  The bare nucleon-nucleon interaction is essential for the production of pair
correlations in nuclei, but an important contribution also arises from the
induced interaction resulting from the exchange of collective vibrations
between nucleons moving in time reversal states close to the Fermi energy. The
pairing field resulting from the summed interaction is strongly peaked at the
nuclear surface. It is possible to reproduce the detailed spatial dependence of
this field using a Local Density Approximation (LDA) and a contact interaction,
with parameters which are quite different from those commonly used in more
phenomenological approaches.
