1. SuperKamiokande Collaboration, Y. Fukuda et al., Phys. Rev. Lett. 81 (1998) 1562; Phys. Lett. B 433 (1998) 9, and 436 (1998) 33; see also K. Scholberg, hep-ex/9905016, May, 1999; for recent analysis, see N. Forego, M.C. Gonzalez-Garcia, J.W.F. Valle, JHEP 7 (2000) 006; Nucl. Phys. B 580 (2000) 58.
2. Z. Maki, M. Nakagawa, S. Sakata, Prog. Theor. Phys. 28 (1962) 870. See also B. Pontecorvo, JETP (USSR) 34 (1958) 247; Zh. Eksp. Teor. Piz. 53 (1967) 1717; V. Gribov, B. Pontecorvo, Phys. Lett. 28B (1969) 493.
3. T. Yanagida, in: A. Sawada, A. Sugamoto (Eds.), Proc. Workshop on Unified Theories and Baryon Number in the Universe (KEK Report No. 79-18, Tsukuba, 1979), p. 95; Prog. Theor. Phys. 64 (1980) 1103; M. Gell-Mann, P. Ramond, R. Slansky, in: P. van Nieuwenhuizen, D.Z. Freedmann (Eds.), Supergravity, North-Holland, Amsterdam 1979, p. 315; R.N. Mohapatra, G. Senjanovic, Phys. Rev. Lett. 44 (1980) 912.
4. A. Zee, Phys. Lett. 93B (1980) 389; 161B (1985) 141; L. Wolfenstein, Nucl. Phys. B 175 (1980) 93.
5. A. Zee, Nucl. Phys. B 264 (1986) 99; K.S. Babu, Phys. Lett. B 203 (1988) 132; D. Chang, W.-Y. Keung, P.B. Pal, Phys. Rev. Lett. 61 (1988) 2420; see also, J. Schechter, J.W.F. Valle, Phys. Rev. D 25 (1982) 2951.