diff --git a/matlab/frank05_rightPreconditioning.m b/matlab/frank05_rightPreconditioning.m
index 36f38a71b..9b547afa8 100644
--- a/matlab/frank05_rightPreconditioning.m
+++ b/matlab/frank05_rightPreconditioning.m
@@ -14,16 +14,24 @@ spy(A);
 
 % isolate the spanning tree part
 A1=A(1:3*nnz(tree),:);
+% add prior
 figure(2)
 spy(A1)
 
-% calculate
+% calculate R1
 tic
 R1 = qr(A1,0);
 toc
 figure(3)
 spy(R1)
 
+% calculate R1
+tic
+R1 = chol(A1'*A1);
+toc
+figure(3)
+spy(R1)
+
 % calculate the entire R factor (expensive)
 tic
 R = qr(A,0);