Note something in relation to step-20.

git-svn-id: https://svn.dealii.org/trunk@28517 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-21/doc/intro.dox b/deal.II/examples/step-21/doc/intro.dox
index d5f8782c55b75e408570701f14374ea00e5bb20d..85ce8f313aa987718f35da9433cee73b7eef5cfe 100644 (file)
--- a/deal.II/examples/step-21/doc/intro.dox
+++ b/deal.II/examples/step-21/doc/intro.dox
@@ -324,7 +324,12 @@ M^S_{ij} &=&
 (S^n,\phi_i)_\Omega +\triangle t \sum_K  \left(F(S^n) q^{n+1}, \phi_i\right)_K.
 @f}
 
-Note the following complication, however: Since the matrix $H_{ij}$
+@note Due to historical accidents, the role of matrices $B$ and $B^T$
+has been reverted in this program compared to step-20. In other words,
+here $B$ refers to the divergence and $B^T$ to the gradient operators
+when it was the other way around in step-20.
+
+The system above presents a complication: Since the matrix $H_{ij}$
 depends on $\mathbf u^{n+1}$ implicitly (the velocities are needed to
 determine which parts of the boundaries $\partial K$ of cells are
 influx or outflux parts), we can only assemble this matrix after we