1. Bondi, H. (1960).Cosmology, (University Press, Cambridge).
2. Adler, A., Bazin, M. and Schiffer, M. (1965).Introduction to General Relativity, (McGraw Hill, New York).
3. Robertson, H.P. (1935).Astrophys. J.,82, 284.
4. Massa, E. (1971).Commun. Math. Phys.,20, 279.
5. Massa, E. (1971).Commun. Math. Phys.,22, 321.