A strong Schottky lemma on $n$ generators for $\mathrm{CAT}(0)$ spaces

Abstract

We give a criterion for a set of $n$ hyperbolic isometries of a $\mathrm{CAT}(0)$ metric space $X$ to generate a free group on $n$ generators. This extends a result by Alperin, Farb and Noskov who proved this for 2 generators under the additional assumption that $X$ is complete and has no fake zero angles. Moreover, when $X$ is locally compact, the group we obtain is also discrete.

arXiv comment7 pages, final version to appear in Münst. J. Math