Degree 2 Lagrange on a triangle

• \(R\) is the reference triangle. The following numbering of the subentities of the reference is used:

• \(\mathcal{V}\) is spanned by: \(1\), \(x\), \(x^{2}\), \(y\), \(x y\), \(y^{2}\)
• \(\mathcal{L}=\{l_0,...,l_{5}\}\)
• Functionals and basis functions:

\(\displaystyle l_{0}:v\mapsto v(0,0)\)

\(\displaystyle \phi_{0} = 2 x^{2} + 4 x y - 3 x + 2 y^{2} - 3 y + 1\)

This DOF is associated with vertex 0 of the reference element.

\(\displaystyle l_{1}:v\mapsto v(1,0)\)

\(\displaystyle \phi_{1} = x \left(2 x - 1\right)\)

This DOF is associated with vertex 1 of the reference element.

\(\displaystyle l_{2}:v\mapsto v(0,1)\)

\(\displaystyle \phi_{2} = y \left(2 y - 1\right)\)

This DOF is associated with vertex 2 of the reference element.

\(\displaystyle l_{3}:v\mapsto v(\tfrac{1}{2},\tfrac{1}{2})\)

\(\displaystyle \phi_{3} = 4 x y\)

This DOF is associated with edge 0 of the reference element.

\(\displaystyle l_{4}:v\mapsto v(0,\tfrac{1}{2})\)

\(\displaystyle \phi_{4} = 4 y \left(- x - y + 1\right)\)

This DOF is associated with edge 1 of the reference element.

\(\displaystyle l_{5}:v\mapsto v(\tfrac{1}{2},0)\)

\(\displaystyle \phi_{5} = 4 x \left(- x - y + 1\right)\)

This DOF is associated with edge 2 of the reference element.