Connectivity properties of group actions on non-positively curved spaces II: The geometric invariants

Abstract

This is the second of two papers but has been written so as to have minimal dependence on the first paper (which is also on this archive). Let G be a group and let M be a CAT(0) proper metric space (e.g. a simply connected complete Riemannian manifold of non-positive sectional curvature or a locally finite tree). Assume G is of type F_n (type F_1 is finitely generated, type F_2 is finitely presented etc.) The "boundary", bdM, of M at infinity has two customary topologies - the compact "cone" topology and the Tits metric topology. We associate with any isometric action of G on M two subsets of bdM, both dependent on n. These subsets consist of those points of bdM at which - in two senses - the action is "controlled (n-1)-connected". One of these sets is open in the Tits metric topology. Even in classical cases like familiar groups of isometries of the hyperbolic plane or of a locally finite tree these sets seem to be new and interesting invariants. The "SIGMA-theory" of Bieri-Neumann-Strebel-Renz is recovered in the special case in which M is G(abelianized) tensor R with the translation action.