Say something on the singularity of the matrix.

git-svn-id: https://svn.dealii.org/trunk@18611 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-34/step-34.cc b/deal.II/examples/step-34/step-34.cc
index 7b44991a3ae5c4406b35631a32ddae26bc795313..07509c97c9e7c50fe7daa759abc246e823330f3b 100644 (file)
--- a/deal.II/examples/step-34/step-34.cc
+++ b/deal.II/examples/step-34/step-34.cc
@@ -880,18 +880,20 @@ void BEMProblem<dim>::assemble_system() {
                                 // reality it only produces an LU
                                 // decomposition) and then apply this inverse
                                 // to the right hand side to yield the
-                                // solution:
+                                // solution.
+                                //
+                                // As mentioned in the introduction,
+                                // the solution is only known up to a
+                                // constant potential. We solve this
+                                // issue by subtracting the mean
+                                // value of the vector from each
+                                // vector entry to normalize it.
 template <int dim>
 void BEMProblem<dim>::solve_system() {
     SparseDirectUMFPACK inverse_matrix;
     inverse_matrix.initialize (system_matrix);
     inverse_matrix.vmult (phi, system_rhs);
 
-//TODO: is this true? it seems to me that the BIE is definite...    
-    // Since we are solving a purely Neumann problem, the solution is
-    // only known up to a constant potential. We solve this issue by
-    // subtracting the mean value of the vector from each vector
-    // entry.
     phi.add(-phi.mean_value());
 }