Conservation of quasi-energy in electrodynamics

We establish a general theorem of quasi-energy conservation for the electromagnetic scattering problem in the time-dependent and time-harmonic cases. This theorem expresses two bilinear relationships between two distinct electromagnetic states, which can be chosen independently one from the other and thus exhibit no causality link between them. The terminology used is borrowed from Tellegen's theorem in the electrical network theory. The Maxwell equations are shown to be equivalent to the theorem of quasi-energy conservation.