1. Robertson, H. P., and Noonan, T. W. (1969).Relativity and Cosmology, (W. B. Saunders, Philadelphia).
2. Eisenhart, L. P. (1933).Continuous Groups of Transformations, (Princeton University Press, Princeton, New Jersey).
3. Petrov, A. Z. (1966).Novye metody v obshchei teorii otnositel'nosti, (Nauka, Moscow).
4. Das, A., and Agrawal, P. (1974).Gen. Rel. Grav.,5, 359.
5. Bekenstein, J. D. (1974).Ann. Phys. N.Y.,82, 535.