  I present an approach to derive the full fermion-boson vertex function in
four-dimensional Abelian gauge theory in terms of a set of normal
(longitudinal) and transverse Ward-Takahashi relations for the fermion-boson
and axial-vector vertices in momentum space in the case of massless fermion.
Such a derived fermion-boson vertex function should be satisfied both
perturbatively and non-perturbatively. I show that, by an explicit computation,
such a derived full fermion-boson vertex function to one-loop order leads to
the same result as one obtained in perturbation theory.
