Quasinormal and hyponormal operators in the $β(H)$-class

Abstract

Following \cite{BR} we study hyponormal operators in the strictly larger class $β(H)$ of operators whose restriction to every non-zero reducing subspace is norm attaining. The Weyl-type characterization of positive operators via the essential spectrum used in the absolutely norm attaining case is replaced by a well-ordering condition on the eigenvalues (Theorem~\ref{thm:positive}). We prove a transfinite, block-diagonal representation $T=\bigoplus_iλ_iS_i$ (with isometries $S_i$) for the $β(H)$-operators whose $|T|$-eigenspaces are reducing, equivalently the quasinormal operators with $|T|\inβ(H)$, and we show by examples that hyponormality alone does not give such a decomposition. For normal operators we obtain a complete representation, and for hyponormal $β(H)$-operators we show that normality is equivalent to the spectrum having planar Lebesgue measure zero. An analogous area-zero criterion for normality is proved for hyponormal operators attaining the minimum modulus on every non-zero reducing subspace.