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2. W. Heisenberg,Introduction to the Unified Field Theory of Elementary Particles (Interscience, Wiley, New York, 1966).
3. J. J. J. Kokkedee,The Quark Model (Benjamin, New York, 1969).
4. S. D. Drell, SLAC-PUB-1683 (November 1975).
5. T. A. de Grond and R. L. Jaffe, Excited States of Confined Quarks, MIT preprint #529 (March 1976).