Geometries on Polygons in the unit disc

Abstract

For a family $\mathcal{C}$ of properly embedded curves in the 2-dimensional disk $\mathbb{D}^{2}$ satisfying certain uniqueness properties, we consider convex polygons $P\subset \mathbb{D}^{2}$ and define a metric $d$ on $P$ such that $(P,d)$ is a geodesically complete metric space whose geodesics are precisely the curves $\left\{ c\cap P\bigm\vert c\in \mathcal{C}\right\}.$ Moreover, in the special case $\mathcal{C}$ consists of all Euclidean lines, it is shown that $P$ with this new metric is not isometric to any convex domain in $\mathbb{R} ^{2}$ equipped with its Hilbert metric. We generalize this construction to certain classes of uniquely geodesic metric spaces homeomorphic to $\mathbb{R}^{2}.$