Characterizations and models for the $C_{1,r}$ class and quantum annulus (2209.00373v2)

Abstract: For fixed $0<r<1$, let $A_r={z \in \mathbb{C} : r<|z|<1}$ be the annulus with boundary $\partial \overline{A}r=\mathbb{T} \cup r\mathbb{T}$, where $\mathbb T$ is the unit circle in the complex plane $\mathbb C$. An operator having $\ov{A}_r$ as a spectral set is called an $A_r$-\textit{contraction}. Also, a normal operator with its spectrum lying in the boundary $\partial \overline{A}_r$ is called an \textit{$A_r$-unitary}. The \textit{$C{1,r}$ class} was introduced by Bello and Yakubovich in the following way: [ C_{1, r}={T: T \ \mbox{is invertible and} \ |T|, |rT^{-1}| \leq 1}. ] McCullough and Pascoe defined the \textit{quantum annulus} $\mathbb Q \mathbb A_r$ by [ \mathbb Q\mathbb A_r = {T \,:\, T \text{ is invertible and } \, |rT|, |rT^{-1}| \leq 1 }. ] If $\mathcal A_r$ denotes the set of all $A_r$-contractions, then $\mathcal A_r \subsetneq C_{1,r} \subsetneq \mathbb Q \mathbb A_r$. We first find a model for an operator in $C_{1,r}$ and also characterize the operators in $C_{1,r}$ in several different ways. We prove that the classes $C_{1,r}$ and $\mathbb Q\mathbb A_r$ are equivalent. Then, via this equivalence, we obtain analogous model and characterizations for an operator in $\mathbb Q \mathbb A_r$.