Every positive integer can be written as the sum of at most 4 square numbers—this is Lagrange’s theorem—or as the sum of 9 or fewer cubes or as the sum of 19 or fewer fourth powers. Waring’s problem (posed in 1770) asks whether, for any given power, there is such a maximum number of those powers needed to express all positive integers as a sum. This was proved true by Hilbert in 1909.