84 lines
2.9 KiB
Text
84 lines
2.9 KiB
Text
import data.nat logic.cast
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open nat
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inductive fin : nat → Type :=
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fz : Π n, fin (succ n),
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fs : Π {n}, fin n → fin (succ n)
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namespace fin
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definition z_cases_on (C : fin zero → Type) (p : fin zero) : C p :=
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by cases p
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definition nz_cases_on {C : Π n, fin (succ n) → Type}
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(H₁ : Π n, C n (fz n))
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(H₂ : Π n (f : fin n), C n (fs f))
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{n : nat}
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(f : fin (succ n)) : C n f :=
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begin
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cases f with (n', n', f'),
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apply (H₁ n'),
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apply (H₂ n' f')
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end
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definition to_nat {n : nat} (f : fin n) : nat :=
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fin.rec_on f
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(λ n, zero)
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(λ n f r, succ r)
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theorem to_nat.lt {n : nat} (f : fin n) : to_nat f < n :=
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fin.rec_on f
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(λ n, calc
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to_nat (fz n) = 0 : rfl
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... < succ n : succ_pos n)
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(λ n f ih, calc
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to_nat (fs f) = succ (to_nat f) : rfl
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... < succ n : succ_lt ih)
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definition lift {n : nat} (f : fin n) : Π m, fin (m + n) :=
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fin.rec_on f
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(λ n m, fz (m + n))
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(λ n f ih m, fs (ih m))
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theorem to_nat.lift {n : nat} (f : fin n) : ∀m, to_nat f = to_nat (lift f m) :=
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fin.rec_on f
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(λ n m, rfl)
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(λ n f ih m, calc
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to_nat (fs f) = succ (to_nat f) : rfl
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... = succ (to_nat (lift f m)) : ih
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... = to_nat (lift (fs f) m) : rfl)
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private definition of_nat_core (p : nat) : fin (succ p) :=
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nat.rec_on p
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(fz zero)
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(λ a r, fs r)
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private theorem to_nat.of_nat_core (p : nat) : to_nat (of_nat_core p) = p :=
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nat.induction_on p
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rfl
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(λ p₁ ih, calc
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to_nat (of_nat_core (succ p₁)) = succ (to_nat (of_nat_core p₁)) : rfl
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... = succ p₁ : ih)
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private lemma of_nat_eq {p n : nat} (H : p < n) : n - succ p + succ p = n :=
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add_sub_ge_left (succ_le_of_lt H)
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definition of_nat (p : nat) (n : nat) : p < n → fin n :=
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λ H : p < n,
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eq.rec_on (of_nat_eq H) (lift (of_nat_core p) (n - succ p))
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theorem of_nat_def (p : nat) (n : nat) (H : p < n) : of_nat p n H = eq.rec_on (of_nat_eq H) (lift (of_nat_core p) (n - succ p)) :=
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rfl
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theorem of_nat_heq (p : nat) (n : nat) (H : p < n) : of_nat p n H == lift (of_nat_core p) (n - succ p) :=
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heq.symm (eq_rec_to_heq (eq.symm (of_nat_def p n H)))
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theorem to_nat.of_nat (p : nat) (n : nat) (H : p < n) : to_nat (of_nat p n H) = p :=
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have aux₁ : to_nat (of_nat p n H) == to_nat (lift (of_nat_core p) (n - succ p)), from
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hcongr_arg2 @to_nat (eq.symm (of_nat_eq H)) (of_nat_heq p n H),
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have aux₂ : to_nat (lift (of_nat_core p) (n - succ p)) = p, from calc
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to_nat (lift (of_nat_core p) (n - succ p)) = to_nat (of_nat_core p) : to_nat.lift
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... = p : to_nat.of_nat_core,
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heq.to_eq (heq.trans aux₁ (heq.from_eq aux₂))
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end fin
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