On rational representations and rational group algebra of $\operatorname{GL}_2(q)$

Abstract

In this article, we study rational representations of $G=\operatorname{GL}_2(q)$, where $q$ is a prime power. Let $ρ$ be an irreducible representation of $G$ over $\mathbb{Q}$. Then $ρ$ affords the character \[ Ω(χ)=m_{\mathbb{Q}}(χ)\sum_{σ\in\operatorname{Gal}(\mathbb{Q}(χ)/\mathbb{Q})}χ^σ, \] for some irreducible complex character $χ$ of $G$, where $m_{\mathbb{Q}}(χ)$ denotes the Schur index of $χ$ over $\mathbb{Q}$, with the converse also holding. We obtain a combinatorial description for the counting of inequivalent irreducible $\mathbb{Q}$-representations of $G$ of distinct degrees. Furthermore, we present a method to construct an irreducible rational matrix representation $ρ$ of $G$ affording the character $Ω(χ)$, where $χ$ is an irreducible complex character of $G$ arising from parabolic induction. Finally, using the results from the rational representations of $G$, we derive an explicit combinatorial formula, depending only on $q$, for the Wedderburn decomposition of $\mathbb{Q}G$.