Posinormal Composition Operators on $H^2$

Abstract

A bounded linear operator $A$ on a Hilbert space is posinormal if there exists a positive operator $P$ such that $A{A}^{\ast }=A^{\ast }PA$. Posinormality of $A$ is equivalent to the inclusion of the range of $A$ in the range of its adjoint $A^{\ast }$. Every hyponormal operator is posinormal, as is every invertible operator. We characterize both the posinormal and coposinormal composition operators $C_{\phi }$ on the Hardy space $H^2$ of the open unit disk $\mathbb{D}$ when $\phi$ 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.

16 pages