An inequality that applies to a function y = f (x ) that is continuous and strictly increasing for x ≧ 0 and satisfies f (0) = 0, with inverse function x = g(y ); it states that, for any positive numbers a and b in the ranges of x and y, respectively, the product ab is equal to or less than the sum of the integral from 0 to a of f (x )dx and the integral from 0 to b of g(y )dy.