We introduce a space of stable meromorphic differentials with poles of prescribed orders and define its tautological cohomology ring. This space, just as the space of holomorphic differentials, is stratified according to the set of multiplicities of zeros of the differential. The main goal of this paper is to compute the Poincar 'e-dual cohomology classes of all strata. We prove that all these classes are tautological and give an algorithm to compute them. In a second part of the paper we study the Picard group of the strata. We use the tools introduced in the first part to deduce several relations in these Picard groups.
We assume that @math . In the appendix of @cite_11 , Farkas and Pandharipande defined a class @math in @math (or @math ): this class is a weighted sum over the classes of irreducible components.
@cite_11
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The class of @math -complete problems is a class of computationally eqivalent counting problems (defined by the author in a previous paper) that are at least as difficult as the @math -complete problems. Here we show, for a large number of natural counting problems for which there was no previous indication of intractability, that they belong to this class. The technique used is that of polynomial time reduction with oracles via translations that are of algebraic or arithmetic nature.