Linear resolvent growth of a weak contraction does not imply its similarity to a normal operator
Kupin, S. ; Treil, S.
Illinois J. Math., Tome 45 (2001) no. 4, p. 229-242 / Harvested from Project Euclid
It was shown in \cite{NB} that if $T$ is a contraction in a Hilbert space with finite defect (i.e., $\|T\|\le 1$ and $\operatorname{rank} (I- T^*T) <\infty$), and if the spectrum $\sigma(T)$ does not coincide with the closed unit disk $\overline{\mathbb{D}}$, then the Linear Resolvent Growth condition $$ \|(\la I - T)^{-1} \|\le\frac{C}{\operatorname{dist}(\la,\si(T))},\ \la\in\bc\backslash \si(T) $$ implies that $T$ is similar to a normal operator. The condition $\operatorname{rank}(I - T^*T)<\infty$ 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 \cite{NB} that this condition can be replaced by the condition $I - T^*T\in \fS_1$, where $\fS_1$ denotes the trace class. In this note we show that this conjecture is not true, and that, in fact, one cannot replace the condition $\operatorname{rank}(I - T^*T)<\infty$ by any reasonable condition of closeness to a unitary operator.
Publié le : 2001-01-15
Classification:  47A10,  47A11,  47A45,  47B15
@article{1258138265,
     author = {Kupin, S. and Treil, S.},
     title = {Linear resolvent growth of a weak contraction does not imply its similarity to a normal operator},
     journal = {Illinois J. Math.},
     volume = {45},
     number = {4},
     year = {2001},
     pages = { 229-242},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1258138265}
}
Kupin, S.; Treil, S. Linear resolvent growth of a weak contraction does not imply its similarity to a normal operator. Illinois J. Math., Tome 45 (2001) no. 4, pp.  229-242. http://gdmltest.u-ga.fr/item/1258138265/