English

On the cone of effective 2-cycles on $\overline{M}_{0,7}$

Algebraic Geometry 2015-09-04 v3

Abstract

Fulton's question about effective kk-cycles on M0,n\overline{M}_{0,n} for 1<k<n41<k<n-4 can be answered negatively by appropriately lifting to M0,n\overline{M}_{0,n} the Keel-Vermeire divisors on M0,k+1\overline{M}_{0,k+1}. In this paper we focus on the case of 22-cycles on M0,7\overline{M}_{0,7}, and we prove that the 22-dimensional boundary strata together with the lifts of the Keel-Vermeire divisors are not enough to generate the cone of effective 22-cycles. We do this by providing examples of effective 22-cycles on M0,7\overline{M}_{0,7} that cannot be written as an effective combination of the aforementioned 22-cycles. These examples are inspired by a blow up construction of Castravet and Tevelev.

Keywords

Cite

@article{arxiv.1501.01736,
  title  = {On the cone of effective 2-cycles on $\overline{M}_{0,7}$},
  author = {Luca Schaffler},
  journal= {arXiv preprint arXiv:1501.01736},
  year   = {2015}
}

Comments

22 pages, 4 figures. Final version. Minor corrections. To appear in the European Journal of Mathematics