On symmetries of iterates of rational functions

Abstract

Let $A$ be a rational function of degree $n\geq 2$. Let us denote by $ G(A)$ the group of M\"obius transformations $\sigma$ such that $ A\circ \sigma=\nu_{\sigma} \circ A$ for some M\"obius transformations $\nu_{\sigma}$, and by $\Sigma(A)$ and ${\rm Aut}(A)$ the subgroups of $ G(A)$ consisting of $\sigma$ such that $ A\circ \sigma= A$ and $ A\circ \sigma= \sigma \circ A$, correspondingly. In this paper, we study sequences of the above groups arising from iterating $A$. In particular, we show that if $A$ is not conjugate to $z^{\pm n},$ then the orders of the groups $ G(A^{\circ k})$, $k\geq 2,$ are finite and uniformly bounded in terms of $n$ only. We also prove a number of results about the groups $\Sigma_{\infty}(A)=\cup_{k=1}^{\infty} \Sigma(A^{\circ k})$ and ${\rm Aut}_{\infty}(A)=\cup_{k=1}^{\infty} {\rm Aut}(A^{\circ k})$, which are especially interesting from the dynamical perspective.