On the Green-Functions of the classical offshell electrodynamics under   the manifestly covariant relativistic dynamics of Stueckelberg
  In previous paper derivations of the Green function have been given for 5D off-shell electrodynamics in the framework of the manifestly covariant relativistic dynamics of Stueckelberg (with invariant evolution parameter $\tau$). In this paper, we reconcile these derivations resulting in different explicit forms, and relate our results to the conventional fundamental solutions of linear 5D wave equations published in the mathematical literature. We give physical arguments for the choice of the Green function retarded in the fifth variable $\tau$. 
I. Aharonovich and L. P. Horwitz
math-ph hep-th math.MP