1. L. Auslander, E. Schenkman, Free groups and Hirsch-Plotkin radicals,Proc. A.M.S.,16 (1965), 784–788.
2. J. Cheeger, D. Ebin,Comparison theorems in Riemannian geometry, North-Holland, 1975.
3. V. Efremovič, The proximity geometry of Riemannian manifolds,Uspehi Mat. Nauk,8 (1953), 189 (Russian).
4. D. Epstein, M. Shub, Expanding endomorphisms of flat manifolds,Topology,7 (1968), 13–141.
5. M. Gromov, Synthetic geometry of Riemannian manifolds,Proc. I.C.M. Helsinki, 1978, vol. I, 415–419 (1979).