Artin conductor

In mathematics, the Artin conductor is a number or ideal associated to a character of a Galois group of a local or global field, introduced by as an expression appearing in the functional equation of an Artin L-function.

Local Artin conductors
Suppose that L is a finite Galois extension of the local field K, with Galois group G. If $$\chi$$ is a character of G, then the Artin conductor of $$\chi$$ is the number
 * $$f(\chi)=\sum_{i\ge 0}\frac{g_i}{g_0}(\chi(1)-\chi(G_i))$$

where Gi is the i-th ramification group (in lower numbering), of order gi, and χ(Gi) is the average value of $$\chi$$ on Gi. By a result of Artin, the local conductor is an integer. Heuristically, the Artin conductor measures how far the action of the higher ramification groups is from being trivial. In particular, if χ is unramified, then its Artin conductor is zero. Thus if L is unramified over K, then the Artin conductors of all χ are zero.

The wild invariant or Swan conductor of the character is


 * $$ f(\chi) - (\chi(1)-\chi(G_0)), $$

in other words, the sum of the higher order terms with i > 0.

Global Artin conductors
The global Artin conductor of a representation $$\chi$$ of the Galois group G of a finite extension L/K of global fields is an ideal of K, defined to be


 * $$\mathfrak{f}(\chi) = \prod_p p^{f(\chi,p)}$$

where the product is over the primes p of K, and f(χ,p) is the local Artin conductor of the restriction of $$\chi$$ to the decomposition group of some prime of L lying over p. Since the local Artin conductor is zero at unramified primes, the above product only need be taken over primes that ramify in L/K.

Artin representation and Artin character
Suppose that L is a finite Galois extension of the local field K, with Galois group G. The Artin character aG of G is the character


 * $$a_G=\sum_\chi f(\chi)\chi$$

and the Artin representation AG is the complex linear representation of G with this character. asked for a direct construction of the Artin representation. showed that the Artin representation can be realized over the local field Ql, for any prime l not equal to the residue characteristic p. showed that it can be realized over the corresponding ring of Witt vectors. It cannot in general be realized over the rationals or over the local field Qp, suggesting that there is no easy way to construct the Artin representation explicitly.

Swan representation
The Swan character swG is given by


 * $$sw_G= a_G -r_G+1 $$

where rg is the character of the regular representation and 1 is the character of the trivial representation. The Swan character is the character of a representation of G. showed that there is a unique projective representation of G over the l-adic integers with character the Swan character.

Applications
The Artin conductor appears in the conductor-discriminant formula for the discriminant of a global field.

The optimal level in the Serre modularity conjecture is expressed in terms of the Artin conductor.

The Artin conductor appears in the functional equation of the Artin L-function.

The Artin and Swan representations are used to defined the conductor of an elliptic curve or abelian variety.