A von Neumann type inequality for an annulus

TL;DR

This paper establishes a sharp von Neumann type inequality for operators with spectra in an annulus, characterizing the induced norm and providing bounds for holomorphic functions applied to these operators.

Contribution

It introduces a new inequality for operators on an annulus, extending von Neumann's inequality with the best possible constant, and characterizes the associated norm via a holomorphic function space.

Findings

The supremum norm of holomorphic functions on operators in r is bounded by  times the supremum norm on the annulus.

The constant = is proven to be optimal.

The induced operator norm is characterized as a multiplier norm on a specific holomorphic function space.

Abstract

Let $A_r=\{r<|z|<1\}$ be an annulus. We consider the class of operators $\mathcal{F}_r:=\{T\in\mathcal{B}(H): r^2T^{-1}(T^{-1})^*+TT^*\le r^2+1,\hspace{0.08 cm}\sigma(T)\subset A_r\}$ and show that for every bounded holomorphic function $\phi$ on $A_r:$ $$\sup_{T\in\mathcal{F}_r}||\phi(T)||\le\sqrt{2}||\phi||_{\infty},$$ where the constant $\sqrt{2}$ is the best possible. We do this by characterizing the calcular norm induced on $H^{\infty}(A_r)$ by $\mathcal{F}_r$ as the multiplier norm of a suitable holomorphic function space on $A_r$.