1. C. W. Misner, K. S. Thorne andJ. A. Wheeler:Gravitation (San Francisco, Cal., 1973).
2. R. Ruffini:International Meeting on Experimental Gravitation, Pavia, Italy, 1976.
3. E. M. Lifshitz andI. M. Khalatnikov:Adv. Phys.,12, 185 (1963).
4. O. Heckmann andE. Schucking: inGravitation, an Introduction to Current Research, edited byL. Witten (New York, N. Y., 1962).
5. A. G. Doroshkevich, Y. B. Zel'dovich andI. D. Novikov:Astrophyz.,5, 15 (1969).