Scrollar invariants, syzygies and representations of the symmetric group II

Abstract

Let $\varphi: C\to \mathbb{P}^1$ be a degree $d$ cover of curves. In work by Castryck, Zhao and the author, we showed how one can attach to each partition $\lambda$ of $d$ a multi-set of scrollar invariants of $\lambda$ with respect to $\varphi$. We studied these invariants when $\varphi$ is simply branched, and related these new scrollar invariants to known geometric data. In this article we show how one can remove this simple branching condition, using the notion of $S_d$-closure as developed by Bhargava and Satriano. With this new framework, we are able to generalize all results from this work to arbitrary covers $C\to \mathbb{P}^1$. In particular, we obtain a syzygy-free interpretation for the splitting types of the syzygy bundles in the Casnati--Ekedahl resolution of an arbitrary cover $\varphi: C\to \mathbb{P}^1$. By using the Maroni bound, we are able to give new general bounds on these splitting types.