Projections,Birkhoff Orthogonality and Angles in Normed Spaces

摘要

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 Aq(x,y)between two vectors x and y in X,such that x is Birkhoff orthogonal to y if and only if Aq(x,y)= π 2.Some other properties of this angle are also discussed.