Abstract. Let p be a prime number. We study certainétale cohomology groups with coefficients associated to a p-adic Artin representation of the Galois group of a number field k. These coefficients are equipped with a modified Tate twist involving a p-adic index. The groups are cofinitely generated, and we determine the additive Euler characteristic. If k is totally real and the representation is even, we study the relation between the behaviour or the value of the p-adic L-function at the point e in its domain, and the cohomology groups with p-adic twist 1 − e. In certain cases this gives short proofs of a conjecture by Coates and Lichtenbaum, and the equivariant Tamagawa number conjecture for classical L-functions. For p = 2 our results involving p-adic L-functions depend on a conjecture in Iwasawa theory.
IntroductionLet k be a number field, p a prime number, E a finite extension of Q p with valuation ring O E , and η : G k → E an Artin character with dual character η ∨ , that is, the character of an Artin representation of G k = Gal(k/k). If S is a finite set of finite primes of k, m ≤ 0 an integer, and σ :in E then this is independent of σ (see Section 3). We call η realizable over E if the corresponding representation can be defined over E. This representation can then be obtained as M (E, η)⊗ OE E ≃ M (E, η)⊗ Zp Q p for some finitely generated torsion-free O E -module M (E, η) on which G k acts (we shall call M (E, η) an O E -lattice for η). If S includes all the finite primes of k at which η is ramified, and O k,S is obtained from the ring of algebraic integers O k of k by inverting all primes in S, then we may view M (E, η) and M (E, η) ⊗ OE E/O E ≃ M (E, η) ⊗ Zp Q p /Z p as sheaves for theétale topology on the open subscheme Spec O k,S of Spec O k . We let α : Spec O k,S → Spec O k be the inclusion, but inétale cohomology groups we shall write O k instead of Spec O k and similarly for O k,S .In the special case that p is odd, E = Q p , m < 0, and L * S (m, η ∨ , k) = 0, according to Conjecture 1 of [11] we should have that theétale cohomology groupsare finite for all i ≥ 0, trivial for i > 3, and that ) for some lattice M (E, η ∨ ) (see Remark 1.5). Using this duality the proof of Báyer-Neukirch relates the right-hand side of (1.1) to the p-adic absolute value of the value of a certain p-adic L-function at m, which equals the left-hand side of (1.1) by an interpolation formula (see (1.7)).