Description:
<jats:p>Linear density response of a dilute gas is calculated using the Fokker-Planck kinetic equation. Exact results for the initial-value problem are compared with calculations based on single-relaxation kinetic models and on hydrodynamic equations. The distinction between collisions which relax the system to local equilibrium and those which relax the system to absolute equilibrium is emphasized. The relation of the dispersion equation in the Fokker-Planck description to that for Maxwell molecules is discussed.</jats:p>