Two-dimensional categorified Hall algebras

In the present paper, we introduce two-dimensional categorified Hall algebras of smooth curves and smooth surfaces. A categorified Hall algebra is an associative monoidal structure on the stable $\infty$-category $\mathsf{Coh}^{\mathsf{b}}(\mathbb{R}\mathcal{M})$ of complexes of sheaves with bounded coherent cohomology on a derived moduli stack $\mathbb{R}\mathcal{M}$. In the surface case, $\mathbb{R}\mathcal{M}$ is a suitable derived enhancement of the moduli stack $\mathcal M$ of coherent sheaves on the surface. This construction categorifies the K-theoretical and cohomological Hall algebras of coherent sheaves on a surface of Zhao and Kapranov-Vasserot. In the curve case, we define three categorified Hall algebras associated with suitable derived enhancements of the moduli stack of Higgs sheaves on a curve $X$, the moduli stack of vector bundles with flat connections on $X$, and the moduli stack of finite-dimensional local systems on $X$, respectively. In the Higgs sheaves case we obtain a categorification of the K-theoretical and cohomological Hall algebras of Higgs sheaves on a curve of Minets and Sala-Schiffmann, while in the other two cases our construction yields, by passing to $\mathsf K_0$, new K-theoretical Hall algebras, and by passing to $\mathsf H_\ast^{\mathsf{BM}}$, new cohomological Hall algebras. Finally, we show that the Riemann-Hilbert and the non-abelian Hodge correspondences can be lifted to the level of our categorified Hall algebras of a curve.Comment: 54 page

GAGA problems for the Brauer group via derived geometry

In this paper we prove that the Brauer group of any (derived) scheme $X$, proper over the spectrum of a quasi-excellent Henselian ring, injects into the Brauer group of the Henselization of $X$ along the base, generalizing a classical result of Grothendieck. We offer two proofs of this fact, one based on a formal GAGA-type theorem for smooth and proper stable $\infty$-categories enriched over the $\infty$-category $\mathrm{QCoh}(X)$ of quasi-coherent $\mathscr{O}_X$-modules, and a second one based on a GAGA-type theorem for perfect complexes on $\mathbb{G}_m$-gerbes.Comment: 33 page