English
If f is monotone on a set s, and a,b ∈ s, then f(max a b) = max(f(a), f(b)).
Русский
Если f монотонно на множестве s и a,b ∈ s, то f(max(a,b)) = max(f(a), f(b)).
LaTeX
$$$ f(\max a b) = \max(f(a), f(b)) $$$
Lean4
theorem map_max (hf : MonotoneOn f s) (ha : a ∈ s) (hb : b ∈ s) : f (max a b) = max (f a) (f b) := by
rcases le_total a b with h | h <;> simp only [max_eq_right, max_eq_left, hf ha hb, hf hb ha, h]