LECTURE NOTES IN ANALYSIS (2011)
《LECTURE NOTES IN ANALYSIS (2011)》 · 第 101 页 · Section 2.8，Corollary 2.38
Euler's criterion. If $p$ is an odd prime and $(a,p)=1$, then $x^2=a\pmod p$ has two solutions or no solution according as $a^{(p-1)/2}=1$ or $-1\pmod p$.