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2. H. Yu and M. Sachs, Int. Jour. Theoret. Phys. 13, 73 (1975);
3. H. Yu, Ph.D. thesis, Department of Physics and Astronomy, State University of New York at Buffalo, 1974.
4. M. Sachs, Lettr. Nuovo Cimento 13, 169 (1975).
5. M. Sachs and S. L. Schwebel, Jour. Math. Phys. 3, 843 (1962).