Applications of Stacks - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-20T10:13:10Z http://mathoverflow.net/feeds/question/42389 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/42389/applications-of-stacks Applications of Stacks James D. Taylor 2010-10-16T14:29:30Z 2010-10-16T15:38:33Z <p>I've been aware of stacks since grad school, and I can usually follow in rough lines a discussion about stacks, but I've often wondered what particular (purely!) scheme-theoretic argument or theorem is significantly simplified by the introduction of stacks. I'm sure there are many, but since I don't deal with stacks on a regular basis I don't encounter them as frequently, and I thought maybe some of you can enlighten me.</p> http://mathoverflow.net/questions/42389/applications-of-stacks/42390#42390 Answer by userN for Applications of Stacks userN 2010-10-16T15:16:49Z 2010-10-16T15:16:49Z <p>The classic application is Deligne &amp; Mumford's <a href="http://archive.numdam.org/ARCHIVE/PMIHES/PMIHES_1969__36_/PMIHES_1969__36__75_0/PMIHES_1969__36__75_0.pdf" rel="nofollow">paper</a> proving the irreducibility of the coarse moduli scheme $\overline{M}_g$ of stable genus g curves over any algebraically closed field. They proved irreducibility first for the moduli stack $\overline{\mathcal{M}}_g$ of curves, and then inferred from this result the irreducibility of the scheme.</p> http://mathoverflow.net/questions/42389/applications-of-stacks/42392#42392 Answer by Greg Muller for Applications of Stacks Greg Muller 2010-10-16T15:38:33Z 2010-10-16T15:38:33Z <p>The DeRham space is a stack $X_{DR}$ associated to a smooth variety $X$, so that modules on $X_{DR}$ are D-modules on $X$. This is accomplished by declaring the maps from $Y$ into $X_{DR}$ are the same as maps from $Y^{red}$ (the reduced scheme) into $X$. This has the effect of identifying points with their infinitesmal neighborhoods.</p> <p>The DeRham space is often most useful as a conceptual tool. However, a specific application of it was by Ben-Zvi and Nevins, who used it (and other tools) to show that certain cusped versions $\widetilde{X}$ of $X$ had equivalent categories of D-modules. The idea being, these cusps were identifying some of the infinitesmal neighborhoods of some of the points, and so they should be intermediate between a variety and its DeRham space.</p>