Magnetoplasmon Excitations from Partially Filled Landau Levels in Two Dimensions

For a noninteracting two-dimensional electron gas in a strong perpendicular magnetic field, the excitation energies are multiples of $\hslash {\omega }_c$. These excitation energies are shifted in an interacting system. Because of the singular nature of the noninteracting-system energy spectrum it has previously been possible to evaluate the excitation energies of the interacting system only when all Landau levels are either completely empty or completely full. In this article we suggest a nonperturbative approach which overcomes this difficulty but recovers existing results in the appropriate limits.