ABSTRACT
In this chapter we also consider periodic boundary conditions. It is impossible to enumerate all the physical problems that require the solution of the Schrödinger equation with such boundary conditions. We only mention well-known models for the study of molecular rotation spectra that are based on a variety of perturbed rigid rotors [78, 79]. For concreteness we concentrate on a particular model and apply perturbation theory in the cases of weak and strong interaction.
