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2. A. S. Eddington, The Mathematical Theory of Relativity, C. U. P., Cambridge (1923).
3. R. C. Tolman, Phys. Rev.,35, 875 (1930).
4. H. Weyl, Raum. Z. Materie, Springer-Verlag, Berlin (1923), §37.
5. L. D. Landau and E. M. Lifshitz, The Classical Theory of Fields, 4th ed., Pergamon Press, Oxford (1975).