The Cohen--Lenstra moments over function fields via the stable homology of non-splitting Hurwitz spaces

Abstract

We compute the average number of surjections from class groups of quadratic function fields over ${\mathbb{F}}_q(t)$ onto finite odd order groups $H$, once $q$ is sufficiently large. These yield the first known moments of these class groups, as predicted by the Cohen--Lenstra heuristics, apart from the case $H=\mathbb{Z}/3\mathbb{Z}$. The key input to this result is a topological one, where we compute the stable rational homology groups of Hurwitz spaces associated to non-splitting conjugacy classes.

Comments welcome! We made some minor edits, mostly to the appendix and Lemma 5.2.2, in order to increase compatibility with a forthcoming article