Cyclotomic character

In number theory, a cyclotomic character is a character of a Galois group giving the Galois action on a group of roots of unity. As a one-dimensional representation over a ring R, its representation space is generally denoted by R(1) (that is, it is a representation χ : G → Aut_R(R(1)) ≈ GL(1, R)).

p-adic cyclotomic character

If p is a prime, and G is the absolute Galois group of the rational numbers, the p-adic cyclotomic character is a group homomorphism

$\chi_p:G\rightarrow\mathbf{Z}_p^\times$

where Z_p^× is the group of units of the ring of p-adic integers. This homomorphism is defined as follows. Let ζ_n be a primitive pⁿ root of unity. Every pⁿ root of unity is a power of ζ_n uniquely defined as an element of the ring of integers modulo pⁿ. Primitive roots of unity correspond to the invertible elements, i.e. to (Z/pⁿ)^×. An element g of the Galois group G sends ζ_n to another primitive pⁿ root of unity

$\theta=\zeta_n^{a_{g,n}}$

where a_g,n ∈ (Z/pⁿ)^×. For a given g, as n varies, the a_g,n form a comptatible system in the sense that they give an element of the inverse limit of the (Z/pⁿ)^×. This inverse limit is exactly Z_p^×. Therefore, the p-adic cyclotomic character sends g to the system (a_g,n)_n, thus encoding the action of g on all p-power roots of unity.

In fact, χ_p is a continuous homomorphism (where the topology on G is the Krull topology, and that on Z_p^× is the profinite topology).

As a compatible system of ℓ-adic representations

By varying ℓ over all prime numbers, a compatible system of ℓ-adic representations is obtained from the ℓ-adic cyclotomic characters (when considering compatible systems of representations, the standard terminology is to use the symbol ℓ to denote a prime instead of p). That is to say, χ = { χ_ℓ }_ℓ is a "family" of ℓ-adic representations

$\chi_\ell:G_\mathbf{Q}\rightarrow\operatorname{GL}_1(\mathbf{Z}_\ell)$

satisfying certain compatibilities between different primes. In fact, the χ_ℓ form a strictly compatible system of ℓ-adic representations.

Geometric realizations

The p-adic cyclotomic character is the p-adic Tate module of the multiplicative group scheme G_m,Q over Q. As such, its representation space can be viewed as the inverse limit of the groups of pⁿth roots of unity in Q.

In terms of cohomology, the p-adic cyclotomic character is the dual of the first p-adic étale cohomology group of G_m. It can also be found in the étale cohomology of a projective variety, namely the projective line: it is the dual of H²_ét( P¹ ).

In terms of motives, the p-adic cyclotomic character is the p-adic realization of the Tate motive Z(1). As a Grothendieck motive, the Tate motive is the dual of H²( P¹ ).^[1]

Properties

The p-adic cyclotomic character satisfies several nice properties.

• It is unramified at all primes ℓ ≠ p (i.e. the inertia subgroup at ℓ acts trivially).
• If Frob_ℓ is a Frobenius element for ℓ ≠ p, then χ_p(Frob_ℓ) = ℓ
• It is crystalline at p.

See also

• Tate twist

References

1. ^ Section 3 of Deligne, Pierre (1979), "Valeurs de fontions L et périodes d'intégrales", in Borel, Armand; Casselman, William (in French), Automorphic Forms, Representations, and L-Functions, Proceedings of the Symposium in Pure Mathematics, 33.2, Providence, RI: AMS, p. 325, ISBN 0-8218-1437-0, MR0546622, http://www.ams.org/online_bks/pspum332/pspum332-ptIV-8.pdf