The law of quadratic reciprocity, conjectured by Euler and Legendre and first satisfactorily proved by Gauss, connects the solvability of two related quadratic equations in modular arithmetic. As a consequence, it allows to determine the solvability of any quadratic equation in modular arithmetic.

Suppose p and q are two different odd primes. If at least one of them is congruent to 1 modulo 4, then the congruence

has a solution x if and only if the congruence

has a solution y. (The two solutions will in general be different.) On the other hand, if both primes are congruent to 3 modulo 4, then the congruence

has a solution x if and only if the congruence

does not have a solution y.

Using the Legendre symbol (p/q), these statements may be summarized as

example showing application of this law would be nice

In a book about reciprocity laws published in 2000, Lemmermeyer collects literature citations for 196 different published proofs for the quadratic reciprocity law.

There are cubic, quartic (biquadratic) and other higher reciprocity laws; but since two of the cube roots of 1 are not real, cubic reciprocity is outside the arithmetic of the rational numbers (and the same applies to higher laws).

External links:

• A "play" comparing two proofs of the quadratic reciprocity law