PARTITION-FUNCTIONS FOR STRONGLY CORRELATED FERMION SYSTEMS

Utilizing the grand canonical partition function as well as the cumula nt summation formula, we consider a systematic approximation scheme fo r a strongly correlated fermion system. As an example, we investigate the single-impurity Anderson model. We are motivated by the fact that for this model there are physical aspects to the approximations used t hat are simply understood. In particular the lowest-order truncation y ields the Kondo temperature as well as a many-particle understanding o f the approximation of Zwicknagl, Zevin, and Fulde to the f spectral f unction.