On sets of irreducible polynomials closed by composition

Abstract

Let S be a set of monic degree 2 polynomials over a finite field and let C be the compositional semigroup generated by S. Inthispaper we establish a necessary and sufficient condition for C to be consisting entirely of irreducible polynomials. The condition we deduce depends on the finite data encoded in a certain graph uniquely determined by the generating set S. Using this machinery we are able both to show examples of semigroups of irreducible polynomials generated by two degree 2 polynomials and to give some non-existence results for some of these sets in infinitely many prime fields satisfying certain arithmetic conditions.