Local topological obstruction for 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 $H_D^2({\mathcal{O}}_X)$ 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 $X_t$.

To appear in Revista Matemática Complutense

References in corpus (1)

• Semi-regular varieties and variational Hodge conjecture