Approximate gravitational field equations

Two useful ways of approximating the exact gravitational field equations are described. In the first, the gravitational potential ψ is arbitrary and the spatial components of the covector field n are small; in the second, ψ is small and the spatial components of n are arbitrary. A canonical stress-momentum is defined which is often easier to use than the symmetric stress-momentum of earlier papers. Applications of the formalism to gravitational radiation, the post-Newtonian theory, and cosmology are outlined.