Brill-Noether theory for moduli spaces of sheaves on algebraic varieties

TL;DR

This paper develops a Brill-Noether theory for moduli spaces of sheaves on algebraic varieties, defining stratifications based on global sections and analyzing their geometric properties and dimensions.

Contribution

It introduces a new Brill-Noether stratification for moduli spaces of sheaves, realized as determinantal varieties, and compares moduli spaces under different polarizations.

Findings

Brill-Noether loci are realized as determinantal varieties.

Expected dimensions of Brill-Noether loci are computed.

Comparison of moduli spaces on Hirzebruch surfaces using stratification.

Abstract

Let $X$ be a smooth projective variety of dimension $n$ and let $H$ be an ample line bundle on $X$. Let $M_{X,H}(r;c_1, ..., c_{s})$ be the moduli space of $H$-stable vector bundles $E$ on $X$ of rank $r$ and Chern classes $c_i(E)=c_i$ for $i=1, ..., s:=min\{r,n\}$. We define the Brill-Noether filtration on $M_{X,H}(r;c_1, ..., c_{s})$ as $W_{H}^{k}(r;c_1,..., c_{s})= \{E \in M_{X,H}(r;c_1, ..., c_{s}) | h^0(X,E) \geq k \}$ and we realize $W_{H}^{k}(r;c_1,..., c_{s})$ as the $k$th determinantal variety of a morphism of vector bundles on $M_{X,H}(r;c_1, ..., c_{s})$, provided $H^i(E)=0$ for $i \geq 2$ and $E \in M_{X,H}(r;c_1, ..., c_{s})$. We also compute the expected dimension of $W_{H}^{k}(r;c_1,..., c_{s})$. Very surprisingly we will see that the Brill-Noether stratification allow us to compare moduli spaces of vector bundles on Hirzebruch surfaces stables with respect to different polarizations. We will also study the Brill-Noether loci of the moduli space of instanton bundles and we will see that they have the expected dimension.