Criteria for extension of commutativity to fractional iterates of holomorphic self-maps in the unit disc

Abstract

Let $\varphi$ be a univalent non-elliptic self-map of the unit disc $\mathbb D$ and let $(\psi_{t})$ be a continuous one-parameter semigroup of holomorphic functions in $\mathbb D$ such that $\psi_{1}\neq\mathrm{id}_{\mathbb D}$ commutes with $\varphi$. This assumption does not imply that all elements of the semigroup $(\psi_t)$ commute with $\varphi$. In this paper, we provide a number of sufficient conditions that guarantee that ${\psi_t\circ\varphi=\varphi\circ\psi_t}$ for all ${t>0}$: this holds, for example, if $\varphi$ and $\psi_1$ have a common boundary (regular or irregular) fixed point different from their common Denjoy-Wolff point $\tau$, or when $\psi_1$ has a boundary regular fixed point ${\sigma\neq\tau}$ at which $\varphi$ is isogonal, or when $(\varphi-\mathrm{id}_{\mathbb D})/(\psi_1-\mathrm{id}_{\mathbb D})$ has an unrestricted limit at $\tau$. In addition, we analyze how $\varphi$ behaves in the petals of the semigroup $(\psi_t)$.