Gauge invariance of massless QED
arXiv:hep-ph/9403399 · doi:10.1016/0370-2693(94)91545-8
Abstract
A simple general proof of gauge invariance in QED is given in the framework of causal perturbation theory. It illustrates a method which can also be used in non-abelian gauge theories.
7 pages, TEX-file, Zuerich University Preprint ZU-TH-33/1993