1. Bishop, R.L., Goldberg, S.I.: Tensor analysis on manifolds, New York: MacMillan 1968
2. Budic, R., Sachs, R. K.: Set causality in general relativity (preprint, 1973)
3. Carter, B.: Phys. Rev.174, 1559 (1968)
4. Carter, B.: Gen. Rel. Grav.1, 349 (1971)
5. Geroch, R.: J. Math. Phys.11, 437 (1970)