1. Collins, C. B. (1986).Can. J. Phys.,64, 191.
2. Carminati, J. (1987).J. Math. Phys.,28, 1848.
3. Lang, J. M., and Collins, C. B. (1987). Observationally homogeneous shear-free perfect fluids, preprint, University of Waterloo.
4. Collins, C. B., and Wainwright, J. (1983).Phys. Rev. D,27, 1209.
5. Ellis, G. F. R. (1971). InGeneral Relativity and Cosmology, Proceedings of the International School of Physics ?Enrico Fermi? Course XLVII (dy1969), Sachs, R. K., ed. (Academic, New York/London), p. 104.