3 papers
math.SP2025
Some Remarks On Krein--von Neumann Extensions
Fritz Gesztesy, Selim Sukhtaiev
We survey various properties of Krein--von Neumann extensions and the reduced Krein--von Neumann operator $\hatt S_K$ in connection with a strictly positive (symmetric) opera…
math.SP2024
Weak Coupling and Spectral Instability for Neumann Laplacians
Jussi Behrndt, Fritz Gesztesy, Henk de Snoo
We prove an abstract criterion on spectral instability of nonnegative selfadjoint extensions of a symmetric operator and apply this to self-adjoint Neumann Laplacians on bounded Li…
math.SP2024
Abstract Left-Definite Theory: A Model Operator Approach, Examples, Fractional Sobolev Spaces, and Interpolation Theory
Christoph Fischbacher, Fritz Gesztesy, Paul Hagelstein +1
We use a model operator approach and the spectral theorem for self-adjoint operators in a Hilbert space to derive the basic results of abstract left-definite theory in a straightfo…