Boundary and Eisenstein Cohomology of SL3(Z)\mathrm{SL}_3(\mathbb{Z})

In this article, several cohomology spaces associated to the arithmetic groups $\mathrm{SL}_3(\mathbb{Z})$ and $\mathrm{GL}_3(\mathbb{Z})$ with coefficients in any highest weight representation $\mathcal{M}_\lambda$ have been computed, where $\lambda$ denotes their highest weight. Consequently, we obtain detailed information of their Eisenstein cohomology with coefficients in $\mathcal{M}_\lambda$. When $\mathcal{M}_\lambda$ is not self dual, the Eisenstein cohomology coincides with the cohomology of the underlying arithmetic group with coefficients in $\mathcal{M}_\lambda$. In particular, for such a large class of representations we can explicitly describe the cohomology of these two arithmetic groups. We accomplish this by studying the cohomology of the boundary of the Borel-Serre compactification and their Euler characteristic with coefficients in $\mathcal{M}_\lambda$. At the end, we employ our study to discuss the existence of ghost classes.Comment: 42 Pages, 1 Figure, 9 Tables, To Appear in Math. Annale

Birational properties of some moduli spaces related to tetragonal curves of genus 7

Let M_{7,n} be the (coarse) moduli space of smooth curves of genus 7 with n marked points defined over the complex field. We denote by M^1_{7,n;4} the locus of points inside M_{7,n} representing curves carrying a g^1_4. It is classically known that M^1_{7,n;4} is irreducible of dimension 17+n. We prove in this paper that M^1_{7,n;4} is rational for 0<= n <= 11.Comment: 20 pages; in the second version we replaced the previous Lemma 4.3 by Lemma 4.5, and fixed the proof of the rationality of the moduli space of unpointed tetragonal genus 7 curves in section 4. Hans-Christian von Bothmer as further author adde