On stacky surfaces and noncommutative surfaces

Abstract

Let $\mathbf{k}$ be an algebraically closed field of characteristic $\ge 7$ or zero. Let $\mathcal{A}$ be a tame order of global dimension $2$ over a normal surface $X$ over $\mathbf{k}$ such that $Z(\mathcal{A})={\mathcal{O}}_X$ is locally a direct summand of $\mathcal{A}$. We prove that there is a $\hat{I}{¼}_N$-gerbe $\mathcal{X}$ over a smooth tame algebraic stack whose generic stabilizer is trivial, with coarse space $X$ such that the category of 1-twisted coherent sheaves on $\mathcal{X}$ is equivalent to the category of coherent sheaves of modules on $\mathcal{A}$. Moreover, the stack $\mathcal{X}$ is constructed explicitly through a sequence of root stacks, canonical stacks, and gerbes. This extends results of Reiten and Van den Bergh to finite characteristic and the global situation. As applications, in characteristic $0$ we prove that such orders are geometric noncommutative schemes in the sense of Orlov, and we study relations with Hochschild cohomology and Connes' convolution algebra.

v2:many minor revisions. Section 6 of v1 is removed. 34 pages