Generalized relativistic velocity addition with spacetime algebra
  Using spacetime algebra, the geometric algebra of spacetime, the general problem of relativistic addition of velocities is addressed. The successive application of non-collinear Lorentz boosts is then studied in Minkowski spacetime. Even spatial vectors, such as the relative velocity of two reference frames, are analyzed in their proper setting: as vectors belonging to a four-dimensional spacetime manifold and not as vectors in ordinary three-dimensional (Euclidean) space. This is a clear and physical result that illustrates the fact that Lorentz boosts do not form a group. The entire derivation is carried through without using the second postulate on the speed of light, thereby stressing that the framework of special relativity does not depend on electromagnetism. 
Carlos R. Paiva, Marco A. Ribeiro