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Neutrinos are abundantly produced in astrophysical environments such as core-collapse supernovae and binary neutron star mergers. Neutrino flavor conversions in the dense media play important roles in the physical and chemical evolutions of the environments. In this book, I present two mechanisms through which neutrinos may change their flavors.

In the first mechanism, neutrinos can experience flavor conversions through interactions with oscillatory perturbations in matter distributions. I show that this mechanism can be understood intuitively as Rabi oscillations. I also derive criteria which can be used to determine whether such parametric resonances exist in a given environment.

In the second mechanism, the whole neutrino medium can experience flavor conversions because of the neutrino self-interactions. Applying the linearized flavor stability analysis method to the dense neutrino medium with discrete and continuous angular emissions, I show that, contrary to a recent conjecture by I. Izaguirre et al., the flavor instabilities are not always associated with the gaps in dispersion relation curves of the collective modes of neutrino oscillations. I also develop a toy model and a numerical scheme which can be used to explore neutrino oscillations in an environment where scattered neutrino fluxes are present.

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