Dynamical Cancellation of Polynomials

Abstract

Extending the work of Bell, Matsuzawa and Satriano, we consider a finite set of polynomials $S$ over a number field $K$ and give a necessary and sufficient condition for the existence of a $N \in \mathbb{N}_{> 0}$ and a finite set $Z \subset \mathbb{P}^1_K \times \mathbb{P}^1_K$ such that for any $(a,b) \in (\mathbb{P}^1_K \times \mathbb{P}^1_K) \setminus Z$ we have the cancellation result: if $k>N$ and $\phi_1,\ldots ,\phi_k$ are maps in $S$ such that $\phi_{k} \circ \dots \circ \phi_1 (a) = \phi_k \circ \dots \circ \phi_1(b)$, then in fact $\phi_N \circ \dots \circ \phi_1(a) = \phi_N \circ \dots \circ \phi_1(b)$. Moreover, the conditions we give for this cancellation result to hold can be checked by a finite number of computations from the given set of polynomials.