On a cosine function defined for smooth normed spaces
  We do further investigation in a certain cosine function defined for smooth Minkowski spaces. We prove that such function is symmetric if and only if the referred space is Euclidean, and also that it can be given in terms of the Gateaux derivative of the norm. As an application we use it to study the ratio between the lengths of tangent segments drawn through an external point to the unit circle of a Radon plane. We also give an easy characterization of such planes in terms of signs of the cosine function. 
Vitor Balestro and Emad Shonoda
math.DG math.MG