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-- this was compiling with mathlib in June 2020 | |
import tactic | |
theorem imo2019Q1 (f : ℤ → ℤ) : | |
(∀ a b : ℤ, f (2 * a) + 2 * (f b) = f (f (a + b))) ↔ | |
(∀ x, f x = 0) ∨ ∃ c, ∀ x, f x = 2 * x + c := | |
begin | |
split, swap, | |
{ -- easy way: f(x)=0 and f(x)=2x+c work. | |
intro h, |