1. A. V. Nosovets, Dep. No. 4184-82, VINITI (1982).
2. O. Buchdahl, Phys. Rev.,115, No. 6, 1323 (1959).
3. K. B. Harrison, Phys. Rev.,138, No. 3, 488 (1965).
4. K. B. Harrison, J. Math. Phys.9, No. 11, 1744 (1968).
5. E. Volker, Zeitschrift fur Naturforschung,22A, 1361 (1967).