D grad ν or grad(Dν)?

Abstract

An introduction to the problem in the title of the paper is attempted, D being a diffusion constant for particles of concentration ν. A general formalism and some special results are outlined which seek to show that, provided one proceeds in an appropriate and consistent manner, either formulation is permissible. The main area of application envisaged is to electronic flow in solids. The mathematical results also hold for atomic flow.