Piecewise geodesic Jordan curves I: weldings, explicit computations, and Schwarzian derivatives

Abstract: We consider Jordan curves $\gamma=\cup_{j=1}^n \gamma_j$ in the Riemann sphere for which each $\gamma_j$ is a hyperbolic geodesic in the complement of the remaining arcs $\gamma\smallsetminus \gamma_j$. These curves are characterized by the property that their conformal welding is piecewise M\"obius. Among other things, we compute the Schwarzian derivatives of the Riemann maps of the two regions in $\hat {\mathbb C}\smallsetminus \gamma$, show that they form a rational function with second order poles at the endpoints of the $\gamma_j,$ and show that the poles are simple if the curve has continuous tangents. Our key tool is the explicit computation of all geodesic pairs, namely pairs of chords $\gamma=\gamma_1\cup\gamma_2$ in a simply connected domain $D$ such that $\gamma_j$ is a hyperbolic geodesic in $D\smallsetminus \gamma_{3-j}$ for both $j=1$ and $j=2$.