QED$_{4}$ Ward Identity for fermionic field in the light-front
  In a covariant gauge we implicitly assume that the Green's function propagates information from one point of the space-time to another, so that the Green's function is responsible for the dynamics of the relativistic particle. In the light front form, which in principle is a change of coordinates, one would expect that this feature would be preserved. In this manner, the fermion's field propagator can be split into a propagating piece and a non-propagating (``contact'') term. Since the latter (``contact'') one does not propagate information, and therefore, assumedly with no harm to the field dynamics we wanted to know what would be the impact of dropping it off. To do that, we investigated its role in the Ward identity in the light front. 
J. H. O. Sales, A. T. Suzuki, and J.D. Bolzan