\begin{definition}[Definition:Concave Real Function/Definition 2]
Let $f$ be a real function which is defined on a real interval $I$.
$f$ is '''concave on $I$''' {{iff}}:
:$\ds \forall x_1, x_2, x_3 \in I: x_1 < x_2< x_3: \frac {\map f {x_2} - \map f {x_1} } {x_2 - x_1} \ge \frac {\map f {x_3} - \map f {x_2} } {x_3 - x_2}$
Hence a geometrical interpretation: the slope of $P_1 P_2$ is greater than that of $P_2 P_3$:
:500px
\end{definition}