Abstract
Systems with two kinds of electrons, one localized and strongly correlated, the other in extended band states, require unconventional perturbational concepts. The application of time ordered perturbation expansions with respect to hybridization is studied. The Brillouin-Wigner scheme is critically investigated and a more general method developed. Particular care is devoted to a discussion of summation methods for diagrams in infinite order, and a new set of integral equations is derived. These avoid explicit regularization procedures and lend themselves naturally to investigations of the Kondo-and Intermediate Valence-regimes. Some general expressions for partition fuction and local Green's function of the impurity problem are studied and related to theT-matrix of compounds. Two exactly solvable cases, the resonant level model and the limit of zero bandwidth, serve as examples and guides for approximations.
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