Unusual perturbation methods for strongly correlated electron systems

We suggest that Pauli-principle violations, which automatically occur within approximations in many body calculations for strongly correlated electron systems, are the cause for well known pathologies at low temperatures. As an example, the (1/N) expansion, where N denotes the degree of degeneracy of the f-level in Anderson's impurity model, is carried out without Pauli-principle violations. The preliminary result shows that the energy shifts due to the hybridization, which are characteristic for slave boson mean-field theories, are contained in the present approach. (orig.)