diff --git a/library/data/nat/power.lean b/library/data/nat/power.lean new file mode 100644 index 000000000..446e7ec78 --- /dev/null +++ b/library/data/nat/power.lean @@ -0,0 +1,62 @@ +/- +Copyright (c) 2015 Microsoft Corporation. All rights reserved. +Released under Apache 2.0 license as described in the file LICENSE. + +Module: data.nat.power +Authors: Leonardo de Moura + +Power +-/ +import data.nat.basic data.nat.div + +namespace nat + +definition pow : nat → nat → nat +| a 0 := 1 +| a (succ b) := a * pow a b + +theorem pow_zero (a : nat) : pow a 0 = 1 := +rfl + +theorem pow_succ (a b : nat) : pow a (succ b) = a * pow a b := +rfl + +theorem one_pow : ∀ (a : nat), pow 1 a = 1 +| 0 := rfl +| (succ a) := by rewrite [pow_succ, one_pow] + +theorem pow_one : ∀ {a : nat}, a ≠ 0 → pow a 1 = a +| 0 h := absurd rfl h +| (succ a) h := by rewrite [pow_succ, pow_zero, mul_one] + +theorem zero_pow : ∀ {a : nat}, a ≠ 0 → pow 0 a = 0 +| 0 h := absurd rfl h +| (succ a) h := by rewrite [pow_succ, zero_mul] + +theorem pow_add : ∀ (a b c : nat), pow a (b + c) = pow a b * pow a c +| a b 0 := by rewrite [add_zero, pow_zero, mul_one] +| a b (succ c) := by rewrite [add_succ, *pow_succ, pow_add a b c, mul.left_comm] + +theorem mul_self_eq_pow_2 (a : nat) : a * a = pow a 2 := +show a * a = pow a (succ (succ zero)), from +by rewrite [*pow_succ, *pow_zero, mul_one] + +theorem pow_cancel_left : ∀ {a b c : nat}, a > 1 → pow a b = pow a c → b = c +| a 0 0 h₁ h₂ := rfl +| a (succ b) 0 h₁ h₂ := + assert aeq1 : a = 1, by rewrite [pow_succ at h₂, pow_zero at h₂]; exact (eq_one_of_mul_eq_one_right h₂), + assert h₁ : 1 < 1, by rewrite [aeq1 at h₁]; exact h₁, + absurd h₁ !lt.irrefl +| a 0 (succ c) h₁ h₂ := + assert aeq1 : a = 1, by rewrite [pow_succ at h₂, pow_zero at h₂]; exact (eq_one_of_mul_eq_one_right (eq.symm h₂)), + assert h₁ : 1 < 1, by rewrite [aeq1 at h₁]; exact h₁, + absurd h₁ !lt.irrefl +| a (succ b) (succ c) h₁ h₂ := + assert ane0 : a ≠ 0, from assume aeq0, by rewrite [aeq0 at h₁]; exact (absurd h₁ dec_trivial), + assert beqc : pow a b = pow a c, by rewrite [*pow_succ at h₂]; exact (mul_cancel_left_of_ne_zero ane0 h₂), + by rewrite [pow_cancel_left h₁ beqc] + +theorem pow_div_cancel : ∀ {a b : nat}, a ≠ 0 → pow a (succ b) div a = pow a b +| a 0 h := by rewrite [pow_succ, pow_zero, mul_one, div_self (pos_of_ne_zero h)] +| a (succ b) h := by rewrite [pow_succ, mul_div_cancel_left _ (pos_of_ne_zero h)] +end nat