Steffensen's inequality
# Steffensen's inequality

Steffensen's inequality is an equation in mathematics named after Johan Frederik Steffensen.

It is an integral inequality in real analysis, stating:

    If ƒ : [a, b] → R is a non-negative, monotonically decreasing, integrable function
    and g : [a, b] → [0, 1] is another integrable function, then
       $\int _{b-k}^{b}f(x)\,dx\leq \int _{a}^{b}f(x)g(x)\,dx\leq \int _{a}^{a+k}f(x)\,dx,$
    where
       $k=\int _{a}^{b}g(x)\,dx.$
