,
assert H4 : m > 0, from pos_of_ne_zero (assume H': m = 0, not_lt_zero _ (!zero_mul ▸ H' ▸ H)),
have H5 : k mod m + 1 ≤ m, from succ_le_of_lt (!mod_lt H4),
assert H6 : m - (k mod m + 1) < m, from sub_lt_self H4 !succ_pos,
calc
(m * n - (k + 1)) div m = (m * n - (k div m * m + k mod m + 1)) div m : eq_div_mul_add_mod
... = (m * n - k div m * m - (k mod m + 1)) div m : by rewrite [*sub_sub]
... = ((n - k div m) * m - (k mod m + 1)) div m :
by rewrite [mul.comm m, mul_sub_right_distrib]
... = ((n - k div m - 1) * m + m - (k mod m + 1)) div m :
by rewrite [H3 at {1}, mul.right_distrib, nat.one_mul]
... = ((n - k div m - 1) * m + (m - (k mod m + 1))) div m : {add_sub_assoc H5 _}
... = (m - (k mod m + 1)) div m + (n - k div m - 1) :
by rewrite [add.comm, (add_mul_div_self H4)]
... = n - k div m - 1 :
by rewrite [div_eq_zero_of_lt H6, zero_add]
private lemma div_div_aux (a b c : nat) : b > 0 → c > 0 → (a div b) div c = a div (b * c) :=
suppose b > 0, suppose c > 0,
nat.strong_induction_on a
(λ a ih,
let k₁ := a div (b*c) in
let k₂ := a mod (b*c) in
assert bc_pos : b*c > 0, from mul_pos `b > 0` `c > 0`,
assert k₂ < b * c, from mod_lt _ bc_pos,
assert k₂ ≤ a, from !mod_le,
or.elim (eq_or_lt_of_le this)
(suppose k₂ = a,
assert i₁ : a < b * c, by rewrite -this; assumption,
assert k₁ = 0, from div_eq_zero_of_lt i₁,
assert a div b < c, by rewrite [mul.comm at i₁]; exact div_lt_of_lt_mul i₁,
begin
rewrite [`k₁ = 0`],
show (a div b) div c = 0, from div_eq_zero_of_lt `a div b < c`
end)
(suppose k₂ < a,
assert a = k₁*(b*c) + k₂, from eq_div_mul_add_mod a (b*c),
assert a div b = k₁*c + k₂ div b, by
rewrite [this at {1}, mul.comm b c at {2}, -mul.assoc,
add.comm, add_mul_div_self `b > 0`, add.comm],
assert e₁ : (a div b) div c = k₁ + (k₂ div b) div c, by
rewrite [this, add.comm, add_mul_div_self `c > 0`, add.comm],
assert e₂ : (k₂ div b) div c = k₂ div (b * c), from ih k₂ `k₂ < a`,
assert e₃ : k₂ div (b * c) = 0, from div_eq_zero_of_lt `k₂ < b * c`,
assert (k₂ div b) div c = 0, by rewrite [e₂, e₃],
show (a div b) div c = k₁, by rewrite [e₁, this]))
lemma div_div_eq_div_mul (a b c : nat) : (a div b) div c = a div (b * c) :=
begin
cases b with b,
rewrite [zero_mul, *div_zero, zero_div],
cases c with c,
rewrite [mul_zero, *div_zero],
apply div_div_aux a (succ b) (succ c) dec_trivial dec_trivial
end
lemma div_lt_of_ne_zero : ∀ {n : nat}, n ≠ 0 → n div 2 < n
| 0 h := absurd rfl h
| (succ n) h :=
begin
apply div_lt_of_lt_mul,
rewrite [-add_one, mul.right_distrib],
change n + 1 < (n * 1 + n) + (1 + 1),
rewrite [mul_one, -add.assoc],
apply add_lt_add_right,
show n < n + n + 1,
begin
rewrite [add.assoc, -add_zero n at {1}],
apply add_lt_add_left,
apply zero_lt_succ
end
end
end nat