The spectral constant for the quantum cross and asymptotically sharp bounds for annuli
J. E. Pascoe
TL;DR
The paper investigates the spectral constant $K(r)$ for evaluating analytic functions on the quantum annulus $Q\mathbb{A}_r$ and its asymptotic behavior as $r\to\infty$. It reduces the problem to the quantum cross via a hyperbola correspondence and proves a miniature dilation theorem on quantum hyperbolae, enabling a norm comparison $\|f(Z,W)\|\le \|f(\hat Z,\hat W)\|$. From this, it derives the bound $\|f(Z,W)\|\le 2\left(1+\frac{2r^2}{r^4-1}\right)\|f\|_{\mathbb{H}_r}$, establishing $K(r)\le 2+O(1/r^2)$ and proving the asymptotic tightness of the Tsikalas bound. In the limit $r\to\infty$, the spectral constant for the quantum cross is exactly $2$, i.e., $K(\infty)=2$.
Abstract
The quantum annulus of type $r$ is the class of invertible operators with singular values in $(1/r,r).$ Given an analytic function on the classical annulus of type $r,$ we may evaluate it on operators in the quantum annulus by The spectral constant gives the maximum ratio betweeen the supremum over the norm of evalutions at operators in the quantum annulus to the supremum over classical evaluations. We show that the limit of the spectral constant as $r$ goes to infinity is $2.$ Via the correspondence between annuli and hyperbolae, our study degenerates the problem to one on the quantum cross, pairs of contractions with product zero, where the spectral constant is exactly $2.$ The essential technique is to rationally dilate $Z$ to $\hat{Z}$ which has $U =(\hat{Z}+(\hat{Z}^{-1})^*)/(r+1/r)$ unitary and estimate $Uf(\hat{Z})U^*$ directly.