1. P. Minkowski, μ → eγ at a rate of one out of 1-billion muon Decays?, Phys. Lett. B 67 (1977) 421 [ INSPIRE ].
2. T. Yanagida, Horizontal symmetry and masses of neutrinos, in the proceedings of the Workshop on the unified theory and the baryon number in the universe, O. Sawada and A. Sugamoto eds., KEK, Japan (1979).
3. M. Gell-Mann, P Ramond and R. Slansky, Complex spinors and unified theories, in Supergravity, P. van Niewenhuizen and D. Freedman eds., North Holland, The Netherlands (1979).
4. R.N. Mohapatra and G. Senjanović, Neutrino mass and spontaneous parity violation, Phys. Rev. Lett. 44 (1980) 912 [ INSPIRE ].
5. J. Schechter and J. Valle, Neutrino masses in SU(2) × U(1) theories, Phys. Rev. D 22 (1980) 2227 [ INSPIRE ].