1. Firmani, C.: 1971,Astrophys. Space Sci. 13, 128.
2. Gomide, F. M.: 1973,Revista Brasileira de Fisica 1, 3.
3. Graves, J. C.: 1971,The Conceptual Foundations of Contemporary Relativity Theory, Cambridge, Mass.
4. Hawking, S. W. and Ellis, G. F. R.: 1973,The Large Scale Structure of Space-Time, Cambridge.
5. Heller, M.: 1970,Acta Phys. Pol. B1, 131.