We study the asymptotic behaviour of eigenvalues and eigenfunctions 2D vibrating systems with mass density perturbed in a vicinity closed curves. The threshold case which resonance frequencies membrane thin inclusion coincide or closely situated is investigated. eigenvalue problem can be realized as family self-adjoint operators acting on varying Hilbert spaces. However so-called limit operator...

The extension theory for semibounded symmetric operators is generalized by including operators acting in a triplet of Hilbert spaces. We concentrate our attention on the case where the minimal operator is essentially self-adjoint in the basic Hilbert space and construct a family of its self-adjoint extensions inside the triplet. All such extensions can be described by certain boundary condition...

In this paper we consider generalized eigenvalue problems for a family of operators with a polynomial dependence on a complex parameter. This problem is equivalent to a genuine non self-adjoint operator. We discuss here existence of non trivial eigenstates for models coming from analytic theory of smoothness for P.D.E. We shall review some old results and present recent improvements on this sub...