Representations of symmetric and alternating groups and their double covers that remain irreducible modulo every prime

Abstract

We classify globally irreducible representations of alternating groups and double covers of symmetric and alternating groups. In order to achieve this classification we also completely characterise irreducible representations of such groups which reduce almost homogeneously in every characteristic. This also allows us to classify irreducible representations that remain irreducible in every characteristic as well as irreducible representations of these groups that can appear as composition factors of globally irreducible representations of groups containing $\mathfrak{A}_n$ or $\hat{\mathfrak{A}}_n$ as normal subgroups. In particular we show that, apart from finitely many exceptions, for any of these questions such representations are either $1$-dimensional or basic spin representations.