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.
Categories: math.FA