Subspace Arrangements over Finite Fields: Cohomological and Enumerative Aspects
✍ Scribed by Anders Björner; Torsten Ekedahl
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 424 KB
- Volume
- 129
- Category
- Article
- ISSN
- 0001-8708
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✦ Synopsis
The enumeration of points on (or off) the union of some linear or affine subspaces over a finite field is dealt with in combinatorics via the characteristic polynomial and in algebraic geometry via the zeta function. We discuss the basic relations between these two points of view. Counting points is also related to the l-adic cohomology of the arrangement (as a variety). We describe the eigenvalues of the Frobenius map acting on this cohomology, which corresponds to a finer decomposition of the zeta function. The l-adic cohomology groups and their decomposition into eigenspaces are shown to be fully determined by combinatorial data. Finally, it is shown that the zeta function is determined by the topology of the corresponding complex variety in some important cases.
1997 Academic Press
1. Introduction
This paper is concerned with subspace arrangements over general (in particular finite) fields and with their enumerative and cohomological invariants. In this introduction we will summarize the main results. We begin with a review of some algebraic geometry.
Let V be a d-dimensional projective variety over the field F q of order q= p : . (We will not assume that a variety is irreducible.