The Ladder Approximation in QED_2
  We study the Schwinger-Dyson equation for the fermion self-energy in massless and massive $QED_2$, in the ladder approximation. When the fermion is massless (and the photon massless or massive), we check the reliability of this approximation by comparing its solutions with the exact ones. They agree only when the photon is massless. For a massive fermion and massless photon, we show that there is no consistent solution at all, the infrared divergences introduced by the approximation forbidding even the trivial solution. When both fermion and photon are massive, we find a non-perturbative (extra) fermion mass generation (which survives in the limit when the bare mass of the fermion tends to zero). We argue that, in this case, the ladder approximation will provide reliable results if the bare masses of the fields are large compared with the (dimensionful) coupling constant. 
C. D. Fosco, T. Matsuyama