# Linear Form Integrators

$\newcommand{\cross}{\times} \newcommand{\inner}{\cdot} \newcommand{\div}{\nabla\cdot} \newcommand{\curl}{\nabla\times} \newcommand{\grad}{\nabla} \newcommand{\ddx}[1]{\frac{d#1}{dx}}$

Linear form integrators are the right-hand side companion to Bilinear Form Integrators that compute the integrals of products of a basis function and a given "right-hand side" function (coefficient) $\,f$ over individual mesh elements (or sometimes over edges or faces). Typically each element is contained in the support of several basis functions, therefore linear integrators simultaneously compute the integrals of all combinations of the relevant basis functions with the given input function $\,f$. This produces a one dimensional array of results that is arranged into a small vector of integral (dual) values called a local element (load) vector.

To put this another way, the LinearForm class builds a global vector, glb_vec, by performing the outer loop in the following pseudocode snippet whereas the LinearFormIntegrator class performs the nested inner loops to compute the local vector, loc_vec.

for each elem in elements
loc_vec = 0.0
for each v_i in elem
loc_vec(i) += w(pt) * rhs(pt) v_i(pt)
end
end
glb_vec += loc_vec
end


There are three types of integrals that typically arise although many other, more exotic, forms are possible:

• Integrals involving Scalar rhs $\,f$ and basis functions: $\int_\Omega\, f v$
• Integrals involving Vector rhs $\,\vec{f}$ and basis functions: $\int_\Omega\, \vec{f}\cdot\vec{v}$
• Integrals involving mix of Scalar and Vector rhs $\,\vec{f}$ and basis functions: $\int_\Omega f\,\vec{\lambda}\cdot\vec{v}$ and $\int_\Omega v\,\vec{\lambda}\cdot\vec{f}$

The LinearFormIntegrator classes allow MFEM to produce a wide variety of local element matrices without modifying the LinearForm class. Many of the possible operators are collected below into tables that briefly describe their action and requirements.

In the tables below the Space column refers to finite element spaces which implement the following methods:

Space Operator Derivative Operator
H1 CalcShape CalcDShape
ND CalcVShape CalcCurlShape
RT CalcVShape CalcDivShape
L2 CalcShape None

Notation: $$\{(f, v)\}_i\equiv \int_\Omega f v_i$$ $$\{(\vec{F}, \vec{v})\}_i\equiv \int_\Omega \lambda \vec{F}\cdot\vec{v}_i$$ For boundary integrators, the integrals are over $\partial \Omega$. Face integrators integrate over the interior and boundary faces of mesh elements and are denoted with $\left<\cdot,\cdot\right>$.

## Scalar Field Operators

### Domain Integrators

Class Name Space Operator Continuous Op. Dimension
DomainLFIntegrator H1, L2 $(f, v)$ $f$ 1D, 2D, 3D
DomainLFGradIntegrator H1 $(\vec{f}, \nabla v)$ $-\nabla \cdot \vec{f}$ 1D, 2D, 3D

### Boundary Integrators

Class Name Space Operator Continuous Op. Dimension
BoundaryLFIntegrator H1, L2 $(f, v)$ $f$ 1D, 2D, 3D
BoundaryNormalLFIntegrator H1, L2 $(\vec{f} \cdot \vec{n}, v)$ $\vec{f} \cdot \vec{n}$ 1D, 2D, 3D
BoundaryTangentialLFIntegrator H1, L2 $(\vec{f} \cdot \vec{\tau}, v)$ $\vec{f} \cdot \vec{\tau}$ 2D
BoundaryFlowIntegrator H1, L2 $\frac{\alpha}{2}\, \left< (\vec{u} \cdot \vec{n})\, f, v \right> - \beta\, \left<\mid \vec{u} \cdot \vec{n} \mid f, v \right>$ $\frac{\alpha}{2} (\vec{u} \cdot \vec{n})\, f - \beta \mid \vec{u} \cdot \vec{n} \mid f$ 1D, 2D, 3D

### Face Integrators

Class Name Space Operator Continuous Op. Dimension
DGDirichletLFIntegrator L2 $\sigma \left< u_D, Q \nabla v \cdot \vec{n} \right> + \kappa \left< \{h^{-1} Q\} u_D, v \right>$ DG essential BCs for $u_D$ 1D, 2D, 3D

## Vector Field Operators

### Domain Integrators

Class Name Space Operator Continuous Op. Dimension
VectorDomainLFIntegrator H1, L2 $(\vec{f}, \vec{v})$ $\vec{f}$ 1D, 2D, 3D
VectorFEDomainLFIntegrator ND, RT $(\vec{f}, \vec{v})$ $\vec{f}$ 2D, 3D
VectorFEDomainLFCurlIntegrator ND $(\vec{f}, \nabla \times \vec{v})$ $\nabla \times \vec{f}$ 2D, 3D
VectorFEDomainLFDivIntegrator RT $(f, \nabla \cdot \vec{v})$ $- \nabla f$ 2D, 3D

### Boundary Integrators

Class Name Space Operator Continuous Op. Dimension
VectorBoundaryLFIntegrator H1, L2 $( \vec{f}, \vec{v} )$ $\vec{f}$ 1D, 2D, 3D
VectorBoundaryFluxLFIntegrator H1, L2 $( f, \vec{v} \cdot \vec{n} )$ $\vec{f}$ 1D, 2D, 3D
VectorFEBoundaryFluxLFIntegrator RT $( f, \vec{v} \cdot \vec{n} )$ $\vec{f}$ 2D, 3D
VectorFEBoundaryTangentLFIntegrator ND $( \vec{n} \times \vec{f}, \vec{v} )$ $\vec{n} \times \vec{f}$ 2D, 3D

### Face Integrators

Class Name Space Operator Continuous Op. Dimension
DGElasticityDirichletLFIntegrator L2 $\alpha\left<\vec{u_D}, \left(\lambda \left(\div \vec{v}\right) I + \mu \left(\nabla\vec{v} + \nabla\vec{v}^T\right)\right) \cdot \vec{n}\right> \\ + \kappa\left< h^{-1} (\lambda + 2 \mu) \vec{u_D}, \vec{v} \right>$ DG essential BCs for $\vec{u_D}$ 1D, 2D, 3D