The renormalization group equations for the general 2 by 2, complex, neutrino
mass matrix are shown to have exact, analytic solutions. Simple formulas are
given for the physical mixing angle, complex phase and mass ratio in terms of
their initial values and the energy scales. We also establish a (complex)
renormalization invariant relating these parameters. The qualitative features
of the physical parameters' renormalization are clearly illustrated in vector
field plots. In both the SM and MSSM, maximal mixing is a saddle point.Comment: 16 pages, 5 figure