1. The importance of this requirement in general relativity is discussed in a paper to appear inPhys. Rev.,116, (1959) Dec. 1 issue.
2. R. Arnowitt andS. Deser:Phys. Rev.,113, 745 (1959).
3. Y. Foures-Bruhat:Journ. Rational Mech. Anal.,4, 951 (1956).
4. See alsoC. W. Misner andJ. A. Wheeler:Ann. Phys.,2, 589 (1957).
5. A. Lichnerowicz:Journ. Math. Pure Appl., (9)23, 37 (1944).