Which named factor, symbolized by gamma, appears in relativistic Doppler formulas?

The named factor symbolized by gamma in relativistic Doppler formulas is the Lorentz factor.

The Lorentz factor is written as γ = 1/√(1 − β²), where β is the relative speed divided by the speed of light. It appears in special-relativity equations because measurements of time and frequency depend on relative motion, especially as speeds approach light speed.

In relativistic Doppler calculations, γ supplies the time-dilation part of the frequency shift. For motion perpendicular to the relevant line of sight, the transverse result is often expressed using γ alone: the observed frequency differs because the moving clock’s rate differs from that of the observer. For motion along the line of sight, γ combines with the ordinary velocity-dependent term.

The other choices belong to different areas. The Hubble parameter describes cosmic expansion, Mach number compares speed with the speed of sound, and Planck’s constant connects photon energy with frequency. None is the standard factor denoted by γ in these formulas.