# On the Theory of Dissipative Extensions

Fischbacher, Christoph Stefan (2017) On the Theory of Dissipative Extensions. Doctor of Philosophy (PhD) thesis, University of Kent,. (KAR id:61093)

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## Abstract

We consider the problem of constructing dissipative extensions of given dissipative

Firstly, we discuss the dissipative extensions of symmetric operators and give a suffcient condition for when these extensions are completely non-selfadjoint. Moreover,

Then, we consider operators A and \widetilde{A} that form a dual pair, which means that A\subset \widetilde{A}^*, respectively \widetilde{A}\subset A^* Assuming that A and (-\widetilde{A}) are dissipative, we present a method of determining the proper dissipative extensions \widehat{A} of this dual pair, i.e. we determine all dissipative operators \widehat{A} such that A\subset \subset\widehat{A}\subset\widetilde{A}^* provided that D(A)\cap D(\widetilde{A}) is dense in H. We discuss applications to symmetric operators, symmetric operators perturbed by a relatively bounded dissipative operator and more singular differential operators. Also, we investigate the stability of the numerical ranges of the various proper dissipative extensions of the dual pair (A,\widetilde{A}).

dissipative. If there exists a dissipative operator (-\widetilde{A}) such that A and \widetilde{A} form a dual

the case, we propose a construction of a dual pair (A_0,\widetilde{A}_0), where A_0\subset A and \widetilde{A}_0\subset\widetilde{A} such that A_K is a proper extension of (A_0,\widetilde{A}_0).

selfadjoint and sectorial extensions of positive symmetric operators as described by Birman, Krein, Vishik and Grubb.

We finish with a discussion of non-proper extensions: Given a dual pair (A,\widetilde{A}) that satisfies certain assumptions, we construct all dissipative extensions of A that have domain contained in D(\widetilde{A}^*). Applying this result, we recover Crandall and Phillip's description

a strictly positive closable quadratic form, we find a criterion for an arbitrary extension of A to be dissipative.

Item Type: Thesis (Doctor of Philosophy (PhD)) Wood, Ian Operator theory, extension theory, dissipative operators, differential operators Q Science > QA Mathematics (inc Computing science) Faculties > Sciences > School of Mathematics Statistics and Actuarial Science Users 1 not found. 30 Mar 2017 11:00 UTC 01 Aug 2019 10:41 UTC https://kar.kent.ac.uk/id/eprint/61093 (The current URI for this page, for reference purposes)