1. A.A. Friedman, The World as Space and Time, “Nauka,” Moscow (1965).
2. Gravitation and Relativity, ed. by H.J. Chin and W.F. Hoffman, W.A. Benjamin, Inc., N.Y.(1964).
3. J.A. Wheeler, et.al., Gravitation Theory and Gravitational Collapse. University of Chicago (1965).
4. K. Menger, Proceedings of the National Academy of Science, (U.S.A.), 28, 235 (1942).
5. D.I. Blokhintsev, Report JINR P2-6094, Dubna (1971).