ABSTRACT
This chapter discusses various kinds of perturbation theory, where the solution can be found in the form of a power series in some small parameter. The simplest case of perturbation theory is the case of a weak external field or a weak interaction between particles. Then the "small parameter" is the ratio of the energy or interaction potential to a typical value of the energy of free motion. A second type of perturbation theory involves perturbation of the boundary conditions. Perturbation theory has played an important role in the development of theoretical physics. The chapter considers the case in which a perturbation is applied to the boundary conditions. It also considers the case in which perturbation acting on a quantum mechanical system varies slowly as a function of time. The chapter shows that the problem of the excitation of an atomic nucleus by the passage nears it of a fast charged particle.
