In mathematics, a reciprocity law is a generalization of the law of quadratic reciprocity to arbitrary monic irreducible polynomials
splits into linear terms when reduced mod
For a general reciprocity law[1]pg 3, it is defined as the rule determining which primes
The early reciprocity laws found in the 19th century were usually expressed in terms of a power residue symbol (p/q) generalizing the quadratic reciprocity symbol, that describes when a prime number is an nth power residue modulo another prime, and gave a relation between (p/q) and (q/p).
Hilbert reformulated the reciprocity laws as saying that a product over p of Hilbert norm residue symbols (a,b/p), taking values in roots of unity, is equal to 1.
Artin reformulated the reciprocity laws as a statement that the Artin symbol from ideals (or ideles) to elements of a Galois group is trivial on a certain subgroup.
Several more recent generalizations express reciprocity laws using cohomology of groups or representations of adelic groups or algebraic K-groups, and their relationship with the original quadratic reciprocity law can be hard to see.
The name reciprocity law is still used in the more general context of splittings.
In terms of the Legendre symbol, the law of quadratic reciprocity states Using the definition of the Legendre symbol this is equivalent to a more elementary statement about equations.
In this terminology the law of quadratic reciprocity is stated as follows.
This establishes the bridge from the name giving reciprocating behavior of primes introduced by Legendre to the splitting behavior of polynomials used in the generalizations.
The law of cubic reciprocity for Eisenstein integers states that if α and β are primary (primes congruent to 2 mod 3) then In terms of the quartic residue symbol, the law of quartic reciprocity for Gaussian integers states that if π and θ are primary (congruent to 1 mod (1+i)3) Gaussian primes then Suppose that ζ is an
The Eisenstein reciprocity law states that for a any rational integer coprime to
Suppose that ζ is an lth root of unity for some odd regular prime l. Since l is regular, we can extend the symbol {} to ideals in a unique way such that The Kummer reciprocity law states that for p and q any distinct prime ideals of Z[ζ] other than (1–ζ).
In terms of the Hilbert symbol, Hilbert's reciprocity law for an algebraic number field states that where the product is over all finite and infinite places.
Over the rational numbers this is equivalent to the law of quadratic reciprocity.
In the language of ideles, the Artin reciprocity law for a finite extension L/K states that the Artin map from the idele class group CK to the abelianization Gal(L/K)ab of the Galois group vanishes on NL/K(CL), and induces an isomorphism Although it is not immediately obvious, the Artin reciprocity law easily implies all the previously discovered reciprocity laws, by applying it to suitable extensions L/K.
For example, in the special case when K contains the nth roots of unity and L=K[a1/n] is a Kummer extension of K, the fact that the Artin map vanishes on NL/K(CL) implies Hilbert's reciprocity law for the Hilbert symbol.
Hasse introduced a local analogue of the Artin reciprocity law, called the local reciprocity law.
One form of it states that for a finite abelian extension of L/K of local fields, the Artin map is an isomorphism from
A power reciprocity law may be formulated as an analogue of the law of quadratic reciprocity in terms of the Hilbert symbols as[3] A rational reciprocity law is one stated in terms of rational integers without the use of roots of unity.
The Langlands program includes several conjectures for general reductive algebraic groups, which for the special of the group GL1 imply the Artin reciprocity law.
Yamamoto's reciprocity law is a reciprocity law related to class numbers of quadratic number fields.