Chiral Symmetry Breaking with the Curtis-Pennington Vertex
  We study chiral symmetry breaking in quenched QED$_4$, using a vertex Ansatz recently proposed by Curtis and Pennington. Bifurcation analysis is employed to establish the existence of a critical coupling and to estimate its value. The main results are in qualitative agreement with the ladder approximation, the numerical changes being minor. 
D. Atkinson, V.P. Gusynin and P. Maris