1. Ellis, G. F. R.: 1971, in R. K. Sachs (ed.),General Relativity and Cosmology, Academic Press, New York, p. 117.
2. Landau, L. D. and Lifshitz, E. M.: 1963,Fluid Mechanics, Vol. 6, p. 505.
3. Roy, S. R. and Prakash, S.: 1977,Ind. J. Pure Appl. Math. 8(6), 723.