Fibrations by plane quartic curves with a canonical moving singularity

TL;DR

This paper classifies special fibrations by plane quartic curves with a canonical moving singularity, revealing unique properties in characteristic two and constructing universal examples with detailed geometric analysis.

Contribution

It provides a classification of fibrations by plane quartic curves with a canonical moving singularity in characteristic two, including universal constructions and detailed geometric descriptions.

Findings

Fibrations exist only in characteristic two.

The generic fibre has tangent lines that are either all bitangents or all non-ordinary inflection tangents.

Constructed universal fibrations and analyzed their minimal regular models.

Abstract

We classify fibrations by integral plane projective rational quartic curves whose generic fibre is regular but admits a non-smooth point that is a canonical divisor. These fibrations can only exist in characteristic two. The geometric generic fibre, which determines the generic behaviour of the special fibres, is an integral plane projective rational quartic curve over the algebraic closure of the function field of the base. It has the remarkable property that the tangent lines at the non-singular points are either all bitangents or all non-ordinary inflection tangents; moreover it is strange, that is, all the tangent lines meet in a common point. We construct two fibrations that are universal in the sense that any other fibration with the aforementioned properties can be obtained from one of them by a base extension. Furthermore, among these fibrations we choose a pencil of plane quartic curves and study in detail its geometry. We determine the corresponding minimal regular model and we describe it as a purely inseparable double covering of a quasi-elliptic fibration.