The quadratic reciprocity theorem is a theorem that tells whether a quadratic equation modulo a prime has a solution.
If p and q are distinct odd primes, then the quadratic reciprocity theorem states that the congruences x^2 congruent q (mod p) x^2 congruent p (mod q) are both solvable or both unsolvable unless both p and q leave the remainder 3 when divided by 4 (in which case one of the congruences is solvable and the other is not).
college level