MODEL-SPACE EFFECTIVE INTERACTIONS IN MANY-BODY PROBLEMS

In treating nuclear many-body problems, one usually reduces the ''full '' many-body problem to a much smaller and more manageable problem, of ten referred to as the model-space problem. Formal theories for carryi ng out this reduction have been developed, and they are briefly review ed here. The reduced many-body problem is characterized by a model-spa ce effective interaction, which is given in the form of a folded-diagr am series. Degenerate and non-degenerate methods for summing up this s eries are discussed and compared. It is pointed out that the applicati on of this effective-interaction approach is not restricted to nuclear systems alone; it may also be useful to other types of many-body prob lems such as that for a system of electrons.