Switch FE_DGQHermite to the more appropriate polynomial basis.

diff --git a/include/deal.II/fe/fe_dgq.h b/include/deal.II/fe/fe_dgq.h
index e8c832e183030932d62111ef8a9e5dc5e8196ab1..cf7ff20b679258503f566f27fc72ab997e34033d 100644 (file)
--- a/include/deal.II/fe/fe_dgq.h
+++ b/include/deal.II/fe/fe_dgq.h
@@ -481,16 +481,20 @@ public:
 
 /**
  * Implementation of scalar, discontinuous tensor product elements based on
- * Hermite polynomials, described by the polynomial space
- * Polynomials::HermiteInterpolation. As opposed to the basic FE_DGQ element,
+ * Hermite-like polynomials, described by the polynomial space
+ * Polynomials::HermiteLikeInterpolation. As opposed to the basic FE_DGQ element,
  * these elements are not interpolatory and no support points are defined.
  *
- * This element is only a Hermite polynomials for degrees larger or equal to
- * three. For degrees zero to two, a usual Lagrange basis is selected.
+ * Note that Hermite polynomials are only available for degrees larger or
+ * equal to three, and thus the beneficial properties of
+ * Polynomials::HermiteLikeInterpolation with only two basis functions having
+ * a non-trivial value or derivative on a face per dimension is only present
+ * for higher degrees. To facilitate usage also for degrees zero to two, a
+ * usual Lagrange basis is constructed by this class.
  *
  * See the base class documentation in FE_DGQ for details.
  *
- * @author Martin Kronbichler, 2017
+ * @author Martin Kronbichler, 2017, 2018
  */
 template <int dim,int spacedim=dim>
 class FE_DGQHermite : public FE_DGQ<dim,spacedim>
@@ -498,15 +502,16 @@ class FE_DGQHermite : public FE_DGQ<dim,spacedim>
 public:
   /**
    * Constructor for tensor product polynomials based on
-   * Polynomials::HermiteInterpolation.
+   * Polynomials::HermiteLikeInterpolation.
    */
   FE_DGQHermite (const unsigned int degree);
 
   /**
-   * Return a list of constant modes of the element. For the Hermite basis of
-   * degree three and larger, it returns one row where the first two elements
-   * (corresponding to the value left and the value at the right) are set to
-   * true and all other elements are set to false.
+   * Return a list of constant modes of the element. Due to the special
+   * construction of the Hermite basis, there is no simple representation of
+   * the constant value 1 in terms of a 0/1 selection, so get_constant_modes()
+   * throws an exception in case it is called for degrees larger or equal to
+   * three.
    */
   virtual std::pair<Table<2,bool>, std::vector<unsigned int> >
   get_constant_modes () const;

diff --git a/source/fe/fe_dgq.cc b/source/fe/fe_dgq.cc
index 31189df18882d919efe132d544837c896650034c..341f3f741c6371a31daede855cd8345e479f930c 100644 (file)
--- a/source/fe/fe_dgq.cc
+++ b/source/fe/fe_dgq.cc
@@ -882,7 +882,7 @@ FE_DGQHermite<dim,spacedim>::FE_DGQHermite (const unsigned int degree)
                          Polynomials::generate_complete_Lagrange_basis
                          (internal::FE_DGQ::get_QGaussLobatto_points(degree))
                          :
-                         Polynomials::HermiteInterpolation::generate_complete_basis(degree))
+                         Polynomials::HermiteLikeInterpolation::generate_complete_basis(degree))
 {}
 
 
@@ -895,17 +895,11 @@ FE_DGQHermite<dim,spacedim>::get_constant_modes () const
     return this->FE_DGQ<dim,spacedim>::get_constant_modes();
   else
     {
-      // The first two basis functions in the Hermite polynomials represent
-      // the value 1 in the left and right end point of the element. Expand
-      // them into the tensor product.
-      AssertThrow(dim<=3, ExcNotImplemented());
-      Table<2,bool> constant_modes(1, this->dofs_per_cell);
-      for (unsigned int i=0; i<(dim>2?2:1); ++i)
-        for (unsigned int j=0; j<(dim>1?2:1); ++j)
-          for (unsigned int k=0; k<2; ++k)
-            constant_modes(0,i*(this->degree+1)*(this->degree+1)+j*(this->degree+1)+k) = true;
+      AssertThrow(false,
+                  ExcMessage("Constant mode cannot be represented by 0/1 vector"));
       return std::pair<Table<2,bool>, std::vector<unsigned int> >
-             (constant_modes, std::vector<unsigned int>(1, 0));
+             (Table<2,bool>(1, this->dofs_per_cell),
+              std::vector<unsigned int>(1, 0));
     }
 }
 

diff --git a/tests/fe/element_constant_modes.cc b/tests/fe/element_constant_modes.cc
index 14be190e6fff2b8675e872374b29dec6fa3e57d2..4fe22916e4700f2e0ffc6372598fef45b614c2e4 100644 (file)
--- a/tests/fe/element_constant_modes.cc
+++ b/tests/fe/element_constant_modes.cc
@@ -50,7 +50,6 @@ void test()
   print_constant_modes(FE_DGQ<dim>(1));
   print_constant_modes(FE_DGQLegendre<dim>(2));
   print_constant_modes(FE_DGQHermite<dim>(2));
-  print_constant_modes(FE_DGQHermite<dim>(3));
   print_constant_modes(FE_DGP<dim>(2));
   print_constant_modes(FE_Q_Hierarchical<dim>(1));
   print_constant_modes(FE_Q_Hierarchical<dim>(2));

diff --git a/tests/fe/element_constant_modes.output b/tests/fe/element_constant_modes.output
index f25e1afb08bac3dd321af96fc3dd740e0a9f39c5..ee07412c5fc81fc71ee5e40b5f9ab004f344493d 100644 (file)
--- a/tests/fe/element_constant_modes.output
+++ b/tests/fe/element_constant_modes.output
@@ -26,9 +26,6 @@ DEAL::
 DEAL::Testing FE_DGQHermite<2>(2)
 DEAL::1 1 1 1 1 1 1 1 1 
 DEAL::
-DEAL::Testing FE_DGQHermite<2>(3)
-DEAL::1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 
-DEAL::
 DEAL::Testing FE_DGP<2>(2)
 DEAL::1 0 0 0 0 0 
 DEAL::
@@ -86,9 +83,6 @@ DEAL::
 DEAL::Testing FE_DGQHermite<3>(2)
 DEAL::1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 
 DEAL::
-DEAL::Testing FE_DGQHermite<3>(3)
-DEAL::1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
-DEAL::
 DEAL::Testing FE_DGP<3>(2)
 DEAL::1 0 0 0 0 0 0 0 0 0 
 DEAL::