Given a function f· and points x, y ∈ X, with x ≠ y and X convex, if f(y) ≥ f(x) implies that f(λx + (1 − λ)y) > f(x) for all 0 < λ < 1, then we say that f is a strictly quasi-concave function. Concave function; Convex function; Quasi-concave function; Quasi-convex function.