residuos cuadráticos quadratic residue

Creator:	Lorenia de la Vega
Language pair:	español al inglés
Discipline:	-
Definition / notes:	In mathematics, a number q is called a quadratic residue modulo n if there exists an integer x such that: q≡x2 (mod n). Otherwise, q is called a quadratic non-residue. For prime moduli, roughly half of the residue classes are of each type. More precisely, for prime p > 2, there are (p − 1)/2 of each kind, excluding 0. They occur in a rather random pattern; this has been exploited in applications to acoustics. In effect, a quadratic residue modulo n is a number that has a square root in modular arithmetic when the modulus is n. This concept plays a large part in classical number theory. Complexity of Finding Square Roots: The problem of finding a square root in modular arithmetic, in other words solving the above for x given q and n, can be a difficult problem. For general composite n, the problem is known to be equivalent to integer factorization of n (an efficient solution to either problem could be used to solve the other efficiently). On the other hand, if we want to know if there is a solution for x less than some given limit c, this problem is NP-complete (Adleman, Manders 1978); however, this is a fixed-parameter tractable problem, where c is the parameter.
URL:	http://mathworld.wolfram.com/QuadraticResidue.html