In number theory, Euler's criterion is a formula for determining whether an integer is a quadratic residue modulo a prime. Precisely,

Let p be an odd prime and a be an integer coprime to p. Then

${\displaystyle a^{\tfrac {p-1}{2}}\equiv {\begin{cases}\;\;\,1{\pmod {p}}&{\text{ if there is an integer }}x{\text{ such that }}a\equiv x^{2}{\pmod {p}},\\-1{\pmod {p}}&{\text{ if there is no such integer.}}\end{cases}}}$

Euler's criterion can be concisely reformulated using the Legendre symbol:

${\displaystyle \left({\frac {a}{p}}\right)\equiv a^{\tfrac {p-1}{2}}{\pmod {p}}.}$

The criterion first appeared in a 1748 paper by Leonhard Euler.