Eisenstein series and the top degree cohomology of arithmetic subgroups of $SL_n/\mathbb{Q}$

Joachim Schwermer

Published 2020-03-10Version 1

The cohomology $H^*(\Gamma, E) $ of a torsion-free arithmetic subgroup $\Gamma$ of the special linear $\mathbb{Q}$-group $\sG = SL_n$ may be interpreted in terms of the automorphic spectrum of $\Gamma$. Within this framework, there is a decomposition of the cohomology into the cuspidal cohomology and the Eisenstein cohomology. The latter space is decomposed according to the classes $\{\sP\}$ of associate proper parabolic $\mathbb{Q}$-subgroups of $\sG$. Each summand $H^*_{\mathrm{\{P\}}}(\Gamma, E)$ is built up by Eisenstein series (or residues of such) attached to cuspidal automorphic forms on the Levi components of elements in $\{\sP\}$. The cohomology $H^*(\Gamma, E) $ vanishes above the degree given by the cohomological dimension $\mathrm{cd}(\Gamma) = \frac{n(n-1)}{2}$. We are concerned with the internal structure of the cohomology in this top degree. On the one hand, we explicitly describe the associate classes $\{\sP\}$ for which the corresponding summand $H^{\mathrm{cd}(\Gamma)}_{\mathrm{\{\sP\}}}(\Gamma, E)$ vanishes. On the other hand, in the remaining cases of associate classes we construct various families of non-vanishing Eisenstein cohomology classes which span $H^{\mathrm{cd}(\Gamma)}_{\mathrm{\{\sQ\}}}(\Gamma, \C)$. Finally, in the case of a principal congruence subgroup $\Gamma(q)$, $q = p^{\nu} > 5$, $p\geq 3$ a prime, we give lower bounds for the size of these spaces if not even a precise formula for its dimension for certain associate classes $\{\sQ\}$.