Limit sets for modules over groups on CAT(0) spaces -- from the Euclidean to the hyperbolic

Abstract

The observation that the 0-dimensional Geometric Invariant $Σ^{0}(G;A)$ of Bieri-Neumann-Strebel-Renz can be interpreted as a horospherical limit set opens a direct trail from Poincaré's limit set $Λ(Γ)$ of a discrete group $Γ$ of Möbius transformations (which contains the horospherical limit set of $Γ$) to the roots of tropical geometry (closely related to $Σ^{0}(G;A)$ when G is abelian). We explore this trail by introducing the horospherical limit set, $Σ(M;A)$, of a G-module A when G acts by isometries on a proper CAT(0) metric space M. This is a subset of the boundary at infinity of M. On the way we meet instances where $Σ(M;A)$ is the set of all conical limit points, the complement of a spherical building, the complement of the radial projection of a tropical variety, or (via the Bieri-Neumann-Strebel invariant) where it is closely related to the Thurston norm.