The structure of limit groups over hyperbolic groups

Abstract

Let $\Gamma$ be a torsion-free hyperbolic group. We study $\Gamma$--limit groups which, unlike the fundamental case in which $\Gamma$ is free, may not be finitely presentable or geometrically tractable. We define model $\Gamma$--limit groups, which always have good geometric properties (in particular, they are always relatively hyperbolic). Given a strict resolution of an arbitrary $\Gamma$--limit group $L$, we canonically construct a strict resolution of a model $\Gamma$--limit group, which encodes all homomorphisms $L\to \Gamma$ that factor through the given resolution. We propose this as the correct framework in which to study $\Gamma$--limit groups algorithmically. We enumerate all $\Gamma$--limit groups in this framework.

Version 2 is 54 pages. Changes made according to comments of referee. To appear in Israel Journal of Mathematics