1. Asgekar, G. G., and Date, T. H. (1975–1976).Proceedings of the International Symposium on Relativity and Unified Field Theory, pp. 269–272, Bose Institute, Calcutta.
2. Carmeli, M. (1977).Group Theory and Relativity, Chap. 10. McGraw-Hill, New York.
3. Chandrasekhar, (1979).General Relativity, An Einstein Centenary Survey, S. W. Hawking and W. Israel, eds., Chap. 7. Cambridge University Press, Cambridge.
4. Collinson, C. D. (1969).Journal of Physics A (General Physics),2, 621–623.
5. Collinson, C. D. (1970).General Relativity and Gravitation,1, 137–142.