A Generation Criterion for Subsets of $SL_n(\mathbb{F}_q)$

Abstract

Let $G_0$ be a either $SL_n(\mathbb{F}_q)$, the special linear group over the finite field with $q$ elements, or $PSL_n(\mathbb{F}_q)$, its projective quotient, and let $\Sigma$ be a symmetric subset of $G_0$, namely, if $x \in \Sigma$ then $x^{-1} \in \Sigma$. We find a certain set $\mathcal{R}(G_0)$ of irreducible representations of $G_0$ whose size is at most $5$, such that $\Sigma$ generates $G_0$ if and only if $\left\lvert\Sigma\right\rvert$ is not an eigenvalue of ${\sum_{\sigma \in \Sigma} \rho(\sigma)}$ for every $\rho \in \mathcal{R}(G_0)$. To achieve this result, let $G$ be either $GL_n(\mathbb{F}_q)$ or $PGL_n(\mathbb{F}_q)$. We consider $\mathcal{X}(G)$, some set of irreducible nontrivial characters of $G$, whose size is at most $5$. We show that for every subgroup $K \le G$ that does not contain $G_0$, the restriction to $K$ of at least one of the characters in $\mathcal{X}(G)$ contains the trivial character as an irreducible summand. We then restrict the characters to $G_0$ and use standard arguments about the Cayley graph of $G$ to imply the result. In addition, we obtain slightly weaker results about the generation of symmetric subsets of $G$. We finish by considering $S_{n}$, the symmetric group on $n$ elements, and presenting $\mathcal{R}(S_{n})$, a set of eight irreducible nontrivial representations of $S_{n}$, such that a symmetric subset $\Sigma \subseteq S_{n}$ generates $S_{n}$ if and only if $\left\lvert\Sigma\right\rvert$ is not an eigenvalue of ${\sum_{\sigma \in \Sigma} \rho(\sigma)}$ for every $\rho \in \mathcal{R}(S_{n})$, which is an improvement upon the previously known set of $12$ irreducible nontrivial representations of $S_{n}$ that satisfies this condition.