Gauge covariant fermion propagator in quenched, chirally-symmetric   quantum electrodynamics
  We discuss the chirally symmetric solution of the massless, quenched, Dyson-Schwinger equation for the fermion propagator in three and four dimensions. The solutions are manifestly gauge covariant. We consider a gauge covariance constraint on the fermion--gauge-boson vertex, which motivates a vertex Ansatz that both satisfies the Ward identity when the fermion self-mass is zero and ensures gauge covariance of the fermion propagator. 
Zhihua Dong, Herman J. Munczek and Craig D. Roberts