Linear resolvent growth of a weak contraction does not imply its similarity to a normal operator

Authors
Citation
S. Kupin et S. Treil, Linear resolvent growth of a weak contraction does not imply its similarity to a normal operator, ILL J MATH, 45(1), 2001, pp. 229-242
Citations number
15
Categorie Soggetti
Mathematics
Journal title
ILLINOIS JOURNAL OF MATHEMATICS
ISSN journal
00192082 → ACNP
Volume
45
Issue
1
Year of publication
2001
Pages
229 - 242
Database
ISI
SICI code
0019-2082(200121)45:1<229:LRGOAW>2.0.ZU;2-Y
Abstract
It was shown in [1] that if T is a contraction in a Hilbert space with fini te defect (i.e., //T// less than or equal to 1 and rank(I - T*T) < infinity ), and if the spectrum sigma (T) does not coincide with the closed unit dis k D, then the Linear Resolvent Growth condition //(lambdaI-T_(-1)// less than or equal to (C)/(dist(lambda,sigma (T))), lam bda is an element of C\sigma (T) implies that T is similar to a normal operator. The condition rank(I - T*T) < infinity measures how close T is to a unitary operator. A natural question is whether this condition can be relaxed. For example, it was conjectured in [1] that this condition can be replaced by the condition I - T*T is an element of G(1) where G(1) denotes the trace cl ass. In this note we show that this conjecture is not true, and that, in fa ct, one cannot replace the condition rank(I - T*T) < infinity by any reason able condition of closeness to a unitary operator.