1. Bardeen, J. M., Carter, B., and Hawking, S. W., Commun. math. Phys., 31, 161–170 (1973).
2. Hawking, S. W., Mon. Not. R. astr. Soc., 152, 75–78 (1971).
3. Penrose, R., in Relativity, Groups and Topology (edit. by de Witt, C. M., and de Witt, B. S). Les Houches Summer School, 1963 (Gordon and Breach, New York, 1964).
4. Hawking, S. W., and Ellis, G. F. R., The Large-Scale Structure of Space-Time (Cambridge University Press, London 1973).
5. Hawking, S. W., in Black Holes (edit. by de Witt, C. M., and de Witt, B. S), Les Houches Summer School, 1972 (Gordon and Breach, New York, 1973).