1. C. Teitelboim, in Einstein Centenary Volume (A. Held, ed., GRG Journal, in press).
2. J.A. Wheeler, in Battelle Rencontres 1967 (C.M. DeWitt and J.A. Wheeler, eds., Benjamin, New York, 1968); also in Analytical Methods in Mathematical Physics (R.P. Gilbert and R. Newton, eds., Gordon and Breach, New York, 1970).
3. C. Teitelboim, Ph.D. Thesis, Princeton University, 1973, unpublished. Ann. Phys. (N.Y.) 79, 542 (1973);
4. S. Hojman, K. Kuchar and C. Teitelboim, Nature, Phys. Sci. 245, 97 (1973);
5. Ann. Phys. (N.Y.) 96, 88 (1976).