Interplay between complex symmetry and Koenigs eigenfunctions

Abstract

We investigate the relationship between the complex symmetry of composition operators $C_{\phi}f=f\circ \phi$ induced on the classical Hardy space $H^2(\mathbb{D})$ by an analytic self-map $\phi$ of the open unit disk $\mathbb{D}$ and its Koenigs eigenfunction. A generalization of orthogonality known as conjugate-orthogonality will play a key role in this work. We show that if $\phi$ is a Schr\"{o}der map (fixes a point $a\in \mathbb{D}$ with $0<|\phi'(a)|<1$) and $\sigma$ is its Koenigs eigenfunction, then $C_{\phi}$ is complex symmetric if and only if $(\sigma^n)_{n\in \mathbb{N}}$ is complete and conjugate-orthogonal in $H^2(\mathbb{D})$. We study the conjugate-orthogonality of Koenigs sequences with some concrete examples. We use these results to show that commutants of complex symmetric composition operators with Schr\"{o}der symbols consist entirely of complex symmetric operators.