<a name="step-39-Intro"></a>

In this program, we use the interior penalty method and Nitsche's weak
boundary conditions to solve Poisson's equation. We use multigrid
methods on locally refined meshes, which are generated using a bulk
criterion and a standard error estimator based on cell and face
residuals. All operators are implemented using the MeshWorker interface.

Like in step-12, the discretization relies on finite element spaces,
which are polynomial inside the mesh cells $K\in \mathbb T_h$, but
have no continuity between cells. Since such functions have two values
on each interior face $F\in \mathbb F_h^i$, one from each side, we
define mean value and jump operators as follows: let
<i>K</i><sub>1</sub> and <i>K</i><sub>2</sub> be the two cells sharing
a face, and let the traces of functions <i>u<sub>i</sub></i> and the
outer normal vectors <b>n</b><i><sub>i</sub></i> be labeled
accordingly. Then, on the face, we let
@f[
	\average{ u } = \frac{u_1 + u_2}2
@f]

Note, that if such an expression contains a normal vector, the
averaging operator turns into a jump. The interior penalty method for the problem
@f[
  -\Delta u = f \text{ in }\Omega \qquad u = u^D \text{ on } \partial\Omega
@f]
becomes
@f{multline*}{
  \sum_{K\in \mathbb T_h} (\nabla u, \nabla v)_K
  \\
  + \sum_{F \in F_h^i} \biggl\{4\sigma_F (\average{ u \mathbf n}, \average{ v \mathbf n })_F
  - 2 (\average{ \nabla u },\average{ v\mathbf n })_F
  - 2 (\average{ \nabla v },\average{ u\mathbf n })_F
  \biggr\}
  \\
  + \sum_{F \in F_h^b} \biggl\{2\sigma_F (u, v)_F
  - (\partial_n u,v)_F
  - (\partial_n v,u)_F
  \biggr\}
  \\
  = (f, v)_\Omega + \sum_{F \in F_h^b} \biggl\{
  2\sigma_F (u^D, v)_F - (\partial_n v,u^D)_F
  \biggr\}.
@f}

Here, $\sigma_F$ is the penalty parameter, which is chosen as follows:
for a face <i>F</i> of a cell <i>K</i>, compute the value
@f[
\sigma_{F,K} = p(p+1) \frac{|F|_{d-1}}{|K|_d},
@f]
where <i>p</i> is the polynomial degree of the finite element
functions and $|\cdot|_d$ and $|\cdot|_{d-1}$ denote the $d$ and $d-1$
dimensional Hausdorff measure of the corresponding
object. If the face is at the boundary, choose $\sigma_F = \sigma_{F,K}$.
For an interior face, we take the average of the two values at this face.

In our finite element program, we distinguish three different
integrals, corresponding to the sums over cells, interior faces and
boundary faces above. Since the MeshWorker::loop organizes the sums
for us, we only need to implement the integrals over each mesh
element. The class MatrixIntegrator below has these three functions
for the left hand side of the formula, the class RHSIntegrator for the
right.

As we will see below, even the error estimate is of the same
structure, since it can be written as
@f{align*}{
  \eta^2 &= \eta_K^2 + \eta_F^2 + \eta_B^2
  \\
  \eta_K^2 &= \sum_{K\in \mathbb T_h} h^2 \|f + \Delta u_h\|^2
  \\
  \eta_F^2 &= \sum_{F \in F_h^i} \biggl\{
    4 \sigma_F \| \average{u_h\mathbf n} \|^2 + h \|\average{\partial_n u_h}\|^2 \biggr\}
  \\
  \eta_B^2 &= \sum_{F \in F_h^b} 2\sigma_F \| u_h-u^D \|^2.
@f}

Thus, the functions for assembling matrices, right hand side and error
estimates below exhibit that these loops are all generic and can be
programmed in the same way.

This program is related to step-12, in that it uses MeshWorker and
discontinuous Galerkin methods. There we solved an advection
problem, while here it is a diffusion problem. Here, we also use multigrid
preconditioning and a theoretically justified error estimator, see
Karakashian and Pascal (2003). The multilevel scheme was discussed in
detail in Kanschat (2004). The adaptive iteration and its convergence
have been discussed (for triangular meshes) in Hoppe, Kanschat, and
Warburton (2009).
