Adrian Sandovici

Extremal extensions for the sum of nonnegative selfadjoint relations

The sum A + B of two nonnegative selfadjoint relations (multivalued operators) A and B is a nonnegative relation. The class of all extremal extensions of the sum A + B is characterized as products of relations via an auxiliary Hilbert space associated with A and B. The so-called form sum extension of A+B is a nonnegative selfadjoint extension, which is constructed via a closed quadratic form associated with A and B. Its connection to the class of extremal extensions is investigated and a criterion for its extremality is established, involving a nontrivial dependence on A and B.

Some basic properties of polynomials in a linear relation in linear spaces

The behavior of the domain, the range, the kernel and the multivalued part of a polynomial in a linear relation is analyzed, respectively.

One-dimensional Perturbations, Asymptotic Expansions, and Spectral Gaps

Let S be a closed symmetric operator or relation with defect numbers (1, 1) and let A be a self-adjoint extension of S. The self-adjoint extensions A(τ), τ ∈ ℝ ∪ {∞}, of S, when parametrized by means of Kreĭn’s formula, can be seen as one-dimensional (graph) perturbations of A. The spectral properties of the self-adjoint extension A(τ) of (the completely nonself-adjoint part of) S can be determined via the analytic properties of the Weyl function (Q-function) Qτ(z) corresponding to S and A(t), and conversely. In order to study the limiting properties of these functions at spectral points, local analogs of the Kac-Donoghue classes of Nevanlinna functions are introduced, giving rise to asymptotic expansions at real points. In the case where the self-adjoint extension A has a (maximal) gap in its spectrum, all the perturbations A(τ) have the same gap in their spectrum with the possible exception of an isolated eigenvalue λ(τ), τ ∈ ℝ ∪{∞}. By means of the Weyl function Q τ(z) the (analytic) properties of this eigenvalue are established.