  We point out that the Hermitian and anti-Hermitian components of the
effective Hamiltonian for decaying neutrinos cannot be simultaneously
diagonalized by unitary transformations for all matter densities. We develop a
formalism for the two-flavor neutrino propagation through matter of uniform
density, for neutrino decay to invisible states. Employing a resummation of the
Zassenhaus expansion, we obtain compact analytic expressions for neutrino
survival and conversion probabilities, to first and second order in the
"mismatch parameter" $\bar{\gamma}$.
