Quasiconvex function

A function \( f: \mathbb{R}^n \to \mathbb{R} \) is called quasiconvex if, for any two points \( x, y \in \mathbb{R}^n \) and for any \( \lambda \in [0, 1] \), the following condition holds: \[ f(\lambda x + (1 - \lambda) y) \leq \max(f(x), f(y)).