  We present an analytic universal impurity solver for strongly correlated
electrons. We extend the many-body perturbation expansion via suitable
two-particle renormalizations from the Fermi-liquid regime to the critical
region of the metal-insulator transition. The reliability of the approximation
in the strong-coupling limit is demonstrated by reproducing the Kondo scale in
the single-impurity Anderson model. We disclose the origin of the Kondo
resonance in terms of Feynman diagrams and find criteria for the existence of
the proper Kondo asymptotic behavior in approximate theories.
