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feat(data/nat/hyperoperation): Defined hyperoperation and added relat…
…ed lemmas (#18116) Added the hyperoperation sequence as defined here: https://en.wikipedia.org/wiki/Hyperoperation Proved main lemmas.
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/- | ||
Copyright (c) 2023 Mark Andrew Gerads. All rights reserved. | ||
Released under Apache 2.0 license as described in the file LICENSE. | ||
Authors: Mark Andrew Gerads, Junyan Xu, Eric Wieser | ||
-/ | ||
import tactic.ring | ||
import data.nat.parity | ||
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/-! | ||
# Hyperoperation sequence | ||
This file defines the Hyperoperation sequence. | ||
`hyperoperation 0 m k = k + 1` | ||
`hyperoperation 1 m k = m + k` | ||
`hyperoperation 2 m k = m * k` | ||
`hyperoperation 3 m k = m ^ k` | ||
`hyperoperation (n + 3) m 0 = 1` | ||
`hyperoperation (n + 1) m (k + 1) = hyperoperation n m (hyperoperation (n + 1) m k)` | ||
## References | ||
* <https://en.wikipedia.org/wiki/Hyperoperation> | ||
## Tags | ||
hyperoperation | ||
-/ | ||
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/-- | ||
Implementation of the hyperoperation sequence | ||
where `hyperoperation n m k` is the `n`th hyperoperation between `m` and `k`. | ||
-/ | ||
def hyperoperation : ℕ → ℕ → ℕ → ℕ | ||
| 0 _ k := k + 1 | ||
| 1 m 0 := m | ||
| 2 _ 0 := 0 | ||
| (n + 3) _ 0 := 1 | ||
| (n + 1) m (k + 1) := hyperoperation n m (hyperoperation (n + 1) m k) | ||
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-- Basic hyperoperation lemmas | ||
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@[simp] lemma hyperoperation_zero (m : ℕ) : hyperoperation 0 m = nat.succ := | ||
funext $ λ k, by rw [hyperoperation, nat.succ_eq_add_one] | ||
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lemma hyperoperation_ge_three_eq_one (n m : ℕ) : hyperoperation (n + 3) m 0 = 1 := | ||
by rw hyperoperation | ||
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lemma hyperoperation_recursion (n m k : ℕ) : | ||
hyperoperation (n + 1) m (k + 1) = hyperoperation n m (hyperoperation (n + 1) m k) := | ||
by obtain (_|_|_) := n; rw hyperoperation | ||
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-- Interesting hyperoperation lemmas | ||
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@[simp] lemma hyperoperation_one : hyperoperation 1 = (+) := | ||
begin | ||
ext m k, | ||
induction k with bn bih, | ||
{ rw [nat_add_zero m, hyperoperation], }, | ||
{ rw [hyperoperation_recursion, bih, hyperoperation_zero], | ||
exact nat.add_assoc m bn 1, }, | ||
end | ||
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@[simp] lemma hyperoperation_two : hyperoperation 2 = (*) := | ||
begin | ||
ext m k, | ||
induction k with bn bih, | ||
{ rw hyperoperation, | ||
exact (nat.mul_zero m).symm, }, | ||
{ rw [hyperoperation_recursion, hyperoperation_one, bih], | ||
ring, }, | ||
end | ||
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@[simp] lemma hyperoperation_three : hyperoperation 3 = (^) := | ||
begin | ||
ext m k, | ||
induction k with bn bih, | ||
{ rw hyperoperation_ge_three_eq_one, | ||
exact (pow_zero m).symm, }, | ||
{ rw [hyperoperation_recursion, hyperoperation_two, bih], | ||
exact (pow_succ m bn).symm, }, | ||
end | ||
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lemma hyperoperation_ge_two_eq_self (n m : ℕ) : hyperoperation (n + 2) m 1 = m := | ||
begin | ||
induction n with nn nih, | ||
{ rw hyperoperation_two, | ||
ring, }, | ||
{ rw [hyperoperation_recursion, hyperoperation_ge_three_eq_one, nih], }, | ||
end | ||
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lemma hyperoperation_two_two_eq_four (n : ℕ) : hyperoperation (n + 1) 2 2 = 4 := | ||
begin | ||
induction n with nn nih, | ||
{ rw hyperoperation_one, }, | ||
{ rw [hyperoperation_recursion, hyperoperation_ge_two_eq_self, nih], }, | ||
end | ||
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lemma hyperoperation_ge_three_one (n : ℕ) : ∀ (k : ℕ), hyperoperation (n + 3) 1 k = 1 := | ||
begin | ||
induction n with nn nih, | ||
{ intros k, | ||
rw [hyperoperation_three, one_pow], }, | ||
{ intros k, | ||
cases k, | ||
{ rw hyperoperation_ge_three_eq_one, }, | ||
{ rw [hyperoperation_recursion, nih], }, }, | ||
end | ||
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lemma hyperoperation_ge_four_zero (n k : ℕ) : | ||
hyperoperation (n + 4) 0 k = if (even k) then 1 else 0 := | ||
begin | ||
induction k with kk kih, | ||
{ rw hyperoperation_ge_three_eq_one, | ||
simp only [even_zero, if_true], }, | ||
{ rw hyperoperation_recursion, | ||
rw kih, | ||
simp_rw nat.even_add_one, | ||
split_ifs, | ||
{ exact hyperoperation_ge_two_eq_self (n + 1) 0, }, | ||
{ exact hyperoperation_ge_three_eq_one n 0, }, }, | ||
end |