We present the exact formula for neutrino oscillations. By resorting to recent results of Quantum Field Theory of fermion mixing, we work out the Green's function formalism for mixed neutrinos. The usual quantum mechanical Pontecorvo formula is recovered in the relativistic limit.Comment: 7 pages, RevTeX, revised version: title changed and some minor change

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We show the presence of a topological (Berry) phase in the time evolution of a mixed state. For the case of mixed neutrinos, the Berry phase is a function of the mixing angle only.Comment: 7 pages, Revte

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The spectral function of pions interacting with a gas of nucleons and ∆ 33 -resonances is investigated using the formalism of Thermo Field Dynamics. After a discussion of the zero ∆-width approximation at finite temperature, we take into account a constant width of the resonance. Apart from a full numerical calculation, we give analytical approximations to the pionic spectral function including such a width. They are found to be different from previous approximations, and require an increase of the effective ∆-width in hot compressed nuclear matter. The results are summarized in an effective dispersion relation for interacting pions.