--- /dev/null
+//-------------------------------------------------------------------------
+// $Id$
+// Version: $Name$
+//
+// Copyright (C) 2010 by the deal.II authors
+//
+// This file is subject to QPL and may not be distributed
+// without copyright and license information. Please refer
+// to the file deal.II/doc/license.html for the text and
+// further information on this license.
+//
+//-------------------------------------------------------------------------
+
+/**
+ * @defgroup constraints Constraints on degrees of freedom
+ * @ingroup dofs
+ *
+ * This module deals with constraints on degrees of
+ * freedom. Constraints typically come from several sources, for
+ * example:
+ * - If you have Dirichlet-type boundary conditions, one usually enforces
+ * them by requiring that that degrees of freedom on the boundary have
+ * particular values, for example $x_12=42$ if the boundary condition
+ * requires that the finite element solution at the location of degree
+ * of freedom 12 has the value 42.
+ *
+ * This class implements dealing with linear (possibly inhomogeneous)
+ * constraints on degrees of freedom. In particular, it handles constraints of
+ * the form $x_{i_1} = \sum_{j=2}^M a_{i_j} x_{i_j} + b_i$. In the context of
+ * adaptive finite elements, such constraints appear most frequently as
+ * "hanging nodes" and for implementing Dirichlet boundary conditions in
+ * strong form. The class is meant to deal with a limited number of
+ * constraints relative to the total number of degrees of freedom, for example
+ * a few per cent up to maybe 30 per cent; and with a linear combination of
+ * $M$ other degrees of freedom where $M$ is also relatively small (no larger
+ * than at most around the average number of entries per row of a linear
+ * system). It is <em>not</em> meant to describe full rank linear systems.
+ */