# Law of quadratic reciprocity

The **law of quadratic reciprocity** predicts whether an odd prime number *p* is a quadratic residue or non-residue modulo another odd prime number [math]q[/math] if we know whether [math]q[/math] is a quadratic residue or non-residue modulo [math]p[/math].

- If at least one of [math]p[/math] or [math]q[/math] are congruent to 1 mod 4: [math]p[/math] is a quadratic residue modulo [math]q[/math] if and only if [math]q[/math] is a quadratic residue modulo [math]p[/math].
- If both of [math]p[/math] or [math]q[/math] are congruent to 3 mod 4: [math]p[/math] is a quadratic residue modulo [math]q[/math] if and only if [math]q[/math] is a quadratic non-residue modulo [math]p[/math].

This theorem was first proved by Carl Friedrich Gauss in 1801.

This does not cover the cases where we want to know whether -1 or 2 are quadratic residues or non-residues modulo [math]p[/math].

- 2 is a quadratic residue modulo [math]p[/math] if and only if [math]p[/math] is congruent to 1 or 7 (mod 8).
- -1 is a quadratic residue modulo [math]p[/math] if and only if [math]p[/math] is congruent to 1 (mod 4).