Linearity of the self-diffusion process

Formal arguments are given that the self-diffusion process, understood as the mutual diffusion process in a system which consists of two mechanically similar species of particles, and which is at total equilibrium if the species labels are ignored, is an inherently linear, but nonlocal, transport process. There are no nonlinear Burnett effects, and the nonlocal diffusion coeffieient is independent of the composition of the mixture. The present state knowledge, from theory and from computer experiments, concerning the various quantities which appear in the formal anlaysis is summarized for both fluid and Lorentz systmes