relativistic train, moving at u, with respect to the ground, has a light source that emits light. An observer on the ground receives this light to be of frequency f_(1), when the train is approaching the observer, and of frequency f_(2), when the train has passes the observer and is now moving away from the observer. Au, in terms of f_(1) and f_(2) Bf_(0), in terms of f_(1) and f_(2)