1. Robinson, I. (1961).J. Math. Phys.,2, 290.
2. Pirani, F. A. E. (1965). ?Introduction to Gravitational Radiation Theory,? in Trautman, A., Pirani, F. A. E., and Bondi, H., Lectures in General Relativity, Brandies 1964 Summer Institute of Theoretical Physics, Vol. I. Prentice-Hall, Englewood Cliffs, New Jersey.
3. Synge, J. L. (1956).Relativity: The Special Theory, (North-Holland, Amsterdam).
4. Mariot, L. (1954).C. R. Acad. Sci.,238, 2055.
5. Bampi, F., and Jacassi, P. (1977).Boll. Unione Mat. Ital.,16, 286.