1. Bieberbach, L., Ueber die Bewegungsgruppen der Euklidischen Räume, I: Math. Ann. 70 (1911) 297–336; II: Math. Ann. 72 (1912) 400–412.
2. Buser, P., Karcher, H., Gromov’s almost flat manifold theorem (to appear).
3. Gromov, M., Almost flat manifolds, J. Differential Geometry 13 (1978) 231–241.
4. Wolf, J., Spaces of constant curvature. McGraw Hill, New York (etc.) 1967.