Reduced polygons in the hyperbolic plane

Abstract

For a hyperplane $H$ supporting a convex body $C$ in the hyperbolic space $\mathbb{H}^d$ we define the width of $C$ determined by $H$ as the distance between $H$ and a most distant ultraparallel hyperplane supporting $C$. The minimum width of $C$ over all supporting $H$ is called the thickness $\Delta (C)$ of $C$. A convex body $R \subset \mathbb{H}^d$ is said to be reduced if $\Delta (Z) < \Delta (R)$ for every convex body $Z$ properly contained in $R$. We describe a class of reduced polygons in $\mathbb{H}^2$ and present some properties of them. In particular, we estimate their diameters in terms of their thicknesses.