Pairing correlations, composite pairs, coupled clusters and all that

The coupled cluster [or exp(S)] formulation of quantum many-body theory is flrst extensively reviewed.  It is then applied to the general problem of pairing correlations within a many-fermion system.  The method is illustrated by restricting ourselves to the exact calculations that can be performed within the various generalized ladder approximations.  It is shown how the formalism provides an efficient and unified framework in which to describe all aspects of pairing at the same level of approximation.  Possible extensions to deal with
composite clusters of more than two particles and to incorporate the effects
of the collective excitations, are sketched