Perturbational Approach to the Anderson Model: New Results from a   Post-Nca Treatment
  We present selected results from combined analytical and numerical studies of the Anderson-impurity model within the framework of infinite order perturbation theory with respect to the hybridization. Our approximation goes considerably beyond the well known non\-cross\-ing approximation (NCA): The re-summations include skeleton diagrams up to infinite order of the cross\-ing variety, including all $1/N^2$ contributions ($N =$ degeneracy of the magnetic state). We demonstrate, in a comparison with NCA theory, changes in the threshold behaviour of local propagators, a qualitative improvement of spectral properties and a clear progress in attaining local Fermi-liquid properties. Further applications of the post-NCA theory are pointed out. 
F.B. Anders and N. Grewe