Beyond Schlessinger: deformation stacks -- Brian Osserman

Many basic examples of deformation theory fall into certain predictable patterns, but have not been studied systematically in an elementary way. Certain statements are intuitively clear: for instance, that for a deformation problem on a scheme $X$ with locally trivial deformations and obstructions, the tangent space is given by $H^1(X,T)$ and obstructions lie in $H^2(X,T)$, where $T$ is the sheaf of infinitesimal automorphisms of the problem. However, even making such statements precise requires a language that goes beyond the deformation functors studied by Schlessinger and many others. In this report on ongoing work, we will begin by reviewing the basics of deformation theory and stacks, and then we will discuss a new stack-based framework for deformation theory which allows a systematic and elementary treatment of the passage from local to global in deformation problems.