Hermite–Hadamard inequality

In mathematics, the Hermite–Hadamard inequality, named after Charles Hermite and Jacques Hadamard and sometimes also called Hadamard's inequality, states that if a function f : [a, b] → R is convex, then the following chain of inequalities hold:

${\displaystyle f\left({\frac {a+b}{2}}\right)\leq {\frac {1}{b-a}}\int _{a}^{b}f(x)\,dx\leq {\frac {f(a)+f(b)}{2}}.}$

The inequality has been generalized to higher dimensions: if ${\displaystyle \Omega \subset \mathbb {R} ^{n}}$ is a bounded, convex domain and ${\displaystyle f:\Omega \rightarrow \mathbb {R} }$ is a positive convex function, then

${\displaystyle {\frac {1}{|\Omega |}}\int _{\Omega }f(x)\,dx\leq {\frac {c_{n}}{|\partial \Omega |}}\int _{\partial \Omega }f(y)\,d\sigma (y)}$

where ${\displaystyle c_{n}}$ is a constant depending only on the dimension.