Berndt, Bruce C..; Dixit, Atul; Kim, Sun; Zaharescu, Alexandru
(American Mathematical Society, 2017-04)
Let $ r_k(n)$ denote the number of representations of the positive integer $ n$ as the sum of $ k$ squares. In 1934, the Russian mathematician A. I. Popov stated, but did not rigorously prove, a beautiful series transformation ...