Quadratic differentials, half-plane structures, and harmonic maps to graphs

TL;DR

This paper generalizes Strebel's theorem by parametrizing meromorphic quadratic differentials with specific pole orders and critical graphs, linking singular-flat metrics and harmonic maps to graphs on punctured Riemann surfaces.

Contribution

It introduces a new parametrization of quadratic differentials with prescribed pole orders and critical graphs, extending classical results and connecting to harmonic maps and Teichmüller theory.

Findings

Parametrization space isomorphic to b5^{k} imes S^1.

Each point corresponds to a unique metric spine as a ribbon-graph.

Establishes a relation between singular-flat geometry and harmonic maps.

Abstract

Let $(\Sigma,p)$ be a pointed Riemann surface of genus $g\geq 1$. For any integer $k\geq 1$, we parametrize the space of meromorphic quadratic differentials on $\Sigma$ with a pole of order $(k+2)$ at $p$, having a connected critical graph and an induced metric composed of $k$ Euclidean half-planes. The parameters form a finite-dimensional space $\mathcal{L} \cong \mathbb{R}^{k} \times S^1$ that describe a model singular-flat metric around the puncture with respect to a choice of coordinate chart. This generalizes an important theorem of Strebel, and associates, to each point in a decorated Teichm\"{u}ller space $\mathcal{T}_{g,1} \times \mathcal{L}$, a unique metric spine of the surface that is a ribbon-graph with $k$ infinite-length edges to $p$. The proofs study and relate the singular-flat geometry on the surface and the infinite-energy harmonic map from $\Sigma\setminus p$ to a $k$-pronged graph, whose Hopf differential is that quadratic differential.