Stability of binomials over finite fields

Abstract

A polynomial $f(x)$ over a field $K$ is said to be stable if all its iterates are irreducible over $K$. L. Danielson and B. Fein have shown that over a large class of fields $K$, if $f(x)$ is an irreducible monic binomial, then it is stable over $K$. In this paper it is proved that this result no longer holds over finite fields. Necessary and sufficient conditions are given in order that a given binomial is stable over $\mathbb{F}_q$. These conditions are used to construct a table listing the stable binomials over $\mathbb{F}_q$ of the form $f(x)=x^d-a$, $a\in\mathbb{F}_q\setminus\{0,1\}$, for $q \leq 27$ and $d \leq 10$. The paper ends with a brief link with Mersenne primes.