Consider a vector in R2,D with norm −R2 , The set of all vectors with this norm are rotated into each other by the O(2,D) Rotations. Use this group to make the vector in the form X=(1,0,0...) It is obvious that the isotropy group that leaves this invariant is O(1,D) and thus we get the equivalence because these vectors with the specified norm are in one to one correspondance with the group transformations modulo the isotropy group.