In ”Methodical Notes” we plan to present, particularly, the problems on oscillations, waves, nonlinear dynamics and chaos. We hope that it will be useful for students and postgraduates studying these fields. Usually, the Books of Problems have a siruciure copying that of physical courses subdivided into Mechanics, Electricity, Optics and so on. Thus, the problems on oscillations and waves fit into different parts. This fact isn’t favorable for the formation of united “oscillatory” manner of thought. Moreover, the fast development of nonlinear studies demands permanently to gain a deeper methodical understanding of new results. We hope that the higher school lecturers will contribute their progressive educational firidings to our Journal. In this issue we publish the problem which may be solved rigorously, but admits an elegant approximate solution.