For a finite group $G$, let $mathrm{diam}(G)$ denote the maximum diameter of a connected Cayley graph of $G$. A well-known conjecture of Babai states that $mathrm{diam}(G)$ is bounded by ${(log_{2} |G|)}^{O(1)}$ in case $G$ is a non-abelian finite simple group. Let $G$ be a finite simple group of Lie type of Lie rank $n$ over the field $F_{q}$. Babais conjecture has been verified in case $n$ is bounded, but it is wide open in case $n$ is unbounded. Recently, Biswas and Yang proved that $mathrm{diam}(G)$ is bounded by $q^{O( n {(log_{2}n + log_{2}q)}^{3})}$. We show that in fact $mathrm{diam}(G) < q^{O(n {(log_{2}n)}^{2})}$ holds. Note that our bound is significantly smaller than the order of $G$ for $n$ large, even if $q$ is large. As an application, we show that more generally $mathrm{diam}(H) < q^{O( n {(log_{2}n)}^{2})}$ holds for any subgroup $H$ of $mathrm{GL}(V)$, where $V$ is a vector space of dimension $n$ defined over the field $F_q$.
Download