  We show that the commutator of lepton mass matrices is invariant under
terrestial matter effects in the four-neutrino mixing scheme. A set of
model-independent sum rules for neutrino masses, which may be generalized to
hold for an arbitrary number of neutrino families, are for the first time
uncovered. Useful sum rules for the rephasing-invariant measures of leptonic CP
violation have also been found. Finally we present a generic formula of
T-violating asymmetries and expect it to be applicable to the future
long-baseline neutrino oscillation experiments.
