Constructing Reducible Brill--Noether Curves II

Abstract

In this paper, we study maps from reducible curves $f : C \cup_\Gamma D \to \mathbb{P}^r$. We restrict our attention to two cases: first, when $f|_D$ factors through a hyperplane $H$ and $f|_C$ is transverse to $H$; and second, when $r = 3$. Degeneration to stable maps of this type have played a crucial role in works of Hartshorne, Ballico, and others, on special cases of the maximal rank conjecture. However, the general problem of studying when such stable maps with specified combinatorial types exist remains open. Here, we give criteria for such Brill--Noether curves of this first type to exist, of specified degree $d$ and genus $g$, such that $f|_C$ is of specified degree $d'$ and genus $g'$. We also give criteria, sharpening earlier results of the author, for the existence of Brill--Noether space curves of specified combinatorial types. As explained in arXiv:1809.05980, these results play a key role in the author's proof of the Maximal Rank Conjecture.