Carmeli, M., Fickler, S. I. and Witten, L., Eds. (1970).Relativity, Proceedings of the Relativity Conference in the Midwest. Plenum Press, New York. Pp. 43–53.
Baierlein, R. F., Sharp, D. H. and Wheeler, J. A. (1962).Phys. Rev.,126, 1864–1865.
Komar A. (1971). To be published (Phys. Rev.).
Choquet-Bruhat, Y., private communication (to A. Komar).