  We analytically discuss the resonance conditions among several neutrinos in
matter. The discriminant for the characteristic equation of the Hamiltonian is
expressed by the coefficients of the equation. The result of the computation
for the discriminants tells us that the neutrino energy and the matter density
are in inverse proportion to each other at the resonance states in not only 2-
but also 3- and 4-neutrino models.
