Sylvester equations and polynomial separation of spectra

Abstract

Sylvester equations $AX-XB=C$ have unique solutions for all $C$ when the spectra of $A$ and $B$ are disjoint. Here $A$ and $B$ are bounded operators in Banach spaces. We discuss the existence of polynomials $p$ such that the spectra of $p(A)$ and $p(B)$ are well separated, either inside and outside of a circle or separated into different half planes. Much of the discussion is based on the following inclusion sets for the spectrum: $V_p(T)=\{\lambda \in \mathbb C \ : \ |p(\lambda)| \le \|p(T)\| \}$ where $T$ is a bounded operator. We also give an explicit series expansion for the solution in terms of $p(M)$, where $M=\begin{pmatrix} A&C\\ &B\end{pmatrix}$, in the case where the spectra of $A$ and $B$ lie in different components of $V_p(M)$ .