 Milne used the letter formula_4 to denote a constant Doppler factor, but also considered a more general case involving non-inertial motion (and therefore a varying Doppler factor).Bondi used the letter formula_1 instead of formula_4 and simplified the presentation (for constant formula_1 only), and introduced the name ""k"-calculus".Consider two inertial observers, Alice and Bob, moving directly away from each other at constant relative velocity.