Iteration of Cox rings of klt singularities

Abstract

Given a klt singularity $(X,\Delta;x)$, we define the iteration of Cox rings of $(X,\Delta;x)$. The first result of this article is that the iteration of Cox rings ${\rm Cox}^{(k)}(X,\Delta;x)$ of a klt singularity stabilizes for $k$ large enough. The second result is a boundedness one, we prove that for a $n$-dimensional klt singularity $(X,\Delta;x)$ the iteration of Cox rings stabilizes for $k\geq c(n)$, where $c(n)$ only depends on $n$. Then, we use Cox rings to establish the existence of a simply connected factorial canonical cover (or scfc cover) of a klt singularity. We prove that the scfc cover dominates any sequence of quasi-\'etale finite covers and reductive abelian quasi-torsors of the singularity. We characterize when the iteration of Cox rings is smooth and when the scfc cover is smooth. We also characterize when the spectrum of the iteration coincides with the scfc cover. Finally, we give a complete description of the regional fundamental group, the iteration of Cox rings, and the scfc cover of klt singularities of complexity one. Analogous versions of all our theorems are also proved for Fano type morphisms. To extend the results to this setting, we show that the Jordan property holds for the regional fundamental group of Fano type morphisms.