1. Regge, T. (1961).Nuovo Cimenta,19, 558.
2. Hawking, S. W. (1979). InGeneral Relativity-an Einstein Centenary Survey, eds. S. W. Hawking and W. Israel. (Cambridge University Press, Cambridge).
3. DeWitt, B. S. (1979). InGeneral Relativity-an Einstein Centenary Survey, eds. S. W. Hawking and W. Israel. (Cambridge University Press, Cambridge).
4. Hawking, S. W. (1968).Nucl. Phys. B,144, 349.
5. Gibbons, G. W., and Hawking, S. W. (1977).Phys. Rev. D,15,