1. The topological Helly theorem,;S. A. Bogatyĭ;\emph{Russian,2002
2. Metric Spaces of Non-Positive Curvature,;M. R. Bridson;Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences],1999
3. Polyhedral Finsler spaces with locally unique geodesics,;D. Burago;\emph{Adv. Math.},2013
4. Spaces with non-positive curvature,;H. Busemann;\emph{Acta Math.},1948
5. Helly type theorems derived from basic singular homology,;H. E. Debrunner;\emph{Amer. Math. Monthly},1970