  We present a detailed discussion of the solution of Nambu-Gor'kov equations
in superfluid nuclei, which provide a consistent framework to deal with the
interplay between particle-hole and particle- particle channel, including the
effects of the fragmentation of the quasiparticle strength and of the pairing
interaction induced by the exchange of collective vibrations. The coupling
between quasiparticle and vibrations is determined from the experimental
polarizability of the low-lying collective surface vibrations. This coupling is
used to renormalize the properties of quasiparticles obtained from a BCS
calculation using the bare nucleon-nucleon interaction. We apply the formalism
to the case of the nucleus 120Sn, showing results for the low-energy spectrum
and the quasiparticle strength distribution in neighbouring odd nuclei and for
the neutron pairing gap.
