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math.FA2025
A generalized Birman-Schwinger principle and applications to one-dimensional Schrödinger operators with distributional potentials
Fritz Gesztesy, Roger Nichols
Given a self-adjoint operator bounded from below in a complex Hilbert space , the corresponding scale of spaces $\mathcal{H}_{+1}(H_0) \subset \mathcal{H} \subse…
math.FA2025
Some Remarks on the Product Formula for Defect Numbers of Closed Operators
Christoph Fischbacher, Fritz Gesztesy, Lance L. Littlejohn
This largely pedagogical paper recalls some facts on defect numbers of products of closed operators employing results from the theory of semi-Fredholm operators and then applies th…