Introduction

This thorn provides a linear extrapolation boundary condition in three dimensions. It is intended as an example for writing physical boundary conditions. (See the ThornGuide for CactusBase/Boundary for details on Cactus boundary conditions.) It is registered under the name LinearExtrap.

The code which actually implements the boundary condition is written in fixed form Fortran 90, in the file LinearExtrapBnd.F. This code was written by Carsten Gundlach, and is taken directly from the thorn AEIThorns/Exact. As such it illustrates a simple way to properly implement a boundary condition using Fortran code which was written long before the current boundary implementation specification.