On the top dimensional cohomology groups of congruence subgroups of $\text{SL}_n(\mathbb{Z})$ (1909.02661v3)

Abstract: Let $\Gamma_n(p)$ be the level-$p$ principal congruence subgroup of $\text{SL}n(\mathbb{Z})$. Borel-Serre proved that the cohomology of $\Gamma_n(p)$ vanishes above degree $\binom{n}{2}$. We study the cohomology in this top degree $\binom{n}{2}$. Let $\mathcal{T}_n(\mathbb{Q})$ denote the Tits building of $\text{SL}_n(\mathbb{Q})$. Lee-Szczarba conjectured that $H^{\binom{n}{2}}(\Gamma_n(p))$ is isomorphic to $\widetilde{H}{n-2}(\mathcal{T}n(\mathbb{Q})/\Gamma_n(p))$ and proved that this holds for $p=3$. We partially prove and partially disprove this conjecture by showing that a natural map $H^{\binom{n}{2}}(\Gamma_n(p)) \rightarrow \widetilde{H}{n-2}(\mathcal{T}_n(\mathbb{Q})/\Gamma_n(p))$ is always surjective, but is only injective for $p \leq 5$. In particular, we completely calculate $H^{\binom{n}{2}}(\Gamma_n(5))$ and improve known lower bounds for the ranks of $H^{\binom{n}{2}}(\Gamma_n(p))$ for $p \geq 5$.