1 files changed, 18 insertions, 0 deletions
diff --git a/README b/README
index 434ec8b..76c047f 100644
--- a/README
+++ b/README
@@ -20,3 +20,21 @@ y directions.
 
 This symmetry boundary condition can be mixed with arbitrary symmetry
 conditions in the z direction, e.g. bitant or periodicity.
+
+
+
+================================================================================
+
+
+
+R90: operator that rotates points (counter-clockwise) by 90 degrees
+
+f(x) = f(R90 x) = f(R180 x) = f(R270 )
+
+
+symmetry about x=0:
+
+f  (-x) =  f  (+x)
+vx (-x) =  vx (+x)
+vy (-x) = -vy (+x)
+vz: same as scalar