Paper detail

Asymptotic geometry in higher products of rank one Hadamard spaces

Given a product X of locally compact rank one Hadamard spaces, we study asymptotic properties of certain discrete isometry groups. First we give a detailed description of the structure of the geometric limit set and relate it to the limit cone; moreover, we show that the action of the group on a quotient of the regular geometric boundary of X is minimal and proximal. This is completely analogous to the case of Zariski dense discrete subgroups of semi-simple Lie groups acting on the associated symmetric space. In the second part of the paper we study the distribution of orbit points in X: As a generalization of the critical exponent we consider the exponential growth rate of the number of orbit points in X with a prescribed "slope". We show in particular that this exponential growth rate is strictly positive in the relative interior of the limit cone and that there exists a unique slope for which it is maximal and equal to the critical exponent. We notice that an interesting class of product spaces as above comes from the second alternative in the Rank Rigidity Theorem for CAT(0)-cube complexes: Given a finite-dimensional CAT(0)-cube complex X and a group G of automorphisms without fixed point in the geometric compactification of X, then either G contains a rank one isometry or there exists a convex F-invariant subcomplex of X which is a product of two unbounded cube subcomplexes; in the latter case one inductively gets a convex G-invariant subcomplex of X which can be decomposed into a finite product of rank one Hadamard spaces. So our results imply in particular that classical properties of discrete subgroups of higher rank Lie groups (as stated e.g. by Y. Benoist and J.F. Quint) also hold for certain discrete isometry groups of reducible CAT(0)-cube complexes.