Algorithms for arithmetic groups with the congruence subgroup property

Abstract

We develop practical techniques to compute with arithmetic groups $H\le \mathrm{SL}(n,\mathbb{Q})$ for $n>2$. Our approach relies on constructing a principal congruence subgroup in $H$. Problems solved include testing membership in $H$, analyzing the subnormal structure of $H$, and the orbit-stabilizer problem for $H$. Effective computation with subgroups of $\mathrm{GL}(n,{\mathbb{Z}}_m)$ is vital to this work. All algorithms have been implemented in GAP.