1. Anderson, J. L.: 1969, in M. Carmeli, S. Fickler, and L. Witten (eds.),Relativity-Proceedings of the Relativity Conference in the Midwest, New York.
2. Hawking, S. W. and Penrose, R.: 1970,Proc. Roy. Soc. Lond. A314, 529.
3. Hopf, E.: 1950,Comm. Pure and Appl. Math. 3, 201.
4. Hoyle, F.: 1958,La Structure et l'évolution de l'univers, Bruxelles, p. 53.
5. Israel, W. and Vardalas, J. N.: 1970,Nuovo Cimento Letters 4, 887.