1. H. Georgi, in Particles and Fields (edited by C. E. Carlson), A.I.P., 1975; H. Fritzsch and P. Minkowski, Ann. Phys. 93, 193 (1975).
2. Y. Tosa and S. Okubo, Phys. Rev.
D23, 2486 (1981).
3. R. N. Mohapatra and B. Sakita, Phys. Rev.
D21, 1062 (1980).
4. F. Wilczek and A. Zee, Phys. Rev.
D25, 553 (1982).
5. M. Gell-Mann, P. Ramond, and R. Slansky, in Supergravity (edited by D. Freedman et al. North-Holland, Amsterdam, 1980;