Angular measures and Birkhoff orthogonality in Minkowski planes

Márton Naszódi, Vilmos Prokaj, Konrad Swanepoel

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 d…