The angle question

The main result of this paper concerns CAT(0) groups Γ which contain an infinite order element in the center. It suffices to consider the case Γ=G×Z here. We show that although the G-factor does not determine a quasi-convex subset inside the space Γ is acting on, we do show that there is a well-defined angle which, for a given action, describes how the G-factor sits inside the space.