Threshold exponents of pseudo-particle spectra of Kondo impurities

Pseudo-particle propagators or resolvents are known to be important ingredients in the theory of a degenerate Anderson impurity. At low temperatures they show conspicuous threshold behavior which is, however, described only approximately in perturbative theories like the non-crossing approximation. In this paper we derive the correct values of the threshold exponents from an argument due to Schotte and Schotte. For degeneracyN=1 we provide a complete perturbative proof of the validity of our results.