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Encyclopedia > Quadratic residue

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

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. Quadratic reciprocity and the Gauss lemma both reason about quadratic residues.


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  Results from FactBites:
 
Reciprocity Laws. Rule of Quadratic Reciprocity - Numericana (1523 words)
Quadratic residues: Half of the nonzero residues modulo an odd prime p.
Euler's criterion: A quadratic residue raised to the power of (p-1)/2 is 1.
In particular, g itself can't be a quadratic residue (the order of g must be p-1, and it would be at most (p-1)/2 if g was congruent to the square of some x, since the order of x divides p-1, by Fermat's little theorem).
quadratic equation: Definition and Much More from Answers.com (1807 words)
In mathematics, a quadratic equation is a polynomial equation of the second degree.
Quadratic equations are called quadratic because quadratus is Latin for "square"; in the leading term the variable is squared.
For quadratic equations with integer coefficients, if the discriminant is a perfect square, then the roots are rational numbers—in other cases they may be quadratic irrationals.
  More results at FactBites »


 

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