On the neutrino mass spectrum and neutrino mixing from oscillation data

Two schemes of mixing of four massive neutrinos with two couples of close neutrino masses separated by a gap of the order of 1 eV can accommodate solar, atmospheric and LSND neutrino oscillation data. It is shown that long-baseline nu(e)->nu(e) and nu(mu)->nu(e) transitions are strongly suppressed in these schemes. The scheme of mixing of three neutrino masses with a mass hierarchy that can describe solar and atmospheric neutrino data is also discussed. It is shown that in this scheme the effective Majorana mass |<m>| that characterizes the matrix element of neutrinoless double-beta decay is less than 10^{-2} eV.