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3. Hansen, R. O., and Newman, E. T. preprint.
4. Penrose, R. (1968). “The Structure of Space-Time,” in:Battell Rencontres in Mathematics and Physics, ed. Dewitt, C. (W. A. Benjamin Inc., New York).
5. Newman, E. T., and Penrose, R. (1966).J. Math Phys., NY,7, 863; Goldberg, J. N., Macfarlane, A. J., Newman, E. T., Rohrlick, F., and Sudarshan, E. C. G. (1967).J. Math. Phys., NY,8, 2155.