  Collective flavor transformations in supernovae, caused by neutrino-neutrino
interactions, are essentially a two-flavor phenomenon driven by the atmospheric
mass difference and the small mixing angle theta_13. In the two-flavor
approximation, the initial evolution depends logarithmically on theta_13 and
the system remains trapped in an unstable fixed point for theta_13 = 0.
However, any effect breaking exact nu_mu-nu_tau equivalence triggers the
conversion. Such three-flavor perturbations include radiative corrections to
weak interactions, small differences between the nu_mu and nu_tau fluxes, or
non-standard interactions. Therefore, extremely small values of theta_13 are in
practice equivalent, the fate of the system depending only on the neutrino
spectra and their mass ordering.
