On the relation between Dencker's polarization set and the gauge-fixing in electrodynamics

The purpose of this short review, based in part on ideas developed in an article by the author and Fagundes [3], is to emphasize that the gauge-fixing condition, necessary to eliminate the spurious degrees of freedom of the electromagnetic field, is elegantly handled by Dencker’s work on the propagation of polarization sets for systems of real principal type.