Clusters, toric ranks, and 2-ranks of hyperelliptic curves in the wild case

Abstract

Given a Galois cover $Y\to X$ of smooth projective geometrically connected curves over a complete discrete valuation field $K$ with algebraically closed residue field, we define a semistable model of $Y$ over the ring of integers of a finite extension of $K$ which we call the \emph{relatively stable model} $\Yrst$ of $Y$, and we discuss its properties, focusing on the case when $Y:y^2=f(x)$ is a hyperelliptic curve viewed as a degree-$2$ cover of the projective line $X := \proj_K^1$. Over residue characteristic different from $2$, it follows from known results that the toric rank (i.e.\ the number of loops in the graph of components) of the special fiber of $\Yrst$ can be computed directly from the knowledge of the even-cardinality clusters of roots of the defining polynomial $f$. We instead consider the ``wild" case of residue characteristic $2$ and demonstrate an analog to this result, showing that each even-cardinality cluster of roots of $f$ gives rise to a loop in the graph of components of the special fiber of $\Yrst$ if and only if the depth of the cluster exceeds some threshold, and we provide a computational description of and bounds for that threshold. As a bonus, our framework also allows us to provide a formula for the $2$-rank of the special fiber of $\Yrst$.

46 pages, 8 sections, 5 figures, 2 tables. Most content extracted from our earlier (longer) preprint [arXiv:2207.12490], but much of Sections 7 and 8 is new material. This is the version now published in Research in Number Theory; many changes have been made since the previous version, particularly in simplifying the set-up and definitions of Subsections 2.2 and 2.3