On contractions that are quasiaffine transforms of unilateral shifts

Abstract

It is known that if $T$ is a contraction of class $C_{10}$ and $I-T^\ast T$ is of trace class, then $T$ is a quasiaffine transform of a unilateral shift. Also it is known that if the multiplicity of a unilateral shift is infinite, the converse is not true. In this paper the converse for a finite multiplicity is proved: if $T$ is a contraction and $T$ is a quasiaffine transform of a unilateral shift of finite multiplicity, then $I-T^\ast T$ is of trace class. As a consequence we obtain that if a contraction $T$ has finite multiplicity and its characteristic function has an outer left scalar multiple, then $I-T^\ast T$ is of trace class. Also, it is known that if a contraction $T$ on a Hilbert space $\mathcal H$ is such that $\|b_λ(T)x\|\geqδ\|x\|$ for every $λ\in\mathbb D$, $x\in\mathcal H$, with some $δ>0$, and $$\sup_{λ\in\mathbb D}\|I-b_λ(T)^\ast b_λ(T)\|_{\frak S_1}<\infty$$ (here $b_λ$ is a Blaschke factor and $\frak S_1$ is the trace class of operators), then $T$ is similar to an isometry. In this paper the converse for a finite multiplicity is proved: if $T$ is a contraction and $T$ is similar to an isometry of finite multiplicity, then $T$ satisfies the above conditions.