In $\mathbb{R}^d$, consider the following equation $$\Delta u -x\cdot \nabla u = f $$ where $f$ can be $C^\infty$ and decay like $e^{-\frac{c|x|^2}{2}}$.
I would like to know fundamental sol. to this equation or some gradient estimate, preferably pointwise. In particular I'm interested in the following quantity \begin{align*} \int_{\mathbb{R}^d}|\nabla u|^2e^{-\frac{|x|^2}{2}}dx. \end{align*}
One useful observation might be $$\nabla \cdot (e^{-\frac{|x|^2}{2}}\nabla u)=f e^{-\frac{|x|^2}{2}} .$$
Thank you!
