# Integrate Vector Laplacian By Parts For FEM

I am currently trying to solve the following, vectorial Poisson equation using the FEM technique:

$$-\nabla^2 \vec{A}=\vec{J} \quad \forall x\in\Omega$$

Now I know in the case of the scalar Poisson equation $$-\nabla^2 \varphi=\rho$$ one can derive the weak form of the PDE by introducing a scalar test function $$v$$ which is multiplied with the PDE and integrated over $$\Omega$$ using integration by parts:

$$\int_{\Omega} (\nabla^2\varphi) v \mathrm{d}x=\int_{\partial\Omega} (\vec{\nabla}\varphi) v \mathrm{d}\vec{\omega}-\int_{\Omega} (\vec{\nabla}\varphi) \cdot(\vec{\nabla}v) \mathrm{d}x$$

My question is how this approach can be applied to the vectorial Poisson equation? In order to do so, one would have to integrate the following expression by parts (where $$\vec{v}$$ is a vectorial test function):

$$\int_{\Omega} (\nabla^2\vec{A})\cdot\vec{v} \mathrm{d}x$$

However, I am struggling to do this integration. I guess one would have to generalize the following identity for scalar functions $$\varphi$$

$$\vec{\nabla}\cdot(v\vec{\nabla}\varphi)=(\nabla^2\varphi) v+(\vec{\nabla}\varphi)\cdot(\vec{\nabla}v)$$

into an identity of the following form for vectorial functions $$\vec{A}$$

$$\vec{\nabla}\cdot(...)=(\nabla^2\vec{A})\vec{v}+(...)$$

• Perhaps you'll find my old post useful: math.stackexchange.com/questions/745000/greens-first-identity. Best, Daniel. Dec 29, 2018 at 15:21
• Thank you for your response. It seems like the vector calculus identity $\nabla \cdot(\mathbf{T} \cdot \vec{\omega} ) = T:\nabla\vec{\omega} + (\nabla \cdot \mathbf{T}) \cdot\vec{\omega}$ which you linked solves the problem. I was able to come to the same solution with some different reasoning. I'll check the identity with some symbolic math software and then formulate an answer to my own question. Dec 29, 2018 at 18:39
• Glad to help @Mantabit. Best, Daniel. Dec 30, 2018 at 18:02

Not sure if this is useful, but have a look at this identity for the divergence of a matrix $$\mathbf{A}$$ acting on a vector $$\vec{v}$$:
$$\vec\nabla \cdot (\mathbf{A}\vec{v}) = (\vec\nabla \cdot \mathbf{A}) \vec{v} + \operatorname{Tr}\left(\mathbf{A}(\vec\nabla\vec{v})\right)$$
Now if you take $$\mathbf{A}$$ to be the Jacobi matrix of your vector field $$\vec{A}$$, i.e. $$\mathbf{A}= \vec{\nabla} \vec{A}$$, you get
$$\vec\nabla \cdot \big((\vec{\nabla} \vec{A})\vec{v}\big) = \left(\vec\nabla \cdot (\vec{\nabla} \vec{A})\right) \vec{v} + \operatorname{Tr}\left((\vec{\nabla} \vec{A})(\vec\nabla\vec{v})\right) = \left(\nabla^2\vec{A}\right) \vec{v} + \operatorname{Tr}\left((\vec{\nabla} \vec{A})(\vec\nabla\vec{v})\right)$$
Not sure if you still need it, but... You could use the identity, $$$$\nabla \times (\nabla \times \boldsymbol{A}) = \nabla (\nabla \cdot \boldsymbol{A}) - \nabla^2 \boldsymbol{A}$$$$
Replacing $$\nabla^2 \boldsymbol{A}$$ by compensating terms, you can use integration by parts easily. The weak form will become something like this: \begin{align*} - \int_\Omega \nabla^2 \boldsymbol{A} \cdot \boldsymbol{v} \; dx &= \int_\Omega \nabla \times ( \nabla \times \boldsymbol{A}) \cdot \boldsymbol{v} \; dx + \int_\Omega \nabla (\nabla \cdot \boldsymbol{A}) \cdot \boldsymbol{v} \; dx \\ &= \int_\Omega (\nabla \times \boldsymbol{A}) \cdot (\nabla \times \boldsymbol{v}) \; dx - \int_\Omega (\nabla \cdot \boldsymbol{A}) (\nabla \cdot \boldsymbol{v}) \; dx + \text{boundary terms} \\ \end{align*}