Bimaximal Neutrino Mixing and Neutrino Mass Matrix
  We show that the bimaximal neutrino mixing pattern suggested by the solar and atmospheric neutrino data can be derived from the maximal, symmetric, four neutrino mixing in the limit that one of the neutrinos is made heavy. Imposing the constraints of no neutrinoless double beta decay and a 20% hot dark matter component of the universe leads to a three neutrino mass matrix recently suggested by Georgi and Glashow. Our result can be useful in constructing theoretical models for the bimaximal pattern. We illustrate this by a simple example. 
R. N. Mohapatra and S. Nussinov