1. A. Einstein:The Meaning of Relativity (Princeton, N. J., 1953), p. 133.
2. E. Schrödinger:Space-Time Structure (Cambridge, 1954).
3. A. Papapetrou:Proc. Roy. Irish Acad.,52, 69 (1948).
4. M. Wyman:Canad. Journ. Math.,2, 427 (1950).
5. H. Takeno, M. Ikeda andS. Abe:Progr. Theor. Phys.,6, 837 (1951).