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-convex function. Concave function; Convex function.