Dune::LagrangeSimplexLocalFiniteElement< D, R, d, k > Class Template Reference

Lagrange finite element for simplices with arbitrary compile-time dimension and polynomial order. More...

#include <dune/localfunctions/lagrange/lagrangesimplex.hh>

Public Types
using	Traits = LocalFiniteElementTraits< Impl::LagrangeSimplexLocalBasis< D, R, d, k >, Impl::LagrangeSimplexLocalCoefficients< d, k >, Impl::LagrangeSimplexLocalInterpolation< Impl::LagrangeSimplexLocalBasis< D, R, d, k > > >
	Export number types, dimensions, etc.

Public Member Functions
	LagrangeSimplexLocalFiniteElement ()
template<typename VertexMap >
	LagrangeSimplexLocalFiniteElement (const VertexMap &vertexmap)
	Constructs a finite element given a vertex reordering.
const Traits::LocalBasisType &	localBasis () const
	Returns the local basis, i.e., the set of shape functions.
const Traits::LocalCoefficientsType &	localCoefficients () const
	Returns the assignment of the degrees of freedom to the element subentities.
const Traits::LocalInterpolationType &	localInterpolation () const
	Returns object that evaluates degrees of freedom.

Static Public Member Functions
static constexpr std::size_t	size ()
	The number of shape functions.
static constexpr GeometryType	type ()
	The reference element that the local finite element is defined on.

Detailed Description

template<class D, class R, int d, int k>
class Dune::LagrangeSimplexLocalFiniteElement< D, R, d, k >

Lagrange finite element for simplices with arbitrary compile-time dimension and polynomial order.

Template Parameters

D	type used for domain coordinates
R	type used for function values
d	dimension of the reference element
k	polynomial order

The Lagrange basis functions $\phi_i$ of order $k>1$ on the unit simplex $G = \{ x \in [0,1]^{d} \,|\, \sum_{j=1}^d x_j \leq 1\}$ are implemented as $\phi_i(x) = \hat{\phi}_i(kx)$ where $\hat{\phi}_i$ is the $i$-th basis function on the scaled simplex $\hat{G} = kG = \{ x \in [0,k]^{d} \,|\, \sum_{j=1}^d x_i \leq k\}$. On $\hat{G}$ the uniform Lagrange points of order $k$ are exactly the points $(i_1,\dots,i_d) \in \mathbb{N}_0^d \cap \hat{G}$ with non-negative integer coordinates in $\hat{G}$. Using the lexicographic enumeration of those points we can identify each $i=(i_1,...,i_d)$ with the flat index $i$ of the Lagrange node and associated basis function.

Since we can map any point $\hat{x} \in \hat{G}$ to its barycentric coordinates $(\hat{x}_1, \dots, \hat{x}_d, k-\sum_{j=1}^d \hat{x}_j)$ we define for any such point its auxiliary $(d+1)$-th coordinate $\hat{x}_{d+1} = k-\sum_{j=1}^d \hat{x}_j$ and use this in particular for Lagrange points $i=(i_1,...,i_d)$. Then the Lagrange basis function on $\hat{G}$ associated to the Lagrange point $i=(i_1,\dots,i_d)$ is given by

$$\hat{\phi}_i(\hat{x}) = \prod_{j=1}^{d+1} L_{i_j}(\hat{x}_j)$$

where we used the barycentric coordinates of $\hat{x}$ and $i$ and the univariate Lagrange polynomials

$$L_n(t) = \prod_{m=0}^{n-1} \frac{t-m}{n-m} = \frac{1}{n!}\prod_{m=0}^{n-1}(t-m).$$

This factorization can be interpreted geometrically. To this end note that any component $1,\dots,d+1$ of the barycentric coordinates is associated to a vertex of the simplex and thus to the opposite facet. Furthermore the Lagrange nodes are all located on hyperplanes parallel to those facets. In the factorized form the term $L_{i_j}(\hat{x}_j)$ guarantees that $\hat{\phi}_i$ is zero on any of these hyperplane that is parallel to the facet associated with $j$ and lying between the Lagrange node $i$ and this facet (excluding $i$ and including the facet).

Using the factorized form, evaluating all basis functions $\hat{\phi}_i$ at a point $\hat{x}$ requires to first evaluate $\alpha_{m,j}=L_m(\hat{x}_j)$ for all $j\in \{1,\dots,d+1\}$ and $m \in \{0,\dots,k\}$ and then computing the products $\hat{\phi}_i(\hat{x})=\prod_{j=1}^{d+1} \alpha_{i_j,j}$ for all admissible multi indices (or, equivalently, Lagrange nodes in barycentric coordinates) $(i_1,\dots,i_{d+1})$ with $\sum_{j=1}^{d+1} i_j = k$. The evaluation of the univariate Lagrange polynomials can be done efficiently using the recursion

$$L_0(t) = 1, \qquad L_{n+1}(t) = L_n(t)\frac{t-n}{n+1} \qquad n\geq 0.$$

Constructor & Destructor Documentation

LagrangeSimplexLocalFiniteElement()

template<class D , class R , int d, int k>

Dune::LagrangeSimplexLocalFiniteElement< D, R, d, k >::LagrangeSimplexLocalFiniteElement ( )

inline

Default-construct the finite element

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The documentation for this class was generated from the following file:

• dune/localfunctions/lagrange/lagrangesimplex.hh