Bogolyubov transformation in a strong coupling theory. III

The paper is concerned with strongly interacting systems that are invariant under translations. If a Bogolyubov transformation is applied, the conservation laws can be taken into account explicitly, a perturbation theory in a small parameter can be constructed, and the stationary states of the system can be found in the first approximation. An equidistant spectrum of the square of the mass is obtained when a classical relativistic particle interacts with a scalar field.