Construction of algebraic covers (1709.03341v3)

Abstract: Let $Y$ be an algebraic variety, $\mathcal{F}$ a locally free sheaf of $\mathcal{O}Y$-modules, and $\mathcal{R}(\mathcal{F})$ the $\mathcal{O}_Y$-algebra $\operatorname{Sym}^\bullet \mathcal{F}$. In this paper we study local properties of sheaves of $\mathcal{O}{\mathcal{R}(\mathcal{F})}$-ideals $\mathcal{I}$ such that $\mathcal{R}(\mathcal{F}))/\mathcal{I}$ is an algebraic cover of $Y$. Following the work of Miranda for triple covers, for $\mathcal{Q}$ a direct summand of $\mathcal{R}(\mathcal{F})$, we say that a morphism $\Phi\colon \mathcal{Q}\rightarrow\mathcal{R}(\mathcal{F})/\langle\mathcal{Q}\rangle$ is a covering homomorphism if it induces such an ideal. As an application we study in detail the case of Gorenstein covering maps of degree $6$ for which the direct image of $\varphi_*\mathcal{O}_X$ admits an orthogonal decomposition. These are deformation of $S_3$-Galois branch covers.