1. Jacobs, K., Nature, 215, 1156 (1967).
2. Wheeler, J. A., in Gravitation and Relativity (edit. by Chiu and Hoffmann) (Benjamin, New York, 1964).
3. Chiu, H. Y., Ann. Phys., 43, 1 (1967).
4. Robertson, Ap. J., 82, 284 (1935); Walker, Proc. Lon. Math. Soc., Ser. 2, 42, 90 (1937).
5. Heckman, O., Schucking, E., in Gravitation (edit. by Witten, L.) (Wiley, New York, 1963) and the references cited there.