Little bit on registering symmetries

git-svn-id: http://svn.cactuscode.org/arrangements/CactusBase/CartGrid3D/trunk@146 c78560ca-4b45-4335-b268-5f3340f3cb52

1 files changed, 22 insertions, 2 deletions

diff --git a/doc/documentation.tex b/doc/documentation.tex
index 6299ed7..8a79610 100644
--- a/doc/documentation.tex
+++ b/doc/documentation.tex
@@ -57,8 +57,28 @@ symmetry domain is chosen (that is, if
 
 \subsection{Registering Symmetry Behaviour}
 
-Each grid function can register how it behaves under a coordinate change
-
+Each grid function can register how it behaves under a coordinate
+change using function calls in {\tt CartGrid3D}. These symmetry
+properties can then be used by other thorns, for example {\tt
+CactusBase/Boundary} uses them to enforce symmetry boundary conditions
+across coordinate axis. Symmetries should obviously be registered
+before they are used, but since they can be different for different grids, 
+they must be registered {\it after} the {\tt CCTK\_STARTUP} timebin. The
+usual place to register symmetries is in the {\tt CCTK\_BASEGRID} timebin.
+
+For example, to register the symmetries of the {\it xy} component of the
+metric tensor from C
+
+{\tt
+\begin{verbatim}
+static int one=1;
+int sym[3];
+sym[0] = -one;
+sym[1] = -one;
+sym[2] =  one;
+SetCartSymVN(cctkGH, sym,"einstein::gxy");
+\end{verbatim}
+}
 
 % Automatically created from the ccl files by using gmake thorndoc
 \include{interface}