On the torsion of the first direct image of a locally free sheaf

TL;DR

This paper explicitly describes the torsion subsheaf of the first direct image of a holomorphic bundle under a proper holomorphic submersion, with applications to complex surface families and class VII surfaces.

Contribution

It provides an explicit description of the torsion subsheaf of R^1π_* for a holomorphic bundle, under the condition R^0π_* = 0, and applies it to complex surface theory.

Findings

Explicit description of torsion subsheaf in R^1π_*

Identification of non-versal loci in families of complex surfaces

Vanishing results for sections of the torsion subsheaf

Abstract

Let $\pi:M\to B$ be a proper holomorphic submersion between complex manifolds and ${\cal E}$ a holomorphic bundle on $M$. We study and describe explicitly the torsion subsheaf $\mathrm{Tors}(R^1\pi_*({\cal E}))$ of the first direct image $R^1\pi_*(\mathcal{E})$ under the assumption $R^0\pi_*(\mathcal{E})=0$. We give two applications of our results. The first concerns the locus of points in the base of a generically versal family of complex surfaces where the family is non-versal. The second application is a vanishing result for $H^0(\mathrm{Tors}(R^1\pi_*(\mathcal{E})))$ in a concrete situation related to our program to prove the existence of curves on class VII surfaces.