On the postcritical set of a rational map

The postcritical set $P(f)$ of a rational map $f:{\mathbb{P}}^1\to {\mathbb{P}}^1$ is the smallest forward invariant subset of ${\mathbb{P}}^1$ that contains the critical values of $f$. In this paper we show that every finite set $X\subset \mathbb P^1(\overline{\mathbb Q})$ can be realized as the postcritical set of a rational map. We also show that every map $F:X\to X$ defined on a finite set $X\subset {\mathbb{P}}^1(\mathbb{C})$ can be realized by a rational map $f:P(f)\to P(f)$,providedweallowsmallperturbationsoftheset$X$. The proofs involve Belyi’s theorem and iteration on Teichm"uller space.