1. K Hayashi and T Shirafuji, Phys. Rev. D 19 3524 (1979)
2. A Einstein, Sitzungsber. Preuss. Akad. Wiss. Berlin (Math. Phys.) 778 (1915)
3. R C Tolman, Relativity, Thermodynamics and Cosmology, (Oxford University Press, Oxford), p. 227 (1934)
4. A Papapetrou, Proc. R. Ir. Acad. A52 11 (1948)
5. L D Landau and E M Lifshitz, The Classical Theory of Fields, (Addison-Wesley Press, Reading, MA) p. 317 (1951)