A number 
that possesses no common divisor with a prime number 
is either a quadratic residue
or nonresidue of 
, depending whether 
is congruent mod 
to 
.
Euler's Quadratic Residue Theorem
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Cite this as:
Weisstein, Eric W. "Euler's Quadratic Residue Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/EulersQuadraticResidueTheorem.html