1. T. Yanagida, in Proceedings of the Workshop on Unified Theory and Baryon Number of the Universe, eds. O. Sawada, A. Sugamoto (KEK, 1979) p. 95; M. Gell-Mann, P. Ramond, R. Slansky, in Supergravity, eds. P. van Nieuwenhuizen, D. Freedman (North-Holland, Amsterdam, 1979).
2. R. Davis Jr., D.S. Harmer, K.C. Hoffman, Phys. Rev. Lett. 20 (1968) 1205; K.S. Hirata et al., Phys. Rev. Lett. 63 (1989) 16; Phys. Rev. Lett. 65 (1990) 1297.
3. K.S. Hirata et al., Phys. Lett. B205 (1988) 416; Phys. Lett. B280 (1992) 146; Y. Fukuda et al., Phys. Lett. B335 (1994) 237; D. Casper et al., Phys. Rev. Lett. 66 (1991) 2561; R. Becker-Szendy et al., Phys. Rev. D46 (1992) 3720.
4. Y. Totsuka, Talk given by Lepton-Photon'97 (Hamburg, July, 1997).
5. S.P. Mikheyev, A.Y. Smirnov, Yad. Fiz. 42 (1985) 1441; Sov. J. Nucl. Phys. 42 (1985) 913; Nuovo Cim. C 9 (1986) 17; L. Wolfenstein, Phys. Rev. D17 (1978) 2369.