SUMMATION OF THE DOMINATING DIAGRAMS IN MANY-BODY PERTURBATION-THEORY

In this paper we present a description of an approach within many-body perturbation theory, which takes into account the infinite series of diagrams that describe screening and the interaction between the excit ed-core electron and the hole. We also present an extension to this th eory that allows us to describe the interaction of a positron with an atom. To do this the infinite series of diagrams that describes virtua l positronium formation is taken into account. Some results of these c alculations are presented.