We discuss the propagation of decaying neutrinos in matter and outline a procedure to obtain the analytic oscillation probabilities in two and three generation framework. For unstable neutrinos, the Hermitian and anti-Hermitian components of the e ective Hamiltonian do not commute and cannot be simultaneously diagonalised by unitary transformations for all matter densities. We present the compact analytic expressions for neutrino probabilities in matter for the two flavour case, obtained using

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... re-summation of the inverse Baker-Campbell-Hausdor (BCH) or Zassenhaus expansion, We also outline briefly how the approximate probabilities for the three flavour case can be obtained assuming only the vacuum mass eigenstate ⌫ 3 decays. We show the baselines and energies where the di erent approximations give good matching with the numerical probabilities.