1. E. Newman and R. Penrose, J. Math. Phys., 3:566 (1962).
2. A. Z. Petrov, Uchen. Zap. Kazan. Univ., Vol. 114, No. 8 (1954).
3. A. Z. Petrov, New Methods in the General Theory of Relativity [in Russian], Nauka, Moscow (1966).
4. F. A. E. Pirani, Phys. Rev., 105:1089 (1957).
5. F. A. E. Pirani, Lectures on General Relativity, in: Brandeis Summer Institute in Theoretical Physics, Vol. 1 (1964), p. 249.