High velocity frame transformations II: A new approach to Mansouri and Sexl theory

General frame transformations between inertial observers are simplified by imposing three kinematical conditions. A theorem is proved that such conditions ultimately correspond to a suitable choice of the coordinates in the two frames. Accordingly, the three kinematical conditions do not imply any genuine restriction. A further, restrictive, condition concerning rotational invariance is imposed, so determining the final form of the frame transformation