1. C. Anantharaman-Delaroche, J. Renault, Monographies de L’Enseignement Mathématique, 36, L’Enseignement Mathmatique, Geneva, 2000.
2. I.R. Aitchison, J.H. Rubinstein, An introduction to polyhedral metrics of non-positive curvature on 3-manifolds, in: S.K. Donaldson, C.B. Thomas (Eds.), Geometry of Low-dimensional Manifolds: 2 Symplectic Manifolds and Jones–Witten Theory, London Mathematical Society Lecture Notes Series, vol. 151, Cambridge University Press, Cambridge, 1990, pp. 127–161.
3. M.R. Bridson, A. Haefliger, Metric spaces of non-positive curvature Grundlehren der mathematischen Wissenschaften, vol. 319, Springer, Berlin, 1999.
4. M. Gromov, Hyperbolic groups, in: S.M. Gersten (Ed.), Essays on Group Theory (Mathematical Sciences Research Institute Publications), Springer, Berlin, 1987.