S. Hassi

Publications131
h-index23
Citations2,062
Highly Influential Citations52
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Boundary-value problems for two-dimensional canonical systems
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].…
Boundary relations and their Weyl families
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…
Lebesgue type decompositions for nonnegative forms
Boundary relations and generalized resolvents of symmetric operators
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+)…
A general factorization approach to the extension theory of nonnegative operators and relations
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…
Square‐integrable solutions and Weyl functions for singular canonical systems
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)…
Contributions to operator theory in spaces with an indefinite metric
A canonical decomposition for linear operators and linear relations
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…
Boundary Relations, Unitary Colligations, and Functional Models
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…
On the class of extremal extensions of a nonnegative operator
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…
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