Alexander Polynomials: Essential Variables and Multiplicities
1 Laboratoire J.A. Dieudonné, UMR du CNRS 6621, Université de Nice Sophia Antipolis, Parc Valrose, 06108 Nice, France
2 Institute of Mathematics Simion Stoilow, P.O. Box 1-764, RO-014700 Bucharest, Romania
3 Department of Mathematics, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA
Correspondence: * Correspondence to be sent to: a.suciu{at}neu.ed
We explore the codimension-one strata in the degree-one cohomology jumping loci of a finitely generated group, through the prism of the multivariable Alexander polynomial. As an application, we give new criteria that must be satisfied by fundamental groups of smooth, quasi-projective complex varieties. These criteria establish precisely which fundamental groups of boundary manifolds of complex line arrangements are quasi-projective. We also give sharp upper bounds for the twisted Betti ranks of a group, in terms of multiplicities constructed from the Alexander polynomial. For Seifert links in homology 3-spheres, these bounds become equalities, and our formula shows explicitly how the Alexander polynomial determines all the characteristic varieties.