From: bangerth <bangerth@0785d39b-7218-0410-832d-ea1e28bc413d>
Date: Wed, 6 Jun 2012 21:22:38 +0000 (+0000)
Subject: Add one more paragraph as suggested by Joerg.
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=66af1126e27e7272f916fee44b03b353631128f4;p=dealii-svn.git

Add one more paragraph as suggested by Joerg.


git-svn-id: https://svn.dealii.org/trunk@25609 0785d39b-7218-0410-832d-ea1e28bc413d
---

diff --git a/deal.II/doc/doxygen/headers/constraints.h b/deal.II/doc/doxygen/headers/constraints.h
index b4493e1564..9a74e6f85c 100644
--- a/deal.II/doc/doxygen/headers/constraints.h
+++ b/deal.II/doc/doxygen/headers/constraints.h
@@ -302,7 +302,7 @@
  * First approach:
  * - Apply the ConstraintMatrix::distribute_local_to_global() function to the
  *   system matrix and the right-hand-side with the parameter
- *   use_inhomogeneities_for_rhs = false (default)
+ *   use_inhomogeneities_for_rhs = false (i.e., the default)
  * - Set the solution to zero in the inhomogeneous constrained components
  *   using the ConstraintMatrix::set_zero() function (or start with a solution
  *   vector equal to zero)
@@ -382,6 +382,15 @@
  *   ConstraintMatrix::distribute() <i>before</i> solving the linear system
  *   (and then, as necessary, a second time after solving).
  *
+ * In addition to these considerations, consider the case where we have
+ * inhomogeneous constraints of the kind $x_{3}=\tfrac 12 x_1 + \tfrac 12$,
+ * e.g., from a hanging node constraint of the form $x_{3}=\tfrac 12 (x_1 +
+ * x_2)$ where $x_2$ is itself constrained by boundary values to $x_2=1$.
+ * In this case, the ConstraintMatrix can of course not figure out what
+ * the final value of $x_3$ should be and, consequently, can not set the
+ * solution vector's third component correctly. Thus, the second approach will
+ * not work and you should take the first.
+ *
  *
  * <h3>Dealing with conflicting constraints</h3>
  *