1. The fundamental group of the orbit space of a discontinuous group
2. N. Bourbaki , Éléments de mathématique Topologie générale Chapitres 1 à 4 (Hermann, Paris, 1971).
3. N. Bourbaki , Groupes et algèbres de Lie (Hermann, Paris, 1968). (French).
4. G. E. Bredon , Introduction to compact transformation groups, Pure and Applied Mathematics 46 (Academic Press, New York, London, 1972).
5. K. S. Brown , Cohomology of groups, Graduate Texts in Mathematics 87 (Springer, Berlin, 1994).