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2. Dicke, R. H.: 1964, in Chiu and Hoffman (eds.),Gravitation and Realtivity, W. A. Benjamin, Inc., New York, Amsterdam, p. 121.
3. Horák, Z.: 1973,Bull. Astron. Inst. Czech. 24, 143.
4. Landau, L. D. and Lifshitz, E. M.: 1962,The Classical Theory of Fields, Pergamon Press Ltd., Oxford, London, Paris, Frankfurt, p. 326.
5. Lawden, D. F.: 1971,An Introduction to the Tensor Calculus and Relativity, Methuen & Co. Ltd., London, p. 175 (exercise 7).