On expansive operators that are quasisimilar to the unilateral shift of finite multiplicity

Abstract

An operator $T$ on a Hilbert space $\mathcal H$ is called expansive, if $\|Tx\|\geq \|x\|$ ($x\in\mathcal H$). Expansive operators $T$ quasisimilar to the unilateral shift $S_N$ of finite multiplicity $N$ are studied. It is proved that $I-T^*T$ is of trace class for such $T$. Also the lattice $\mathrm{Lat}T$ of invariant subspaces of an expansive operator $T$ quasisimilar to $S_N$ is studied. It is proved that $\dim\mathcal M\ominus T\mathcal M\leq N$ for every $\mathcal M\in\mathrm{Lat}T$. It is shown that if $N\geq 2$, then there exist $\mathcal M_j\in\mathrm{Lat}T$ ($j=1,\ldots, N$) such that the restriction $T|_{\mathcal M_j}$ of $T$ on $\mathcal M_j$ is similar to the unilateral shift $S$ of multiplicity $1$ for every $j=1,\ldots, N$, and $\mathcal H=\vee_{j=1}^N\mathcal M_j$. For $N=1$, that is, for $T$ quasisimilar to $S$, there exist two spaces $\mathcal M_1$, $\mathcal M_2\in\mathrm{Lat}T$ such that $T|_{\mathcal M_j}$ is similar to $S$ for $j=1,2$, and $\mathcal H=\mathcal M_1\vee\mathcal M_2$. Example of an expansive operator $T$ quasisimilar to $S$ is given such that intertwining transformations do not give an isomorphism of $\mathrm{Lat}T$ and $\mathrm{Lat}S$.