Degenerations of complete collineations and geometric Tevelev degrees of ${\mathbb{P}}^r$

Abstract

We consider the problem of enumerating maps $f$ of degree $d$ from a fixed general curve $C$ of genus $g$ to ${\mathbb{P}}^r$ satisfying incidence conditions of the form $f(p_i)\in X_i$, where $p_i\in C$ are general points and $X_i\subset {\mathbb{P}}^r$ are general linear spaces. We give a complete answer in the case where the $X_i$ are points, where the counts, the ``Tevelev degrees'' of ${\mathbb{P}}^r$, were previously known only when $r=1$, when $d$ is large compared to $r,g$, or virtually in Gromov-Witten theory. We also give a complete answer in the case $r=2$ with arbitrary incidence conditions. Our main approach studies the behavior of complete collineations under various degenerations.

small corrections. Accepted version to appear in J. Reine Angew. Math