1. Latin indices range and sum over 1, 2, 3, 4. The metric is of signature +2.
2. L. P. Eisenhart:Riemannian Geometry (Princeton, N. J., 1966), p. 113.
3. For the general theory of parallel planes see, for instance,A. G. Walker:Quart. Journ. Math.,20, 135 (1949);T. J. Willmore:An Introduction to Differential Geometry (London, 1959), p. 242.
4. L. Witten, Editor:Gravitation: an Introduction to Current Research (New York, N. Y., and London, 1962), p. 86.
5. V. D. Zakharov:Gravitational Waves in Einstein's Theory (Jerusalem, London, and New York, N. Y., 1973), p. 90.