Improve documentation of positive entries for constrained entries.

git-svn-id: https://svn.dealii.org/trunk@30784 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/doc/doxygen/headers/constraints.h b/deal.II/doc/doxygen/headers/constraints.h
index 999a97659e60e4744cdfc3ca5b4d8facc87857f0..09255160533b89cdc8cba4ac3fa4c632561fa280 100644 (file)
--- a/deal.II/doc/doxygen/headers/constraints.h
+++ b/deal.II/doc/doxygen/headers/constraints.h
@@ -26,7 +26,7 @@
  * Constraints typically come from several sources, for example:
  * - If you have Dirichlet-type boundary conditions, $u|_{\partial\Omega}=g$,
  *   one usually enforces
- *   them by requiring that that degrees of freedom on the boundary have
+ *   them by requiring that degrees of freedom on the boundary have
  *   particular values, for example $x_{12}=42$ if the boundary condition
  *   $g(\mathbf x)$ requires that the finite element solution $u(\mathbf x)$
  *   at the location of degree
@@ -113,6 +113,13 @@
  * below. The resulting linear systems are equivalent to those one gets after
  * calling the ConstraintMatrix::condense() functions.
  *
+ * @note Both ways of applying constraints set the value of the matrix
+ * diagonals to constrained entries to a <i>positive</i> entry of the same
+ * magnitude as the other entries in the matrix. As a consequence, you need to
+ * set up your problem such that the weak form describing the main matrix
+ * contribution is not <i>negative definite</i>. Otherwise, iterative solvers
+ * such as CG will break down or be considerably slower as GMRES.
+ *
  * @note While these two ways are <i>equivalent</i>, i.e., the solution of
  * linear systems computed via either approach is the same, the linear
  * systems themselves do not necessarily have the same matrix and right
@@ -121,7 +128,6 @@
  * as a result of the way in which we compute them; they are, however, always
  * chosen in such a way that the solution to the linear system is the same.
  *
- *
  * <h4>Condensing matrices and sparsity patterns</h4>
  *
  * As mentioned above, the first way of using constraints is to build linear