The function y = f (x) is called convex downward (or concave upward) if for any two points x1 and x2 in [a, b], the following inequality holds: If this inequality is strict for any x1, x2 ∈ [a, b], such that x1 ≠ x2, then the function f (x) is called strictly convex downward on the interval [a, b]. Similarly, we define a concave function.