Introduction to Analytic Number Theory
《Introduction to Analytic Number Theory》 · 第 306 页 · Section 14.1「Introduction」
Waring's problem is to determine whether, for a given positive integer $k$, there is an integer $s$ (depending only on $k$) such that the equation $n=x_1^k+x_2^k+\cdots+x_s^k$ has solutions for every $n>1$.