Local systems and Suzuki groups

Abstract

We study geometric monodromy groups $G_{\geo,\sF_q}$ of the local systems $\sF_q$ on the affine line over $\F_2$ of rank $D=\sqrt{q}(q-1)$, $q=2^{2n+1}$, constructed in \cite{Ka-ERS}. The main result of the paper shows that $G_{\geo,\sF_q}$ is either the Suzuki simple group $\tw2 B_2(q)$, or the special linear group $\SL_D$. We also show that $\sF_8$ has geometric monodromy group $\tw2B_2(8)$, and arithmetic monodromy group $\Aut(\tw2 B_2(8))$ over $\F_2$, thus establishing \cite[Conjecture 2.2]{Ka-ERS} in full in the case $q=8$.