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Boundary-value problems for two-dimensional canonical systems

- S. Hassi, H. Snoo, H. Winkler
- Mathematics
- 1 December 2000

The two-dimensional canonical systemJy′=−ℓHy where the nonnegative Hamiltonian matrix functionH(x) is trace-normed on (0, ∞) has been studied in a function-theoretic way by L. de Branges in [5]–[8].… Expand

Boundary relations and their Weyl families

- V. Derkach, S. Hassi, M. Malamud, H. Snoo
- Mathematics
- 1 December 2006

The concepts of boundary relations and the corresponding Weyl families are introduced. Let S be a closed symmetric linear operator or, more generally, a closed symmetric relation in a Hilbert space… Expand

Lebesgue type decompositions for nonnegative forms

- S. Hassi, Z. Sebestyén, H. Snoo
- Mathematics
- 15 December 2009

Boundary relations and generalized resolvents of symmetric operators

- V. Derkach, S. Hassi, M. Malamud, H. Snoo
- Mathematics
- 9 October 2006

The Kreĭn-Naĭmark formula provides a parametrization of all selfadjoint exit space extensions of a (not necessarily densely defined) symmetric operator in terms of maximal dissipative (in ℂ+)… Expand

A general factorization approach to the extension theory of nonnegative operators and relations

- S. Hassi, A. Sandovici, H. Snoo, H. Winkler
- Mathematics
- 2007

The Krein-von Neumann and the Friedrichs extensions of a nonnegative linear operator or relation (i.e., a multivalued operator) are characterized in terms of factorizations. These factorizations lead… Expand

Square‐integrable solutions and Weyl functions for singular canonical systems

- J. Behrndt, S. Hassi, Henk de Snoo, Rudi Wietsma
- Mathematics
- 1 August 2011

Boundary value problems for singular canonical systems of differential equations of the form \documentclass{article}\usepackage{amssymb}\begin{document}\pagestyle{empty} $$ Jf^{\prime }(t)-H(t)… Expand

Contributions to operator theory in spaces with an indefinite metric

- S. Hassi, M. Kaltenbäck, H. D. Snoo
- Mathematics
- 1998

A canonical decomposition for linear operators and linear relations

- S. Hassi, Z. Sebestyén, H. D. Snoo, F. H. Szafraniec
- Mathematics
- 2 November 2006

An arbitrary linear relation (multivalued operator) acting from one Hilbert space to another Hilbert space is shown to be the sum of a closable operator and a singular relation whose closure is the… Expand

Boundary Relations, Unitary Colligations, and Functional Models

- J. Behrndt, S. Hassi, H. Snoo
- Mathematics
- 1 March 2009

Abstract.Recently a new notion, the so-called boundary relation, has been introduced involving an analytic object, the so-called Weyl family. Weyl families and boundary relations establish a link… Expand

On the class of extremal extensions of a nonnegative operator

- Y. Arlinskii̇̆, S. Hassi, Z. Sebestyén, H. Snoo
- Mathematics
- 2001

A nonnegative selfadjoint extension Aof a nonnegative operator A is called extremal if inf {(A)(ϕ) - f),ϕ - f) : ∈ dom A} = 0 for all ϕ ∈ dom A.A new construction of all extremal extensions of a… Expand

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