Local topological obstruction for divisors

TL;DR

This paper develops a local topological obstruction theory for divisors on smooth projective varieties, linking it to geometric obstructions and applying it to study deformation phenomena and loci such as the Noether-Lefschetz locus.

Contribution

It introduces a new local topological obstruction framework replacing the classical one, and compares it with geometric obstructions, providing insights into deformation and lifting problems.

Findings

Established a local topological obstruction theory using $H^2_D(\

Demonstrated the relation between topological and geometric obstructions in divisor deformations.

Provided examples where cohomology classes deform but divisors do not lift as effective Cartier divisors.

Abstract

Given a smooth, projective variety $X$ and an effective divisor $D\,\subseteq\, X$, it is well-known that the (topological) obstruction to the deformation of the fundamental class of $D$ as a Hodge class, lies in $H^2(\mathcal{O}_X)$. In this article, we replace $H^2(\mathcal{O}_X)$ by $H^2_D(\mathcal{O}_X)$ and give an analogous topological obstruction theory. We compare the resulting local topological obstruction theory with the geometric obstruction theory (i.e., the obstruction to the deformation of $D$ as an effective Cartier divisor of a first order infinitesimal deformations of $X$). We apply this to study the jumping locus of families of linear systems and the Noether-Lefschetz locus. Finally, we give examples of first order deformations $X_t$ of $X$ for which the cohomology class $[D]$ deforms as a Hodge class but $D$ does not lift as an effective Cartier divisor of $X_t$.