Brill-Noether loci of pencils with prescribed ramification on moduli of curves and on Severi varieties on $K3$ surfaces

Under the assumption that the adjusted Brill-Noether number $\widetildeρ$ is at least $-g$, we prove that the Brill-Noether loci in $\mathcal{M}_{g,n}$ of pointed curves carrying pencils with prescribed ramification at the marked points have a component of the expected codimension with pointed curves having Brill-Noether varieties of pencils of the minimal dimension. As an application, the map from the Hurwitz scheme to $\mathcal{M}_g$ is dominant if $n+\widetildeρ \geq 0$ and generically finite otherwise, settling a variation of a classical problem of Zariski. In the second part of the paper, we study the analogous loci of curves in Severi varieties on $K3$ surfaces, proving existence of curves with non-general behaviour from the point of view of Brill-Noether theory. This extends previous results of Ciliberto and the first named author to the ramified case. We apply these results to study correspondences and cycles on $K3$ surfaces in relation to Beauville-Voisin points and constant cycle curves.

💡 Research Summary

The paper studies two intertwined problems: (1) the existence and dimension of loci of pointed curves carrying a pencil (a (g^1_k)) with prescribed ramification, and (2) the analogous problem for nodal curves on K3 surfaces, i.e. curves in Severi varieties.

Adjusted Brill–Noether number. For a genus‑(g) curve with (n) marked points and prescribed ramification orders (e_1,\dots,e_n) the authors introduce the adjusted Brill–Noether number
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