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1 module root2 where
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3 open import Data.Nat
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4 open import Data.Nat.Properties
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5 open import Data.Empty
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6 open import Data.Unit using (⊤ ; tt)
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7 open import Relation.Nullary
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8 open import Relation.Binary.PropositionalEquality
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9 open import Relation.Binary.Definitions
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10
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11 open import gcd
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12 open import even
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13 open import nat
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14
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15 record Rational : Set where
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16 field
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17 i j : ℕ
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18 coprime : gcd i j ≡ 1
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19
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20 even→gcd=2 : {n : ℕ} → even n → gcd n 2 ≡ 2
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21 even→gcd=2 {zero} en = refl
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22 even→gcd=2 {suc (suc zero)} en = refl
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23 even→gcd=2 {suc (suc (suc (suc n)))} en = begin
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24 gcd (suc (suc (suc (suc n)))) 2 ≡⟨⟩
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25 gcd (suc (suc n)) 2 ≡⟨ even→gcd=2 {suc (suc n)} en ⟩
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26 2 ∎ where open ≡-Reasoning
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27
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28 even^2 : {n : ℕ} → even ( n * n ) → even n
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29 even^2 {n} en with even? n
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30 ... | yes y = y
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31 ... | no ne = ⊥-elim ( odd4 ((2 * m) + 2 * m * suc (2 * m)) (n+even {2 * m} {2 * m * suc (2 * m)} ee3 ee4) (subst (λ k → even k) ee2 en )) where
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32 m : ℕ
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33 m = Odd.j ( odd3 n ne )
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34 ee3 : even (2 * m)
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35 ee3 = subst (λ k → even k ) (*-comm m 2) (n*even {m} {2} tt )
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36 ee4 : even ((2 * m) * suc (2 * m))
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37 ee4 = even*n {(2 * m)} {suc (2 * m)} (even*n {2} {m} tt )
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38 ee2 : n * n ≡ suc (2 * m) + ((2 * m) * (suc (2 * m) ))
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39 ee2 = begin n * n ≡⟨ cong ( λ k → k * k) (Odd.is-twice (odd3 n ne)) ⟩
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40 suc (2 * m) * suc (2 * m) ≡⟨ *-distribʳ-+ (suc (2 * m)) 1 ((2 * m) ) ⟩
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41 (1 * suc (2 * m)) + 2 * m * suc (2 * m) ≡⟨ cong (λ k → k + 2 * m * suc (2 * m)) (begin
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42 suc m + 1 * m + 0 * (suc m + 1 * m ) ≡⟨ +-comm (suc m + 1 * m) 0 ⟩
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43 suc m + 1 * m ≡⟨⟩
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44 suc (2 * m)
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45 ∎) ⟩ suc (2 * m) + 2 * m * suc (2 * m) ∎ where open ≡-Reasoning
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46
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47 e3 : {i j : ℕ } → 2 * i ≡ 2 * j → i ≡ j
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48 e3 {zero} {zero} refl = refl
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49 e3 {suc x} {suc y} eq with <-cmp x y
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50 ... | tri< a ¬b ¬c = ⊥-elim ( nat-≡< eq (s≤s (<-trans (<-plus a) (<-plus-0 (s≤s (<-plus a ))))))
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51 ... | tri≈ ¬a b ¬c = cong suc b
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52 ... | tri> ¬a ¬b c = ⊥-elim ( nat-≡< (sym eq) (s≤s (<-trans (<-plus c) (<-plus-0 (s≤s (<-plus c ))))))
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54 open Factor
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55
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56 -- gcd-div : ( i j k : ℕ ) → (if : Factor k i) (jf : Factor k j )
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57 -- → remain if ≡ 0 → remain jf ≡ 0
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58 -- → Dividable k ( gcd i j )
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59
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60 root2-irrational : ( n m : ℕ ) → n > 1 → m > 1 → 2 * n * n ≡ m * m → ¬ (gcd n m ≡ 1)
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61 root2-irrational n m n>1 m>1 2nm = rot13 ( gcd-div n m 2 (s≤s (s≤s z≤n)) {!!} {!!} ) where
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62 rot13 : {m : ℕ } → Dividable 2 m → m ≡ 1 → ⊥
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63 rot13 d refl with Dividable.factor d | Dividable.is-factor d
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64 ... | zero | ()
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65 ... | suc n | ()
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66 rot11 : {m : ℕ } → even m → Factor 2 m
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67 rot11 {zero} em = record { factor = 0 ; remain = 0 ; is-factor = refl }
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68 rot11 {suc zero} ()
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69 rot11 {suc (suc m) } em = record { factor = suc (factor fc ) ; remain = remain fc ; is-factor = isfc } where
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70 fc : Factor 2 m
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71 fc = rot11 {m} em
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72 isfc : suc (factor fc) * 2 + remain fc ≡ suc (suc m)
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73 isfc = begin
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74 suc (factor fc) * 2 + remain fc ≡⟨ cong (λ k → k + remain fc) (*-distribʳ-+ 2 1 (factor fc)) ⟩
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75 ((1 * 2) + (factor fc)* 2 ) + remain fc ≡⟨⟩
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76 ((1 + 1) + (factor fc)* 2 ) + remain fc ≡⟨ cong (λ k → k + remain fc) (+-assoc 1 1 _ ) ⟩
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77 (1 + (1 + (factor fc)* 2 )) + remain fc ≡⟨⟩
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78 suc (suc ((factor fc * 2) + remain fc )) ≡⟨ cong (λ x → suc (suc x)) (is-factor fc) ⟩
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79 suc (suc m) ∎ where open ≡-Reasoning
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80 rot5 : {n : ℕ} → n > 1 → n > 0
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81 rot5 {n} lt = <-trans a<sa lt
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82 rot1 : even ( m * m )
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83 rot1 = subst (λ k → even k ) rot4 (n*even {n * n} {2} tt ) where
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84 rot4 : (n * n) * 2 ≡ m * m
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85 rot4 = begin
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86 (n * n) * 2 ≡⟨ *-comm (n * n) 2 ⟩
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87 2 * ( n * n ) ≡⟨ sym (*-assoc 2 n n) ⟩
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88 2 * n * n ≡⟨ 2nm ⟩
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89 m * m ∎ where open ≡-Reasoning
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90 E : Even m
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91 E = e2 m ( even^2 {m} ( rot1 ))
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92 rot2 : 2 * n * n ≡ 2 * Even.j E * m
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93 rot2 = subst (λ k → 2 * n * n ≡ k * m ) (Even.is-twice E) 2nm
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94 rot3 : n * n ≡ Even.j E * m
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95 rot3 = e3 ( begin
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96 2 * (n * n) ≡⟨ sym (*-assoc 2 n _) ⟩
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97 2 * n * n ≡⟨ rot2 ⟩
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98 2 * Even.j E * m ≡⟨ *-assoc 2 (Even.j E) m ⟩
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99 2 * (Even.j E * m) ∎ ) where open ≡-Reasoning
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100 rot7 : even n
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101 rot7 = even^2 {n} (subst (λ k → even k) (sym rot3) ((n*even {Even.j E} {m} ( even^2 {m} ( rot1 )))))
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102 f2 : Factor 2 n
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103 f2 = rot11 rot7
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104 f3 : ( n : ℕ) → (e : even n ) → remain (rot11 {n} e) ≡ 0
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105 f3 zero e = refl
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106 f3 (suc (suc n)) e = f3 n e
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107 fm : Factor 2 m
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108 fm = record { factor = Even.j E ; remain = 0 ; is-factor = fm1 } where
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109 fm1 : Even.j E * 2 + 0 ≡ m
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110 fm1 = begin
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111 Even.j E * 2 + 0 ≡⟨ +-comm _ 0 ⟩
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112 Even.j E * 2 ≡⟨ *-comm (Even.j E) 2 ⟩
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113 2 * Even.j E ≡⟨ sym ( Even.is-twice E ) ⟩
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114 m ∎ where open ≡-Reasoning
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