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 KEK lectures, O. Sawada and A. Sugamoto eds., KEK, Tsukuba Japan (1979) [Conf. Proc. C 7902131 (1979) 95] [ INSPIRE ].
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, Amsterdam The Netherlands (1979) [Conf. Proc. C 790927 (1979) 315] [ INSPIRE ].
4. R.N. Mohapatra and G. Senjanovic, 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 ].