## Computations of cuspidal cohomology of congruence subgroups of SL(3,$${\mathbb{Z}})$$.(English)Zbl 0552.10015

In the classical case of congruence subgroups $$\Gamma$$ of $$SL_ 2({\mathbb{Z}})$$ the space of cusp forms with respect to $$\Gamma$$ is well understood. Via the Eichler-Shimura isomorphism the cusp cohomology of $$\Gamma$$ isolates special types (depending on the chosen coefficients) of cusp forms. This relation between an analytically defined subspace of the cohomology of $$\Gamma$$ and cuspidal automorphic forms with respect to $$\Gamma$$ is also valid more generally for an arithmetic subgroup $$\Gamma$$ of a semi-simple algebraic group G over $${\mathbb{Q}}$$. It is this cohomological interpretation of cusp forms which allows one to introduce geometrical methods in the study of cusp forms.
Using this approach the paper under review contains explicit computations of the dimension of the cusp cohomology of the congruence groups $$\Gamma_ 0(p)=\{(a_{ij})\in SL_ 3({\mathbb{Z}})| a_{i1}\equiv 0 mod p,\quad 2\leq i\leq 3\}$$ of $$SL_ 3({\mathbb{Z}})$$ for primes $$p\leq 113$$. For all but four primes $$\leq 113$$, this dimension turns out to be zero. For $$p=53$$, 61, 79, 89 it is two. In these cases also the action of the Hecke algebra is considered.
Since the cusp cohomology of $$\Gamma$$ can be interpreted in this case as the image of the cohomology of the arithmetic quotient $$\Gamma$$ $$\setminus X$$ $$(X=SL_ 3({\mathbb{R}})/SO(3))$$ with compact supports in the usual cohomology the computational problem is converted into finite- dimensional linear algebra. The authors present explicit algorithms to perform these computations. [A general non-vanishing result for the cusp cohomology of arithmetic subgroups of $$SL_ 3({\mathbb{Z}})$$ may be found in R. Lee and the reviewer, Invent. Math. 73, 189-239 (1983; Zbl 0525.10014)].
Reviewer: J.Schwermer

### MSC:

 11F27 Theta series; Weil representation; theta correspondences 22E40 Discrete subgroups of Lie groups 11F67 Special values of automorphic $$L$$-series, periods of automorphic forms, cohomology, modular symbols 68W99 Algorithms in computer science

Zbl 0525.10014
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### References:

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