1. S. Weinberg: Phys. Rev. Lett.19, 1264 (1967)
2. A. Salam: in Elementary particle theory, relativistic groups and analyticity, Ed. M. Svarthalm M. Stockholm: 1968
3. G.G. Branco, G. Senjanovic: Universität Bonn, Physikalisches Institut Preprint; H. Georgi, S.L. Glashow: Phys. Rev.D7, 2457 (1973)
4. J.C. Pati, A. Salam: Phys. Rev. D10, 275 (1974); R.N. Mohapatra, J.C. Pati: Phys. Rev.D11, 566, 2558 (1975); G. Senjanovic, R.N. Mohapatra: Phys. Rev.D12, 1502 (1975)
5. B. Ponte Corvo: Soviet Phys. JETP53, 1717 (1967); V. Gnibov, B. Pante Corvo: Phys. Lett.28B, 493 (1969)