The dimension of the image of the Abel map associated with normal surface singularities

TL;DR

This paper develops algorithms to determine the dimension of the image of the Abel map for normal surface singularities, linking algebraic geometry, topology, and combinatorics, especially in generic analytic structures.

Contribution

It introduces two algorithms and combinatorial formulas for the Abel map's image dimension in the context of normal surface singularities, extending to twisted line bundles.

Findings

Algorithms for the Abel map image dimension are provided.

Combinatorial formulas are derived for generic analytic structures.

The work relates the codimension of the Abel map image to cohomological invariants.

Abstract

Let $(X,o)$ be a complex normal surface singularity with rational homology sphere link and let $\widetilde{X}$ be one of its good resolutions. Fix an effective cycle $Z$ supported on the exceptional curve and also a possible Chern class $l'\in H^2(\widetilde{X},\mathbb{Z})$. Define ${\rm Eca}^{l'}(Z)$ as the space of effective Cartier divisors on $Z$ and $c^{l'}(Z):{\rm Eca}^{l'}(Z)\to {\rm Pic}^{l'}(Z)$, the corresponding Abel map. In this note we provide two algorithms, which provide the dimension of the image of the Abel map. Usually, $\dim {\rm Pic}^{l'}(Z)=p_g$, $\dim\,{\rm Im} (c^{l'}(Z))$ and ${\rm codim}\,{\rm Im} (c^{l'}(Z))$ are not topological, they are in subtle relationship with cohomologies of certain line bundles. However, we provide combinatorial formulae for them whenever the analytic structure on $\widetilde{X}$ is generic. The ${\rm codim}\,{\rm Im} (c^{l'}(Z))$ is related with $\{h^1(\widetilde{X},\mathcal{L})\}_{\mathcal{L}\in {\rm Im} (c^{l'}(Z))}$; in order to treat the `twisted' family $\{h^1(\widetilde{X},\mathcal{L}_0\otimes \mathcal{L})\}_{\mathcal{L}\in {\rm Im} (c^{l'}(Z))}$ we need to elaborate a generalization of the Picard group and of the Abel map. The above algorithms are also generalized.