Holomorphic differentials of Klein four covers

TL;DR

This paper investigates how the ramification structure of Klein four covers in characteristic two influences the decomposition of holomorphic differentials into indecomposable modules, providing a complete classification in certain cases.

Contribution

It offers a complete decomposition of holomorphic differentials for Klein four covers with totally ramified points and refutes a previous theorem by showing the diversity of indecomposable modules involved.

Findings

Decomposition of holomorphic differentials explicitly determined for certain covers.

Existence of infinitely many non-isomorphic indecomposable modules in the decomposition.

Refutation of a previous theorem regarding module classification.

Abstract

Let $k$ be an algebraically closed field of characteristic two, and let $G$ be isomorphic to $\mathbb{Z}/2\times\mathbb{Z}/2$. Suppose $X$ is a smooth projective irreducible curve over $k$ with a faithful $G$-action, and assume that the cover $X\to X/G$ is totally ramified, in the sense that it is ramified and every branch point is totally ramified. We study to what extent the lower ramification groups of the closed points of $X$ determine the isomorphism types of the indecomposable $kG$-modules and the multiplicities with which they occur as direct summands of the space $\mathrm{H}^0(X,\Omega_{X/k})$ of holomorphic differentials of $X$ over $k$. In the case when $X/G=\mathbb{P}^1_k$, we completely determine the decomposition of $\mathrm{H}^0(X,\Omega_{X/k})$ into a direct sum of indecomposable $kG$-modules. Moreover, we show that the isomorphism classes of indecomposable $kG$-modules that actually occur as direct summands belong to an infinite list of non-isomorphic indecomposable $kG$-modules that contain modules of arbitrarily large $k$-dimension. In particular, our results show that [14, Theorem 6.4] is incorrect.