1. F. Gursey, Erice Lectures 1977.
2. H.J. Lipkin, in Symmetry Principles at High Energy, Fifth Coral Gables Conf., eds. A. Permutter, C.A. Hurst and B. Kursunoglu (Benjamin, 1968), p. 261.
3. S. Weinberg, Phys. Rev. Letters 19 (1967) 1264.
4. A. Salam, in Elementary Particle Theory, ed. N. Svartholm (Almquist and Forlag, Stockholm, 1968).
5. E.S. Abers and B.W. Lee, Phys. Reports 9C (1973) 1.