feat(library/data/nat/primes): add is_prime

library/data/nat/primes.lean

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+/-
+Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura
+
+Prime numbers
+-/
+import data.nat.fact data.nat.bquant data.nat.power logic.identities
+open bool
+
+namespace nat
+open decidable
+
+definition is_prime [reducible] (p : nat) := p ≥ 2 ∧ ∀ m, m ∣ p → m = 1 ∨ m = p
+
+definition is_prime_ext (p : nat) := p ≥ 2 ∧ ∀ m, m ≤ p → m ∣ p → m = 1 ∨ m = p
+local attribute is_prime_ext [reducible]
+
+lemma is_prime_ext_iff_is_prime (p : nat) : is_prime_ext p ↔ is_prime p :=
+iff.intro
+  begin
+    intro h, cases h with h₁ h₂, constructor, assumption,
+    intro m d, exact h₂ m (le_of_dvd (lt_of_succ_le (le_of_succ_le h₁)) d) d
+  end
+  begin
+    intro h, cases h with h₁ h₂, constructor, assumption,
+    intro m l d, exact h₂ m d
+  end
+
+definition decidable_is_prime [instance] (p : nat) : decidable (is_prime p) :=
+decidable_of_decidable_of_iff _ (is_prime_ext_iff_is_prime p)
+
+lemma ge_two_of_is_prime {p : nat} : is_prime p → p ≥ 2 :=
+assume h, obtain h₁ h₂, from h, h₁
+
+lemma pred_prime_pos {p : nat} : is_prime p → pred p > 0 :=
+assume h,
+have h₁ : p ≥ 2, from ge_two_of_is_prime h,
+lt_of_succ_le (pred_le_pred h₁)
+
+lemma succ_pred_prime {p : nat} : is_prime p → succ (pred p) = p :=
+assume h, succ_pred_of_pos (lt_of_succ_le (le_of_succ_le (ge_two_of_is_prime h)))
+
+lemma divisor_of_prime {p m : nat} : is_prime p → m ∣ p → m = 1 ∨ m = p :=
+assume h d, obtain h₁ h₂, from h, h₂ m d
+
+lemma gt_one_of_pos_of_prime_dvd {i p : nat} : is_prime p → 0 < i → i mod p = 0 → 1 < i :=
+assume ipp pos h,
+have h₁ : p ∣ i, from dvd_of_mod_eq_zero h,
+have h₂ : p ≥ 2, from ge_two_of_is_prime ipp,
+have h₃ : p ≤ i, from le_of_dvd pos h₁,
+lt_of_succ_le (le.trans h₂ h₃)
+
+end nat