Javier Carvajal-Rojas - Finite torsors over strongly $F$-regular singularities

epiga:7532 - Épijournal de Géométrie Algébrique, March 1, 2022, Volume 6 - https://doi.org/10.46298/epiga.2022.7532
Finite torsors over strongly $F$-regular singularities

Authors: Javier Carvajal-Rojas

We investigate finite torsors over big opens of spectra of strongly $F$-regular germs that do not extend to torsors over the whole spectrum. Let $(R,\mathfrak{m},k)$ be a strongly $F$-regular $k$-germ where $k$ is an algebraically closed field of characteristic $p>0$. We prove the existence of a finite local cover $R \subset R^{\star}$ so that $R^{\star}$ is a strongly $F$-regular $k$-germ and: for all finite algebraic groups $G/k$ with solvable neutral component, every $G$-torsor over a big open of $\mathrm{Spec} R^{\star}$ extends to a $G$-torsor everywhere. To achieve this, we obtain a generalized transformation rule for the $F$-signature under finite local extensions. Such formula is used to show that that the torsion of $\mathrm{Cl} R$ is bounded by $1/s(R)$. By taking cones, we conclude that the Picard group of globally $F$-regular varieties is torsion-free. Likewise, it shows that canonical covers of $\mathbb{Q}$-Gorenstein strongly $F$-regular singularities are strongly $F$-regular.


Volume: Volume 6
Published on: March 1, 2022
Accepted on: March 1, 2022
Submitted on: June 1, 2021
Keywords: Mathematics - Algebraic Geometry,Mathematics - Commutative Algebra,13A35, 13A50, 14B05, 14L15, 14L30, 16T05


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