Specializations of indecomposable polynomials

Abstract

We address some questions concerning indecomposable polynomials and their behaviour under specialization. For instance we give a bound on a prime $p$ for the reduction modulo $p$ of an indecomposable polynomial $P(x)\in \Zz[x]$ to remain indecomposable. We also obtain a Hilbert like result for indecomposability: if $f(t_1,...,t_r,x)$ is an indecomposable polynomial in several variables with coefficients in a field of characteristic $p=0$ or $p>\deg(f)$, then the one variable specialized polynomial $f(t_1^\ast+\alpha_1^\ast x,...,t_r^\ast+\alpha_r^\ast x,x)$ is indecomposable for all $(t_1^\ast, ..., t_r^\ast, \alpha_1^\ast, ...,\alpha_r^\ast)\in \bar k^{2r}$ off a proper Zariski closed subset.