Reducible fibers of polynomials over integers: the Hilbert-Siegel problem over $\mathbb{Q}$ and its applications

Abstract

Given a polynomial $f\in \mathbb{Q}[X]$, the Hilbert--Siegel problem asks for a description of the set of all values $t_0\in \mathbb{Z}$, possibly up to a finite set, such that $f(X)-t_0$ is reducible. Reducibility occurs trivially for $t_0$ in the value set of some non-linear left-composition factor of $f$. The problem therefore reduces to determining the polynomials having infinitely many further reducible values. The biggest step towards a solution so far was taken in joint work of the author with Behajaina and Neftin. We strengthen some of the methods obtained in that work to present the complete solution of the Hilbert--Siegel problem over $\mathbb{Q}$. We give applications to the problem of newly reducible iterates of polynomials, and to an unlikely intersection problem on simultaneous reducible fibers of two polynomial maps over $\mathbb{Q}$, generalizing a result by Bilu and Tichy.