I am looking for a paper or a book that says about the existence and some estimates (like these in the non-degenerate case) of the fundamental solutions for degenerate elliptic equations $L = -divA\nabla$ over the domain ${\mathbb R}^{n+1}_+$, where $\lambda^{-1}\omega(x)\xi^2 \leq A(x)\xi\xi \leq \lambda\omega(x)\xi^2$ and $\omega \in A_2$

Wheeden and Chanillo had some kind of that results but only for bounded domain (they called Green function) on their paper "Existence and estimates of Greens functions for degenerate elliptic equations." However, they did not have the estimates for the size or the smoothness of the Green function.

Thanks.

Update: It looks like the problem is still untouched and we may not have fundamental solution for unbounded domain.