Electrodynamics of a two-electron atom with retardation and self-interaction effects

We study the linear stability of a circular orbit in a two-electron atom, with the inclusion of retardation and self-interaction effects. We calculate all the eigenvalues of the linear stability of the circular orbit, expanded up to third order in (v/c). We discuss how the linear stability results can be used to construct a resonant normal form constant of motion, which is important for the short time stability of the orbits (e.g. emission of sharp spectral lines). We calculate the magnitudes related to some of those orbits and compare to the quantum atomic results.