Tests for complete $K$-spectral sets

Michael A. Dritschel, Daniel Estévez, Dmitry Yakubovich

Published 2015-10-28Version 1

Let $\Phi$ be a family of functions analytic in some neighborhood of a complex domain $\Omega$, and let $T$ be a Hilbert space operator whose spectrum is contained in $\overline\Omega$. Our typical result shows that under some extra conditions, if the closed unit disc is complete $K'$-spectral for $\phi(T)$ for every $\phi\in \Phi$, then $\overline\Omega$ is complete $K$-spectral for $T$ for some constant $K$. In particular, we prove that under a geometric transversality condition, the intersection of finitely many $K'$-spectral sets for $T$ is again $K$-spectral for some $K\ge K'$. These theorems generalize and complement results by Mascioni, Stessin, Stampfli, Badea-Beckerman-Crouzeix and others. We also extend to non-convex domains a result by Putinar and Sandberg on the existence of a skew dilation of $T$ to a normal operator with spectrum in $\partial\Omega$. As a key tool, we use the results from our previous paper on traces of analytic uniform algebras.