1. J. Ehlers andW. Kundt:Gravitation. An Introduction to Current Research, edited byL. Witten, Chapter 2 (New York, 1962). For more references see for exampleJ. L. Synge:Relativity, The General Theory (1960).
2. See for exampleM. Trümper:Akad. Wiss. Mainz Math. Nat.,12, 969 (1962);Journ. Math. Phys.,6, 584 (1965). Trumper has considered gravitational fields for vacuum spaces. In this note, we shall extend the formalism for nonempty spaces also. In this connection see furtherR. M. Misra andR. A. Singh:Journ. Math. Phys.,7, 1836 (1966).
3. A. Z. Petrov:Recent Developments in General Relativity (London, 1962), p. 375.
4. R. M. Misra andR. A. Singh:Three-Dimensional Formulation of the Gravitational Null Fields—II,Journ. Math. Phys. (to appear).
5. A. S. Lanczos:Ann. Math.,39, 842 (1938).