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3. S. Weinberg, Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity, (Wiley, New York, 1972).
4. A. Einstein, Collection of Science Works (Nauka, Moscow, 1965), Vol. 1 [in Russian].
5. D. Hilbert, Math. Ann. 92(1–2), 1 (1924).