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

Abstract

Let $(Σ,p)$ 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 $Σ$ 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 Teichmüller 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 $Σ\setminus p$ to a $k$-pronged graph, whose Hopf differential is that quadratic differential.