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Quadratic residue

In mathematics, a number q is called a quadratic residue modulo p if there exists an integer x such that:

<math>{x^2}\equiv{q}\mbox{ (mod }p\mbox{)}.</math>

Otherwise, q is called a quadratic non-residue.

In effect, a quadratic residue modulo p is a number that has a square root in modular arithmetic when the modulus is p. The Law of quadratic reciprocity says something about quadratic residues and primes.

Quadratic residues are used in the Legendre symbol.

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