 If Alice and Bob were to swap roles, and Bob sent flashes of light to Alice, the Principle of Relativity (Einstein's first postulate) implies that the "k"-factor from Bob to Alice would be the same value as the "k"-factor from Alice to Bob, as all inertial observers are equivalent.So the "k"-factor depends only on the relative speed between the observers and nothing else.Consider, now, a third inertial observer Dave who is a fixed distance from Alice, and such that Bob lies on the straight line between Alice and Dave.