The zero-momentum limit of thermal green functions
  The zero momentum limit of thermal self-energies calculated in perturbation theory depends on the order in which the time and the space components of the momentum are taken to zero. We show that this is an artifact of the perturbative calculation, and in fact the limit is well-defined when higher orders of the perturbation expansion are properly taken into account. 
J. F. Nieves and P. B. Pal