Some inverse problems of the theory of oscillations

Attention is given to the problem of determining the properties of a medium from a wave field known on some manifold for a certain set of frequencies. This problem is reduced to the inverse problem of determining the right part of the Helmholtz equation. Uniqueness theorems for solving this problem are presented, and it is noted that the method of proof can be used as an inversion algorithm. It is shown how certain inverse wave-diffraction problems can be reduced to the aforementioned problem for the wave equation.