The perturbation of the Dirac sea to first order in the external potential is
calculated in an expansion around the light cone. It is shown that the
perturbation consists of a causal contribution, which describes the singular
behavior of the Dirac sea on the light cone and contains bounded line integrals
over the potential and its partial derivatives, and a non-causal contribution,
which is a smooth function. As a preparatory step, we construct a formal
solution of the inhomogeneous Klein-Gordon equation in terms of an infinite
series of line integrals.
More generally, the method presented can be used for an explicit analysis of
Feynman diagrams of the Dirac, Klein-Gordon, and wave equations in position
space.