Uniqueness and nonuniqueness of the solution of boundary-contact acoustics problems

One establishes uniqueness theorems and one gives examples of nonuniqueness for a series of boundary-contact acoustics problems, i.e. diffraction problems under boundary conditions, described by a higher order operator and vanishing at isolated points.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 128, pp. 21–32, 1983.