Modified multigrid for 3D elliptic equations with cross-derivatives

A portable Fortran code that solves the general nonseparable three-dimensio nal linear elliptic partial differential equation (PDE) with cross-derivati ve terms on a rectangular region is described. Boundary conditions can be a ny combination of periodic, specified, or mixed derivative. A multigrid sch eme, modified to handle complexities introduced by cross-derivative terms a nd nonnormal derivative components in boundary conditions, is utilized. Suc cessful application is illustrated with an analytically prescribed abstract example and simulation of an idealized geophysical fluid flow. (C) 2001 El sevier Science Inc. All rights reserved.