Intersections of several disks of the Riemann sphere as K-spectral sets

TL;DR

This paper proves that the intersection of multiple spectral disks on the Riemann sphere forms a complete K-spectral set for a bounded operator, extending spectral set theory and answering a longstanding question for annuli.

Contribution

It establishes that the intersection of several spectral disks is a complete K-spectral set, with explicit bounds, generalizing previous results and solving a 1974 problem for annuli.

Findings

Intersection of spectral disks is a complete K-spectral set.

Explicit bound for K depending on the number of disks.

Positive answer to Shields' question for annuli.

Abstract

We prove that if $n$ closed disks $D_1, D_2, ..., D_n$, of the Riemann sphere are spectral sets for a bounded linear operator $A$ on a Hilbert space, then their intersection $D_1\cap D_2...\cap D_n$ is a complete $K$-spectral set for $A$, with $K\leq n+n(n-1)/\sqrt3$. When $n=2$ and the intersection $X_1\cap X_2$ is an annulus, this result gives a positive answer to a question of A.L. Shields (1974).