Invariants recovering the reduction type of a hyperelliptic curve

Abstract

Tate's algorithm tells us that for an elliptic curve $E$ over a local field $K$ of residue characteristic $\ge 5$, $E/K$ has potentially good reduction if and only if $\text{ord}(j_E)\ge 0$. It also tells us that when $E/K$ is semistable the dual graph of the special fibre of the minimal regular model of $E/K^{\text{unr}}$ can be recovered from $\text{ord}(j_E)$. We generalise these results to hyperelliptic curves of genus $g\ge 2$ over local fields of odd residue characteristic $K$ by defining a list of absolute invariants that determine the potential stable model of a genus $g$ hyperelliptic curve $C$. They also determine the dual graph of the special fibre of the minimal regular model of $C/K^{\text{unr}}$ if $C/K$ is semistable. This list depends only on the genus of $C$, and the absolute invariants can be written in terms of the coefficients of a Weierstrass equation for $C$. We explicitly describe the method by which the valuations of the invariants recover the dual graphs. Additionally, we show by way of a counterexample that if $g\ge 2$, there is no list of invariants whose valuations determine the dual graph of the special fibre of the minimal regular model of a genus $g$ hyperelliptic curve $C$ over a local field $K$ of odd residue characteristic when $C$ is not assumed to be semistable.

40 pages