Add to documentation.

git-svn-id: https://svn.dealii.org/trunk@23011 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/include/deal.II/fe/mapping_q1.h b/deal.II/include/deal.II/fe/mapping_q1.h
index 245c90fe107556877202a50792803aa4224a5f15..74e2871984055153803bc2bc751fff734cbad45e 100644 (file)
--- a/deal.II/include/deal.II/fe/mapping_q1.h
+++ b/deal.II/include/deal.II/fe/mapping_q1.h
@@ -219,6 +219,21 @@ class MappingQ1 : public Mapping<dim,spacedim>
                                          * i.e. $J_{ij}=dx_i/d\hat x_j$.
                                          *
                                          * Computed on each cell.
+                                         *
+                                         * @note If dim<spacedim,
+                                         * then the Jacobian matrix
+                                         * is rectangular, with
+                                         * spacedim rows and dim
+                                         * columns. We nevertheless
+                                         * store it as a square
+                                         * rank-2 tensor. The unused
+                                         * columns are internally
+                                         * used to store temporary
+                                         * data, such as the
+                                         * "missing" vectors spanning
+                                         * the full space (for
+                                         * example the cell normal
+                                         * vector).
                                          */
        std::vector<Tensor<2,spacedim> > contravariant;