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3. Hall, G. S. (1984)Diff. Geom. 12, 53. This reference contains an extensive bibliography on the classification of the Ricci tensor on 4D Lorentzian spacetime manifolds.
4. Hall, G. S. (1984).Arab. J. Sci. Eng. 9, 87.
5. Hall, G. S., and Negm, D. A. (1986).Int. J. Theor. Phys. 25, 405.