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Algebra Seminar

Monday, April 3, 2023 - 3:30pm

Daebeom Choi

University of Pennsylvania


University of Pennsylvania


Finding the maximal dimension of complete subvarieties of moduli space of smooth $n$-pointed curves of genus $g$ is a long-standing open question. Here we show that for $g\ge 3\cdot 2^{d-1}$, if the characteristic of the base field is greater than $2$, then $\rm{M}_g$ contains a complete subvariety of dimension $d$. Furthermore, in positive characteristic, we construct a complete surface in $\rm{M}_{g,n}$ for $g\ge 3$ and $n\ge 1$, which contain a general point. These results are consequences of lifting conjectures introduced here. In particular, we translate the existence of complete subvarieties to properties of line bundles on $\rm{M}_{g,n}$. Our method reframes Zaal's approach, with increased efficiency via Keel's results on semi-ample line bundles in positive characteristic. This reinterpretation reveals differences in the geometry of the space depending on whether one works in characteristic $0$ or in characteristic $p$.