Gauge Invariance and Construction of the Fermion-Boson Vertex in QED3
  We study the specific role that the Ward-Green-Takahashi identity plays in the restoration of the gauge independence of the physical observables related to the phenomenon of Dynamical Symmetry Breaking in QED3, the chiral condensate and the fermion mass, in the framework of the Schwinger-Dyson equations (SDE). We see in the lack of gauge independence of the physical observables the necessity of including further gauge invariance constraints and study the gauge covariance of the Fermion Propagator (FP) through its Landau-Khalatnikov-Fradkin transformation. From this we stress that 1) in order to achieve full gauge independence of the physical observables we must include the Full Vertex in the corresponding SDE for the FP, and 2) that Perturbation Theory (PT) is the only scheme where this gauge independence as well as all of the gauge invariance constraints are satisfied order by order. Therefore we carry out the construction of the fermion-boson vertex taking PT as a guide upto the first order of approximation. We completely relate this vertex to the FP and propose a non perturbative form for it. Furthermore, we write this non perturbative vertex in a suitable form for its future implementation in similar studies. 
A. Raya