# Dune Core Modules (2.9.0)

Reference Elements

## Classes

struct  Dune::Geo::ReferenceElements< ctype_, dim >

class  Dune::Geo::ReferenceElement< Implementation >
This class provides access to geometric and topological properties of a reference element. More...

## Typedefs

template<typename... T>
using Dune::Transitional::ReferenceElement = unspecified-type
Returns the type of reference element for the argument types T... More...

template<typename T , int dim>
using Dune::ReferenceElement = unspecified-type
Returns the type of reference element for the argument type T. More...

## Functions

template<typename... T>
unspecified value type Dune::referenceElement (T &&... t)
Returns a reference element for the objects t.... More...

template<typename T , int dim>
auto Dune::referenceElement (const Dune::GeometryType &gt, Dune::Dim< dim >={})
Returns a reference element of dimension dim for the given geometry type and coordinate field type. More...

template<typename T , int dim, std::enable_if_t< IsNumber< std::decay_t< T >>::value, int > = 0>
auto Dune::referenceElement (const T &, const Dune::GeometryType &gt, Dune::Dim< dim >)
Returns a reference element of dimension dim for the given geometry type and coordinate field type. More...

# Introduction

In the following we will give a definition of reference elements and subelement numbering. This is used to define geometries by prescribing a set of points in the space $$\mathbf{R}^w$$.

The basic building block for these elements is given by a recursion formula which assigns to each set $$E \subset \mathbf{R}^d$$ either a prism element $$E^\vert\subset \mathbf{R}^{d+1}$$ or a pyramid element $$E^\circ\subset \mathbf{R}^{d+1}$$ with $$E^\vert = \lbrace (x,\bar{x}) \mid x \in E, \bar{x} \in [0,1] \rbrace$$ and $$E^\circ = \lbrace ((1-\bar{x})x,\bar{x}) \mid x \in E, \bar{x} \in [0,1] \rbrace$$. The recursion starts with a single point $$P_0=0\in\mathbf{R}^0$$.

For $$d=1,2,3$$ this leads to the following elements

• $$d=1$$: $$L_1 = P_0^\vert = P_0^\circ = [0,1]$$ is a line.
• $$d=2$$: $$Q_2 = L_1^\vert$$ is a cube and $$S_2 = L_1^\circ$$ is a simplex.
• $$d=3$$: $$Q_3 = Q_2^\vert$$ is a cube, $$S_3 = S_2^\circ$$ is a simplex, $$\mathrm{pyramid}_3 = Q_2^\circ$$ is a pyramid, and $$\mathrm{prism}_3 = S_2^\vert$$ is a prism.

In general if $$Q_d$$ is a cube then $$Q_d^\vert$$ is also a cube and if $$S_d$$ is a simplex then $$S_d^\circ$$ is also a simplex.

Based on the recursion formula we can also define a numbering of the subentities and also of the sub-subentities of $$E^\vert$$ or $$E^\circ$$ based on a numbering of $$E$$. For the subentities of codimension $$c$$ we use the numbering

• $$E^\vert$$: the first numbers are assigned to the entities parallel to the $$x_d-$$axis in the same order as the subentites of the same codimension in $$E$$; then to the subentities of codimension $$c-1$$ in the bottom followed by those in the top.
• $$E^\circ$$: in this case we first number the subentities of codimension $$c-1$$ in the bottom, followed by each subentity based on a subentity of codimension $$c$$ in $$E$$.

For the subentity of codimension $$cc$$ in a codimension $$c$$ subentity $$E'$$ we use the numbering induced by the numbering the reference element corresponding to $$E'$$.

Here is a graphical representation of the reference elements:

1. One-dimensional reference element. For d=1 the simplex and cube are identical
2. Two-dimensional reference simplex (a.k.a. triangle)
3. Three-dimensional reference simplex (a.k.a. tetrahedron)
 Face Numbering Edge Numbering
4. Two-dimensional reference cube (a.k.a. quadrilateral)
5. Three-dimensional reference cube (a.k.a. hexahedron)
 Face Numbering Edge Numbering
6. Prism reference element
 Face Numbering Edge Numbering
7. Pyramid reference element
 Face Numbering Edge Numbering

In addition to the numbering and the corner coordinates of a reference element $$\hat{E}$$ we also define the barycenters $$b(\hat{E})$$, the volume $$|\hat{E}|$$ and the normals $$n_i(\hat{E})$$ to all codimension one subelements.

The recursion formula is also used to define mappings from reference elements $$E$$ to general polytop given by a set of coordinates for the corner points - together with the mapping $$\Phi$$, the transpose of the Jacobian $$D\Phi^T(x)\in\mathbf{R}^{d,w}$$ is also defined where $$d$$ is the dimension of the reference element and $$w$$ the dimension of the coordinates. This suffices to define other necessary parts of a Dune geometry by LQ-decomposing $$D\Phi^T$$: let $$D\Phi^T(x) = LQ$$ be given with a lower diagonal matrix $$L\in \mathbf{R}^{d,d}$$ and a matrix $$Q\in\mathbf{R}^{d,w}$$ which satisfies $$Q Q^T = I$$:

• Jacobian inverse transpose

$D\Phi(x)^{-T}:=Q^T L^{-1}.$

• Integration element

$\sqrt{\mathrm{det}(D\Phi(x)^T D\Phi(x))} = \sqrt{\mathrm{det}(LL^T)} = \Pi_{i=1}^d l_{ii}.$

• Volume

$\int_{\hat{E}} \Pi_{i=1}^d l_{ii}(x) d\;x = |\hat{E}| \Pi_{i=1}^d l_{ii}(b(\hat{E})).$

(Here some assumptions on the degree of the integration element is used.)

The next sections describe the details of the construction.

# Reference Topology

We define the set $$\mathcal{T}$$ of reference topologies by the following rules:

• $$\mathcal{T}$$ contains an element $$\mathbf{p}$$ that we call the point topology.
• For $$T \in \mathcal{T}$$, $$\mathcal{T}$$ contains an element $$T^\vert$$ that we call the prism over $$T$$.
• For $$T \in \mathcal{T}$$, $$\mathcal{T}$$ contains an element $$T^\circ$$ that we call the pyramid over $$T$$.

For each reference topology $$T$$ we define the following values:

• Dimension: The point topology has dimension zero and the dimension of a prism or a pyramid topology over $$t$$ has dimension $$d(t)+1$$.
• Size: For $$c=0,\dots,d$$ with $$d=d(T)$$ we define the number $$s_c(T)$$ through
• $$s_0(T)=1$$.
• If $$T=t^\vert$$ then $$s_d(T)=2s_{d-1}(t)$$ and for $$c\in \{1,\dots,d-1\}$$ we have $$s_c(T)=s_{c}(t)+2s_{c-1}(t)$$.
• If $$T=t^\circ$$ then $$s_d(T)=s_{d-1}(t)+1$$ and for $$c\in \{1,\dots,d-1\}$$ we have $$s_c(T)=s_{c-1}(t)+s_{c}(t)$$.
• Subtopology: Given a reference topology $$T$$ of dimension $$d$$ and a codimension $$c=0,\dots,d$$ we now define the subtopology $$S_{c,i}(T)\in\mathcal{T}$$ for $$i=1,\dots,s_c(T)$$:
• $$S_{0,1}(T)=T$$ and $$S_{d,i}(T)=\mathbf{p}$$
• For $$T=t^\vert$$ and $$c\in \{1,\dots,d-1\}$$ we define using the abbreviations $$p=s_{c}(t),q=s_{c-1}(t)$$

$S_{c,i}(T) = \left\{\begin{array}{ll} S_{c,i}(t)^\vert & \mbox{for}\; i=1,\dots,p, \\ S_{c-1,i-p}(t) & \mbox{for}\; i=p+1,\dots,p+q, \\ S_{c-1,i-p-q}(t) & \mbox{for}\; i=p+q+1,\dots,p+2q. \end{array}\right.$

• For $$T=t^\circ$$ and $$c\in \{1,\dots,d-1\}$$ we define using the abbreviations $$p=s_{c}(t),q=s_{c-1}(t)$$

$S_{c,i}(T) = \left\{\begin{array}{ll} S_{c-1,i}(t) & \mbox{for}\; i=1,\dots,q, \\ S_{c,i-q}(t)^\circ & \mbox{for}\; i=q+1,\dots,q+p. \end{array}\right.$

Notice that the number of vertices (i.e., subtopologies of codimension $$d(T)$$) of a topology $$T$$ does not uniquely identify the topology. To see this, consider the topologies $$T_1 = \mathbf{p}^{\vert\vert\vert\circ\circ}$$ and $$T_2 = \mathbf{p}^{\circ\circ\circ\circ\vert}$$. For these topologies we have $$s_d( T_1 ) = s_d( T_2 ) = 10$$.

# Reference Domains

For each reference topology $$T$$ we associate the set of corners $$\mathcal{C}(T):=(p_i(T))_{1=1}^{s_d(T)}\subset\mathbf{R}^d(T)$$ defined through

• $$T=\mathbf{p}$$: $$p_0(T)=0\in\mathbf{R}^0$$
• $$T=t^\vert$$: $$p_k(T)=(p_k(t),0),p_{d'+k}(T)=(p_k(t),1)$$ for $$k=1,\dots,d'$$, with $$d'=s_d(t)$$.
• $$T=t^\circ$$: $$p_k(T)=(p_k(t),0)$$ for $$k=1,\dots,d-1$$ and $$p_d(T)=e_d$$ with $$d=s_d(T)$$

The convex hall of the set of points $$\mathcal{C}(T)$$ defines the reference domain $$\mathrm{domain}(T)$$ for the reference topology $$T$$; it follows that

• $$\mathrm{domain}( \mathbf{p} ) := \lbrace 0 \rbrace \subset \mathbf{R}^0$$,
• $$\mathrm{domain}( T^\vert ) := \mathrm{domain}( T ) \times [0,1]$$,
• $$\mathrm{domain}( T^\circ ) := \lbrace ((1-\bar{x})x,\bar{x}) \mid x \in \mathrm{domain}( T ), \bar{x} \in [0,1] \rbrace$$.

# Reference Elements and Mappings

A pair $$E=(T,\Phi)$$ of a topology $$T$$ and a map $$\Phi:\mathrm{domain}( T ) \to \mathbf{R}^w$$ with $$w\geq d(T)$$ is called an element.

The reference element is the pair $$E(T)=(T,{\rm id})$$.

For a given set of points $$\mathcal{C}:=(p_i)_{1=1}^{s_d(T)}\subset\mathbf{R}^w$$ we define a mapping $$\Phi_T(\mathcal{C},\cdot)\mathrm{domain}( T ) \to \mathbf{R}^w$$ through $$\Phi_T(\mathcal{C};p_i(T))=p_i$$ for all $$p_i(T)\in \mathcal{C}(T)$$. This mapping can be expressed using the recursive definition of the reference topologies through:

• $$\Phi_{\mathbf{p}}( (p_0) ; x ) = p_0$$,
• $$\Phi_{T^\vert}( (p_1,\ldots,p_{s_d(T^\vert)} ; (x,\bar{x}) ) = (1 - \bar{x}) \Phi_T( p_1,\ldots,p_{s_d(T)} ; x ) + \bar{x} \Phi_T( p_{s_d(T)+1},\dots,p_{s_d(T^\vert)} ; x )$$ with $$x\in \mathrm{domain}( T )$$ and $$\bar{x}\in[0,1]$$.
• $$\Phi_{T^\circ}( p_1,\ldots,p_{s_d(T^\circ)}) ; (x,\bar{x}) ) = (1 - \bar{x}) \Phi_T( p_1,\ldots,p_{s_d(T)-1} ; x ) + \bar{x} p_{s_d(T^\circ)}$$ with $$x\in \mathrm{domain}( T )$$ and $$\bar{x}\in[0,1]$$.

# Numbering of Subelements

Given a reference topology $$T$$, a codimension $$c\in\{0,\dots,d(T)\}$$ and a subtopology $$S_{c,i}(T)$$ we define a subset of the corner set $$\mathcal{C}(T)$$ $$\mathcal{C}_{c,i}(T)= (p_{k_j}(T))_{j=1}^{s_{d(S_{c,i}(T)}(S_{c,i}(T))}$$ given by the subsequence $$\mathcal{K}_{c,i}(T)=(k_j)_{j=1}^{s_{d(S_{c,i}(T)}(S_{c,i}(T))}$$ of $$\{1,dots,s_{d(T)}(T)\}$$:

$$\mathcal{C}_{0,0}(T) = \mathcal{C}(T)$$, $$\mathcal{C}_{d(T),i}(T) = (p_i(T))$$, and for $$c\in\{1,\dots,d(T)-1\}$$ we define $$\mathcal{K}_{c,i}(T)=(k_j)_{j=1}^{s_{d(S_{c,i}(T)}(S_{c,i}(T))}$$ through the recursion

• $$T=t^\vert$$:
For $$i=1,\dots,s_c(T)$$ we define $$\mathcal{K}_{c,i}(T)=(k_1,\dots,k_n, k_1+s_{d(t)}(t)\dots,k_n+s_{d(t)}(t))$$ with $$\mathcal{K}_{c,i}(T)= \{k_1,\dots,k_n\}$$.
For $$i=s_(T)+1,\dots,s_c(T)+s_{c+1}(T)$$ we define $$\mathcal{K}_{c,i}(T)=(k_1,\dots,k_n)$$ with $$\mathcal{K}_{c-1,i}(T)= \{k_1,\dots,k_n\}$$.
For $$i=s_c(T)+s_{c+1}(T)+1,\dots,s_c(T)+2s_{c+1}(T)$$ we define $$\mathcal{K}_{c,i}(T)=(k_1+s_{d(t)}(t),\dots,k_n+s_{d(t)}(t))$$ with $$\mathcal{K}_{c-1,i}(T)= \{k_1,\dots,k_n\}$$.
• $$T=t^\circ$$:
For $$i=1,\dots,s_{c-1}(T)$$ we define $$\mathcal{K}_{c,i}(T)=(k_1,\dots,k_n)$$ with $$\mathcal{K}_{c-1,i}(T)= \{k_1,\dots,k_n\}$$.
For $$i=s_{c-1}(T)+1,\dots,s_{c-1}(T)+s_c(T)$$ we define $$\mathcal{K}_{c,i}(T)=(k_1,\dots,k_n,s_{d(T)}(T))$$ with $$\mathcal{K}_{c-1,i}(T)= \{k_1,\dots,k_n\}$$.

Given these subsets we define subreference elements $$E_{c,i}(T)=(S_{c,i}(T),\Phi_{c,i}(T))$$ of $$E(T)$$ given by the following mapping $$\Phi_{c,i}(T,\cdot)=\Phi(S_{c,i}(T),\mathcal{C}_{c,i}(T),\cdot)$$.

Furthermore we define a numbering of the subreference elements of each subreference element in $$T$$. This is the number $$k=\mathrm{index}_{c,i,cc,ii}(T)$$ for $$c\in\{0,\dots,d(T)\}$$, $$i\in\{1,\dots,s_c(T)\}$$, and $$cc\in\{0,\dots,d(S_{c,i})\}$$, $$ii\in\{1,\dots,s_{cc}(S_{c,i})\}$$ for which

$\Phi_{c,i}(S_{c,i}(T))\circ \Phi_{cc,ii}(S_{cc,ii}(S_{c,i}(T))) = \Phi_{c+cc,k}(T).$

## ◆ ReferenceElement [1/2]

template<typename... T>
 using Dune::Transitional::ReferenceElement = typedef unspecified-type

Returns the type of reference element for the argument types T...

This type alias can be used to get the type of a reference element if you want to store the element as a class member or need to access nested type information. Normally, it will return the type of reference element that a call to referenceElement(T...) would return, for example for a geometry:

Cell::Geometry geo = cell.geometry();
unspecified value type referenceElement(T &&... t)
Returns a reference element for the objects t....
unspecified-type ReferenceElement
Returns the type of reference element for the argument type T.
Definition: referenceelements.hh:497

There is also a special shorthand signature for the default reference elements:

constexpr int dim = ...;
auto geometry_type = ...;
Dune::ReferenceElement<double,Dune::Dim<dim>> = referenceElement<double,dim>(geometry_type);
Dune::ReferenceElement

## ◆ ReferenceElement [2/2]

template<typename T , int dim>
 using Dune::ReferenceElement = typedef unspecified-type

Returns the type of reference element for the argument type T.

This type alias can be used to get the type of a reference element if you want to store the element as a class member or need to access nested type information. Normally, it will return the type of reference element that a call to referenceElement(T) would return, for example for a geometry:

Cell::Geometry geo = cell.geometry();

In the long run, we want to support multiple types T here, but for now that does not work due to backwards compatibility reasons. You can also still obtain the type of a standard reference element using Dune::ReferenceElement<ctype,dim>, but this is deprecated in DUNE 2.6. If you need support for ReferenceElement with multiple type arguments, you can use Dune::Transitional::ReferenceElement for now, which supports multiple types, but not the backwards compatibility mode and will become the default after the release of DUNE 2.6. There is also a special shorthand signature for the default reference elements:

Deprecated:
Using the syntax Dune::ReferenceElement<ctype,dim> is deprecated in DUNE 2.6. You have the following alternatives:
Dune::Transitional::ReferenceElement

## ◆ referenceElement() [1/3]

template<typename T , int dim>
 auto Dune::referenceElement ( const Dune::GeometryType & gt, Dune::Dim< dim > = {} )

Returns a reference element of dimension dim for the given geometry type and coordinate field type.

This function allows you to obtain a reference element for a given coordinate type, dimension and GeometryType:

auto gt = ...;
auto ref_el = referenceElement<ctype,dim>(gt);
auto ref_el = referenceElement<ctype>(gt,Dune::Dim<dim>());
bool gt(const T &first, const T &second, typename EpsilonType< T >::Type epsilon)
test if first greater than second
Definition: float_cmp.cc:158
Static tag representing a dimension.
Definition: dimension.hh:16

## ◆ referenceElement() [2/3]

template<typename T , int dim, std::enable_if_t< IsNumber< std::decay_t< T >>::value, int > = 0>
 auto Dune::referenceElement ( const T & , const Dune::GeometryType & gt, Dune::Dim< dim > )

Returns a reference element of dimension dim for the given geometry type and coordinate field type.

This function allows you to obtain a reference element for a given coordinate type, dimension and GeometryType:

auto gt = ...;
auto ref_el = referenceElement(ctype(),gt,Dune::Dim<dim>());

## ◆ referenceElement() [3/3]

template<typename... T>
 unspecified value type Dune::referenceElement ( T &&... t )

Returns a reference element for the objects t....

The freestanding function referenceElement is a generic entry point for getting reference elements for arbitrary objects that support the operation. As it relies on argument-dependent lookup, the function should be called without any qualifying namespace. Note, however, that the versions of referenceElement() for a dimension and GeometryType with explicit template arguments cannot be found by ADL, so you have to explicitly pull them in or qualify the call using Dune:::

{
// option 1: using
auto ref_el = referenceElement<double,dim>(geometry_type);
}
{
// option 2: explicitly put in Dune::
auto ref_el = Dune::referenceElement<double,dim>(geometry_type);
}
{
// option 3: use version without explicit template arguments
auto ref_el = referenceElement(double(),geometry_type,Dune::Dim<dim>());
}

The returned object is guaranteed to have value semantics, so you can copy it around and store it by value.

The grid geometries in dune-grid support this function, and thus most people will use this function as

for (const auto& cell : elements(grid_view))
{
auto geo = cell.geometry();
auto ref_el = referenceElement(geo);
// do some work...
}

This does of course also work for entities of other codimensions.

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