95 lines
3.7 KiB
Text
95 lines
3.7 KiB
Text
/-
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Copyright (c) 2015 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Module: data.nat.sqrt
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Authors: Leonardo de Moura
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Very simple (sqrt n) function that returns s s.t.
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s*s ≤ n ≤ s*s + s + s
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-/
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import data.nat.order data.nat.sub
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namespace nat
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open decidable
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-- This is the simplest possible function that just performs a linear search
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definition sqrt_aux : nat → nat → nat
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| 0 n := 0
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| (succ s) n := if (succ s)*(succ s) ≤ n then succ s else sqrt_aux s n
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theorem sqrt_aux_succ_of_pos {s n} : (succ s)*(succ s) ≤ n → sqrt_aux (succ s) n = (succ s) :=
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assume h, if_pos h
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theorem sqrt_aux_succ_of_neg {s n} : ¬ (succ s)*(succ s) ≤ n → sqrt_aux (succ s) n = sqrt_aux s n :=
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assume h, if_neg h
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theorem sqrt_aux_of_le : ∀ {s n : nat}, s * s ≤ n → sqrt_aux s n = s
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| 0 n h := rfl
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| (succ s) n h := by rewrite [sqrt_aux_succ_of_pos h]
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definition sqrt (n : nat) : nat :=
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sqrt_aux n n
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theorem sqrt_aux_lower : ∀ {s n : nat}, s ≤ n → sqrt_aux s n * sqrt_aux s n ≤ n
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| 0 n h := h
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| (succ s) n h := by_cases
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(λ h₁ : (succ s)*(succ s) ≤ n, by rewrite [sqrt_aux_succ_of_pos h₁]; exact h₁)
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(λ h₂ : ¬ (succ s)*(succ s) ≤ n,
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assert aux : s ≤ n, from lt.step (lt_of_succ_le h),
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by rewrite [sqrt_aux_succ_of_neg h₂]; exact (sqrt_aux_lower aux))
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theorem sqrt_lower (n : nat) : sqrt n * sqrt n ≤ n :=
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sqrt_aux_lower (le.refl n)
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theorem sqrt_aux_upper : ∀ {s n : nat}, n ≤ s*s + s + s → n ≤ sqrt_aux s n * sqrt_aux s n + sqrt_aux s n + sqrt_aux s n
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| 0 n h := h
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| (succ s) n h := by_cases
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(λ h₁ : (succ s)*(succ s) ≤ n,
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by rewrite [sqrt_aux_succ_of_pos h₁]; exact h)
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(λ h₂ : ¬ (succ s)*(succ s) ≤ n,
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assert h₃ : n < (succ s) * (succ s), from lt_of_not_le h₂,
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assert h₄ : n ≤ s * s + s + s, by rewrite [succ_mul_succ_eq at h₃]; exact h₃,
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by rewrite [sqrt_aux_succ_of_neg h₂]; exact (sqrt_aux_upper h₄))
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theorem sqrt_upper (n : nat) : n ≤ sqrt n * sqrt n + sqrt n + sqrt n :=
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have aux : n ≤ n*n + n + n, from le_add_of_le_right (le_add_of_le_left (le.refl n)),
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sqrt_aux_upper aux
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theorem sqrt_aux_eq : ∀ {s n}, s ≥ n → sqrt_aux s (n*n) = n
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| 0 n h :=
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assert neqz : n = 0, from eq_zero_of_le_zero h,
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by rewrite neqz
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| (succ s) n h := by_cases
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(λ h₁ : (succ s)*(succ s) ≤ n*n,
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assert h₂ : (succ s)*(succ s) ≥ n*n, from mul_le_mul h h,
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assert h₃ : (succ s)*(succ s) = n*n, from le.antisymm h₁ h₂,
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assert h₄ : ¬ succ s > n, from
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assume ssgtn : succ s > n,
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assert h₅ : (succ s)*(succ s) > n*n, from mul_lt_mul_of_le_of_le ssgtn ssgtn,
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have h₆ : n*n > n*n, by rewrite [h₃ at h₅]; exact h₅,
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absurd h₆ !lt.irrefl,
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have sslen : succ s ≤ n, from le_of_not_lt h₄,
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assert sseqn : succ s = n, from le.antisymm sslen h,
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by rewrite [sqrt_aux_succ_of_pos h₁]; exact sseqn)
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(λ h₂ : ¬ (succ s)*(succ s) ≤ n*n,
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or.elim (eq_or_lt_of_le h)
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(λ sseqn, by rewrite [sseqn at h₂]; exact (absurd !le.refl h₂))
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(λ sgen : s ≥ n,
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by rewrite [sqrt_aux_succ_of_neg h₂]; exact (sqrt_aux_eq sgen)))
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private theorem le_squared : ∀ (n : nat), n ≤ n*n
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| 0 := !le.refl
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| (succ n) :=
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have aux₁ : 1 ≤ succ n, from succ_le_succ !zero_le,
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assert aux₂ : 1 * succ n ≤ succ n * succ n, from mul_le_mul aux₁ !le.refl,
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by rewrite [one_mul at aux₂]; exact aux₂
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theorem sqrt_eq (n : nat) : sqrt (n*n) = n :=
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sqrt_aux_eq !le_squared
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theorem mul_square_cancel {a b : nat} : a*a = b*b → a = b :=
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assume h,
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assert aux : sqrt (a*a) = sqrt (b*b), by rewrite h,
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by rewrite [*sqrt_eq at aux]; exact aux
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end nat
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