1. M. Rees, R. Ruffini andJ. A. Wheeler:Black Holes, Gravitational Waves and Cosmology: an Introduction to Current Research (New York, N. Y., 1974).
2. See, for instance,E. Amaldi andG. Pizzella:Relativity Quanta and Cosmology in the Development of the Scientific Thought of Einstein (New York, N. Y., 1979).
3. R. P. Giffard:Phys. Rev. D,14, 2478 (1976).
4. J. P. Richard:Acta Astronaut.,5, 63 (1978).
5. G. Pizzella:Nuovo Cimento C,2, 209 (1979).