1. Ehlers, J. (1971). ?General Relativity and Kinetic Theory,? inGeneral Relativity and Cosmology, Proceedings of the course 47 of the International School of Physics ?Enrico Fermi?, ed. Sachs, R. K., Academic, New York.
2. Vaidya, P. C. (1951).Proc. Indian Acad. Sci.,A33, 264.
3. Noerdlinger, P. D. (1976).Gen. Rel. Grav.,7, 239.
4. Evans, A. B. (1977).Gen. Rel. Grav.,8, 155.