1. M. Gell-Mann, R. J. Oakes andB. Renner:Phys. Rev.,175, 2195 (1968).
2. M. Gell-Mann:Proceedings of the Third Hawaii Topical Conference on Particle Physics (Los Angeles, Cal., 1969); H. Fritzsch and M. Gell-Mann:Proceedings of the International Conference on Duality and Symmetry in Hadron Physics (Jerusalem, 1971), p. 317.
3. D. G. Sutherland:Phys. Lett.,23, 384 (1966).
4. S. Weinberg:Proceedings of the XVII International Conference on High-Energy Physics, Vol.3 (Chilton, 1974), p. 59. For an extensive review of the difficulties connected with the η see, for instance,R. N. Mohapatra andJ. C. Pati:Phys. Rev. D,8, 4212 (1973) and references therein. In particular:J. S. Bell andD. G. Sutherland :Nucl. Phys.,4 B, 315 (196S);R. N. Mohapatra:Nuovo Cimento,2 A, 707 (1971).
5. S. Weinberg:Phys. Rev. Lett.,31, 494 (1973);Phys. Rev. D,8, 605, 4482 (1973);D. J. Gross andF. Wilczek:Phys. Rev. D,8, 3633 (1973).