A Note on Spectral Mapping Theorems for Subnormal Operators

TL;DR

This paper constructs a specific example of a subnormal operator to demonstrate a counterexample to a spectral mapping theorem, resolving an open question from 1984.

Contribution

It provides the first known example showing the failure of spectral mapping theorems for subnormal operators, answering a long-standing open problem.

Findings

Existence of a compact set and measure with specific properties

Construction of a function invertible in one algebra but not in another

Counterexample to spectral mapping theorem for subnormal operators

Abstract

For a compact subset $K\subset \mathbb C$ and a positive finite Borel measure $\mu$ supported on $K,$ let $\text{Rat}(K)$ denote the space of rational functions with poles off $K,$ let $R^\infty (K,\mu)$ be the weak-star closure of $\text{Rat}(K)$ in $L^\infty (\mu),$ and let $R^2 (K,\mu)$ be the closure of $\text{Rat}(K)$ in $L^2(\mu).$ We show that there exists a compact subset $K\subset \mathbb C,$ a positive finite Borel measure $\mu$ supported on $K,$ and a function $f\in R^\infty (K,\mu)$ such that $R^\infty (K,\mu)$ has no non-trivial direct $L^\infty$ summands, $f$ is invertible in $R^2 (K,\mu)\cap L^\infty(\mu),$ and $f$ is not invertible in $R^\infty (K,\mu).$ The result answers an open question concerning spectral mapping theorems for subnormal operators raised by J. Dudziak in 1984.