Projections, Birkhoff Orthogonality and Angles in Normed Spaces

Abstract

Let $X$ be a Minkowski plane, i.e., a real two dimensional normed linear space. We use projections to give a definition of the angle $A_q(x, y)$ between two vectors $x$ and $y$ in $X$, such that $x$ is Birkhoff orthogonal to $y$ if and only if $A_q(x, y) = \frac{π}{2}$. Some other properties of this angle are also discussed.