Euler's criterion

Etymology
Named after (1707–1783), Swiss mathematician and physicist, who devised the criterion in 1748.

Proper noun

 * 1)  A theorem which states that, given a positive integer a which is coprime to an odd prime number p, a is a quadratic residue of p if and only if a^{(p-1)/2} is congruent to 1 modulo p.

Usage notes

 * The calculation on the left side of the congruence — or the result thereof — can be denoted with the Legendre symbol.