1. Bel, L. (1958).C. R. Acad. Sci. Paris,246, 1094; Robinson, I. (unpublished).
2. Deser, S. (1976).Phys. Lett.,64B, 463.
3. Deser, S. (1967).C. R. Acad. Sci. Paris,264, 805.
4. Lichnerowicz, A. (1955).Theories Relativistes de la Gravitation, Masson, Paris.
5. See for example, Misner, C. W., Thorne, K. S., and Wheeler, J. A. (1973). Section 33.1,Gravitation, Freeman, San Francisco.