Angular measures and Birkhoff orthogonality in Minkowski planes

Abstract

Let $x$ and $y$ be two unit vectors in a normed plane ${\mathbb{R}}^2$. We say that $x$ is Birkhoff orthogonal to $y$ if the line through $x$ in the direction $y$ supports the unit disc. A B-measure (Fankhänel 2011) is an angular measure $\mu$ on the unit circle for which $\mu (C)=\pi /2$ whenever $C$ is a shorter arc of the unit circle connecting two Birkhoff orthogonal points. We present a characterization of the normed planes that admit a B-measure.