1. Relations related to betweenness;Adeleke;Mem. Amer. Math. Soc.,1998
2. Manifolds of Non-positive Curvature;Ballmann,1999
3. B. Bekka, P. de la Harpe, A. Valette, Kazhdan's property (T), preprint, 2002.
4. Topologie générale, quatrième édition;Bourbaki,1965
5. B. Bowditch, Relatively hyperbolic groups, preprint, Southampton, 1997.