1. H. Weyl:Ann. der Phys.,59, 101 (1919).
2. H. Weyl:Space-Time-Matter (New York, N. Y., 1922).
3. J. L. Synge andA. Schild:Tensor Calculus, Mathematical Expositions (Toronto, 1949).
4. P. G. Bergmann:Introduction to the Theory of Relativity (New York, N. Y., 1947).
5. The Bianchi identities follow directly from the definition of the covariant derivative. Since this definition is postulated for any action, the Bianchi identities are action independent.