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