Bentham's Utilitarianism is defined as pursuing "the greatest happiness of the greatest number", but it contains two optimization goals, "greatest happiness" and "greatest number" and in common cases, you cannot guarantee both are satisfied. The decision that causes the greatest happiness for some people maybe is not the one causing happiness for the greatest number of people, and the decision causing happiness for the greatest number of people maybe is not the one causing the greatest happiness among some people. Is there a math expression showing what exactly Bentham refers to?
It seems better defined as "the greatest summation of happiness". For the set of decisions $d_1, ..., d_n$, and for m people whose utility functions are U_1(), ..., U_m() respectively, choose the decision d that:
d = argmax_{d \in {d_1, ..., d_n}} \sum_{i=1}^m U_i(d)