Posinormal Composition Operators on $H^2$

TL;DR

This paper characterizes posinormal and coposinormal composition operators on the Hardy space for linear-fractional selfmaps, revealing operators with complex posinormality properties and their power behaviors.

Contribution

It provides a complete characterization of posinormal and coposinormal composition operators on $H^2$ for linear-fractional maps, highlighting novel examples with non-preserved posinormality in powers.

Findings

Characterization of posinormal composition operators on $H^2$

Identification of operators that are both posinormal and coposinormal

Existence of composition operators with powers that are not posinormal

Abstract

A bounded linear operator $A$ on a Hilbert space is posinormal if there exists a positive operator $P$ such that $AA^{*} = A^{*}PA$. Posinormality of $A$ is equivalent to the inclusion of the range of $A$ in the range of its adjoint $A^*$. Every hyponormal operator is posinormal, as is every invertible operator. We characterize both the posinormal and coposinormal composition operators $C_\varphi$ on the Hardy space $H^2$ of the open unit disk $\mathbb{D}$ when $\varphi$ is a linear-fractional selfmap of $\mathbb{D}$. Our work reveals that there are composition operators that are both posinormal and coposinormal yet have powers that fail to be posinormal.