非線形問題と複素幾何学
《非線形問題と複素幾何学》 · 第 15 页 · Section 2.1「Dirichlet convolution and switching divisors」
The left side of (2.5) can be visualized geometrically as the number of integral points in the disc of radius $\sqrt{x}$ and (2.5) can alternatively be deduced by the Gauss method of packing the disc with unit squares. One can also view the Gauss circle problem as an additive convolution problem.