On Culler-Shalen Seminorms and Dehn Filling

Abstract

Consider the exterior M of a hyperbolic knot lying in a closed, connected, orientable 3-manifold. Culler and Shalen defined norm on H_1(dM;R) using the SL(2,C) character variety of pi_1(M). The Culler-Shalen norm encodes many topological properties of M; in particular it provides information about Dehn fillings of M. Their construction may be applied to arbitrary curves in the SL_2(C)-character variety of a connected, compact, orientable, irreducible 3-manifold whose boundary is a torus, though in this generality one can only guarantee that it will define a seminorm. The first half of this paper is devoted to the development of the general theory of Culler-Shalen seminorms defined for curves of PSL_2(C)-characters. By working over PSL_2(C) we obtain a theory that is more generally applicable than its SL_2(C) counterpart, while being only mildly more difficult to set up. In the second half of this paper we apply the theory of Culler-Shalen seminorms to study the Dehn filling operation. In particular we examine the relationship between fillings which yield manifolds having a positive dimensional PSL_2(C)-character variety with those that yield manifolds having a finite or cyclic fundamental group. In one interesting application of this work we show that manifolds resulting from a nonintegral surgery on a knot in the 3-sphere tend to have a zero-dimensional PSL_2(C)-character variety. As a consequence we obtain an infinite family of closed, orientable, hyperbolic Haken manifolds which have zero-dimensional PSL_2(C)-character varieties.