rfl
m : ℕ
f : ℕ → ℕ
hf : ∀ (n : ℕ), f (f n) = n + (2 * m + 1)
f_injective : Function.Injective f
NN : Set ℕ := Set.univ
A : Set ℕ := NN \ f '' NN
B : Set ℕ := f '' A
hid : f '' (NN \ f '' NN) = f '' NN \ f '' (f '' NN)
⊢ f '' (NN \ f '' NN) = B
no goals