# A recursion principle based on even and odd numbers. #

def Nat.evenOddRec {P : Sort u_1} (h0 : P 0) (h_even : (n : ) → P nP (2 * n)) (h_odd : (n : ) → P nP (2 * n + 1)) (n : ) :
P n

Recursion principle on even and odd numbers: if we have P 0, and for all i : ℕ we can extend from P i to both P (2 * i) and P (2 * i + 1), then we have P n for all n : ℕ. This is nothing more than a wrapper around Nat.binaryRec, to avoid having to switch to dealing with bit0 and bit1.

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@[simp]
theorem Nat.evenOddRec_zero {P : Sort u_1} (h0 : P 0) (h_even : (i : ) → P iP (2 * i)) (h_odd : (i : ) → P iP (2 * i + 1)) :
Nat.evenOddRec h0 h_even h_odd 0 = h0
@[simp]
theorem Nat.evenOddRec_even {P : Sort u_1} (h0 : P 0) (h_even : (i : ) → P iP (2 * i)) (h_odd : (i : ) → P iP (2 * i + 1)) (H : h_even 0 h0 = h0) (n : ) :
Nat.evenOddRec h0 h_even h_odd (2 * n) = h_even n (Nat.evenOddRec h0 h_even h_odd n)
@[simp]
theorem Nat.evenOddRec_odd {P : Sort u_1} (h0 : P 0) (h_even : (i : ) → P iP (2 * i)) (h_odd : (i : ) → P iP (2 * i + 1)) (H : h_even 0 h0 = h0) (n : ) :
Nat.evenOddRec h0 h_even h_odd (2 * n + 1) = h_odd n (Nat.evenOddRec h0 h_even h_odd n)
noncomputable def Nat.evenOddStrongRec {P : Sort u_1} (h_even : (n : ) → ((k : ) → k < 2 * nP k)P (2 * n)) (h_odd : (n : ) → ((k : ) → k < 2 * n + 1P k)P (2 * n + 1)) (n : ) :
P n

Strong recursion principle on even and odd numbers: if for all i : ℕ we can prove P (2 * i) from P j for all j < 2 * i and we can prove P (2 * i + 1) from P j for all j < 2 * i + 1, then we have P n for all n : ℕ.

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• One or more equations did not get rendered due to their size.
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