A four-neutrino texture implying bimaximal flavor mixing

A four--neutrino effective texture is described, where a sterile neutrino mixes nearly maximally with the electron neutrino and so, is responsible for the deficit of solar $\nu_e $'s (according to the large-angle MSW solution or vacuum solution, of which the former is selected {\it a posteriori}). But, while maximal mixing of muon neutrino with tauon neutrino causes the deficit of atmospheric $\nu_\mu $'s, the original magnitude of LSND effect is reduced by as much as four orders, becoming unobservable