Question

Let X ---> Y be a finite surjective morphism of smooth, projective, connected varieties over a finite field F_q. Can one describe the zeta function Z(X, t) in terms of the zeta-function Z(Y,t) of Y? Can one say anything at all about how they are related? What if we assume the morphism is finite etale?

Answer 1

I don't think you can say much of anything of consequence unless you have better control over things. As an example, consider the kth power map A^1 -> A^1. This is finite, and surjective and even etale if you throw out 0, and has degree k, but the zeta functions are the same.

Another way of rerephrasing Ilya's comment is that it's not just varieties at have zeta functions; all mixed sheaves have them. The zeta function of X is the zeta function of the pushforward of the constant sheaf on X to Y. Now it may be that you can say something useful about this sheaf (for example, if you have a cover, it is a local system, and you can think about L-functions of pi_1 representations) but in general, that sheaf could be pretty horrible.

Answer 2

You should be looking not at just zeta functions, but at the L-functions.

Then yes, for a finite etale Galois morphism the identity should be

        Z(Y) = Z(X) * L(X, \pi_1) * L(X, \pi_2) * ...

(where the product is over summands of the regular representation of Galois group of the morphism, Z being the L-function of the trivial representation.) This is in no way restricted to finite fields — in fact the idea as well as the notation comes from theorem about Dirichlet L-functions.

The proof is that by a definition of what is L-function it can be written either for a trivial mixed sheaf on Y (LHS) or for its pushforward on X (RHS).