On the neutrino mass spectrum and neutrino mixing from oscillation data

Two schemes of mixing of four massive neutrinos with two couples of close n eutrino masses separated by a gap of the order of 1 eV can accommodate sola r, atmospheric and LSND neutrino oscillation data. It is shown that long-ba seline (v) over bar(e) --> (v) over bar(e) and v(mu) --> v(e) transitions a re strongly suppressed in these schemes. The scheme of mixing of three neut rino masses with a mass hierarchy that can describe solar and atmospheric n eutrino data is also discussed. It is shown that in this scheme the effecti ve Majorana mass \[m]\ that characterizes the matrix element of neutrinoles s double-beta decay is less than similar to 10(-2) eV.