Power counting degree versus singular order in the Schwinger model
  The importance of a rigorous definition of the singular degree of a distribution is demonstrated on the case of two-dimensional QED (Schwinger model). Correct mathematical treatment of second order vacuum polarization in the perturbative approach is crucial in order to obtain the Schwinger mass of the photon by resummation. 
Andreas Aste, G"unter Scharf and Urs Walther