Definition:Concave Real Function/Definition 3

Definition
Let $f$ be a real function which is defined on a real interval $I$. $f$ is concave on $I$ iff:


 * $\displaystyle \forall x_1, x_2, x_3 \in I: x_1 < x_2< x_3: \frac {f \left({x_2}\right) - f \left({x_1}\right)} {x_2 - x_1} \ge \frac {f \left({x_3}\right) - f \left({x_1}\right)} {x_3 - x_1}$

Hence a geometrical interpretation: the slope of $P_1 P_2$ is greater than that of $P_1 P_3$:


 * ConcaveFunction3.png

Also see

 * Equivalence of Definitions of Concave Real Function