Coarse Alexander duality for pairs and applications
Abstract
For a group G (of type F) acting properly on a coarse Poincar\'e duality space X, Kapovich-Kleiner introduced a coarse version of Alexander duality between G and its complement in X. More precisely, the cohomology of G with group ring coefficients is dual to a certain Cech homology group of the family of increasing neighborhoods of a G-orbit in X. This duality applies more generally to coarse embeddings of certain contractible simplicial complexes into coarse PD(n) spaces. In this paper we introduce a relative version of this Cech homology that satisfies the Eilenberg-Steenrod Exactness Axiom, and we prove a relative version of coarse Alexander duality. As an application we provide a detailed proof of the following result, first stated by Kapovich-Kleiner. Given a 2-complex formed by gluing k halfplanes along their boundary lines and a coarse embedding into a contractible 3-manifold, the complement consists of k deep components that are arranged cyclically in a pattern called a Jordan cycle. We use the Jordan cycle as an invariant in proving the existence of a 3-manifold group that is virtually Kleinian but not itself Kleinian.