• [1]

    D.J. Booth,J. Phys. A: Math. Gen., 14(1981), 2325.[2] S.Weinberg,Gravitation and Cosmology, London, Wiley, (1972), 175[3] L.D. Landau, E.M.Lifshitz, The classical theory of fields, Oxford, Pergamon, (1975), 326