Geometric constants for quantifying the difference between orthogonality   types
  This paper is devoted to introduce new geometric constants that quantify the difference between Roberts orthogonality and Birkhoff orthogonality in normed planes. We start by characterizing Roberts orthogonality in two different ways: via bisectors of two points and using certain linear transformations. Each of these characterizations yields one of those geometric constants that we will study. 
Vitor Balestro, Horst Martini and Ralph Teixeira
math.MG math.FA