On algebraic dependencies between Poincaré functions

Abstract

Let $A$ be a rational function of one complex variable, and $z_0$ its repelling fixed point with the multiplier $λ.$ Then a Poincaré function associated with $z_0$ is a function $\mathcal{P}_{A,z_0,λ}$ meromorphic on $\mathbb C$ such that $\mathcal{P}_{A,z_0,λ}(0)=z_0$, $\mathcal{P}_{A,z_0,λ}'(0)\neq 0,$ and $\mathcal{P}_{A,z_0,λ}(λz)=A\circ \mathcal{P}_{A,z_0,λ}(z).$ In this paper, we investigate the following problem: given Poincaré functions $\mathcal{P}_{A_1,z_1,λ_1}$ and $\mathcal{P}_{A_2,z_2,λ_2}$, find out if there is an algebraic relation $f(\mathcal{P}_{A_1,z_1,λ_1},\mathcal{P}_{A_2,z_2,λ_2})=0$ between them and, if such a relation exists, describe the corresponding algebraic curve. We provide a solution, which can be viewed as a refinement of the classical theorem of Ritt about commuting rational functions. We also reprove and extend previous results concerning algebraic dependencies between Böttcher functions.