1. For instance, G.F.R. Ellis, in ”Proceedings of the Tenth International Con- ference on General Relativity and Gravitation”, edited by B. Bertotti, F. De Felice and A. Pascolini, Reidel, Dordrecht, 1984. See also, A. Krasiński, ”In-homogeneous Cosmological Models”, Cambridge University Press, Cambridge, 1997, Chapter 8, and the references therein.
2. M. Kac, Am. Math. Mon. 73(4), 1966, 1.
3. M. Seriu, Physical Review D53, 1996, 6902.
4. M. Seriu, in ”Proceedings of the 8th Workshop on General Relativity and Gravitation”, K,Oohara et. al. (eds.), Niigata University, 1999.
5. M. Seriu, Communications in Mathematical Physics 209, 2000, 393.