# Integral representation of solution of an elliptic PDE in divergence form

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Posted online: 2021-05-26 16:33:17Z by Harish Shrivastava8

Cite as: P-210526.1

• Analysis of PDEs
• Classical Analysis and ODEs

### Problem's Description

Suppose we have a second order elliptic differential operator $$L(v) = -\text{div}(A(x) \nabla v)$$ $A(x)$ is a bounded and strictly positive definite matrix with Hölder continuous entries. And suppose $\Omega$ is a $C^1$ domain (can be considered more regular if required).

From [2], Theorem 1, we know that there exists a Poisson Kernel $K(x,y)$ for the above operator and domain $\Omega$. That is the solution $u$ such that $Lu=0$ in $\Omega$ can be written as $$u(x) = \int_{\partial \Omega}u(y) K(x,y) d\mathcal H^{N-1}(y)$$

On the other hand, we have from [1] we have the existence of Green's function $G$ for the operator $L$ in the domain $\Omega$. Natural question to ask is just like we have a integral representation for harmonic functions, can we say that the Poisson Kernel $K(x,y)$ and the co-normal derivative of Green's function on the boundary i.e. $\left ( A(y)\nabla_y G(x,y)\right )\cdot \nu$ are equal on $\partial \Omega$?

In particular, since the Green's function $G$ of $L$ exists for the domain $\Omega$, then for $u$ such that $Lu=0$ is the following expression true? $$u(x) = \int_{\partial \Omega}u(y) \left ( A(y)\nabla_y G(x,y)\right )\cdot \nu_y\,d\mathcal H^{N-1}(y).$$

More precisely, is the following true?

$$K(x,y) = \left ( A(y)\nabla_y G(x,y)\right )\cdot \nu_y\; \forall y \in \partial \Omega,\; x\in \Omega$$

The above claim hods true when $A(x)= Id$, i.e. when the operator $L$ is simple Laplacian. Under what condition/s we can say the same for a general second order elliptic operator in divergence form?

1. ## Article The Green function for uniformly elliptic equations

Manuscripta Mathematica 37 (1), 303-342, 1982

2. ## Article Necessary and sufficient conditions for absolute continuity of elliptic-harmonic measure

Annals of Mathematics 119, 121-141, 1984