Memory functions for kinetic theory models of density fluctuations in fluids

Space-time memory functions for density correlation functions are derived from several simple kinetic equations and found to be simply expressed in terms of the memory functions for a free-particle system. Spectral and temporal behavior which illustrate the effects of free molecular flow, mean-field interactions, and intermolecular collisions are examined, and the manner in which improvements over linearized hydrodynamics are obtained is discussed.