Operators on anti-dual pairs: Generalized Schur complement

Tarcsay, Zsigmond; Titkos, Tamás

doi:10.1016/j.laa.2020.02.031

Cited by 2 publications

(1 citation statement)

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“…So, extending A to a positive operator

\overset{A}{true}

is equivalent to find

X \geq 0

such that the operator matrix (1.1) is positive. (For a more general completion problem for block operators see [8, 37]. ) This form also helps us to demonstrate that such an extension need not exist even in the simplest case.…”

Section: Introductionmentioning

confidence: 94%

Operators on anti‐dual pairs: Generalized Krein–von Neumann extension

Tarcsay

Titkos

2021

Mathematische Nachrichten

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The main aim of this paper is to generalize the classical concept of positive operator, and to develop a general extension theory, which overcomes not only the lack of a Hilbert space structure, but also the lack of a normable topology. The concept of anti-duality carries an adequate structure to define positivity in a natural way, and is still general enough to cover numerous important areas where the Hilbert space theory cannot be applied. Our running example -illustrating the applicability of the general setting to spaces bearing poor geometrical features -comes from noncommutative integration theory. Namely, representable extension of linear functionals of involutive algebras will be governed by their induced operators. The main theorem, to which the vast majority of the results is built, gives a complete and constructive characterization of those operators that admit a continuous positive extension to the whole space. Various properties such as commutation, or minimality and maximality of special extensions will be studied in detail.

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“…So, extending A to a positive operator