Algebraic Number Theory
《Algebraic Number Theory》 · 第 1 页 · Section 1.1「Generating functions and asymptotics」
By choosing an appropriate contour, and estimating the integrand, we can determine the asymptotic size of $r_{k,s}(n)$ as $n\to\infty$. This is the germ of the Hardy–Littlewood circle method.