Harnack inequality for hypoelliptic second order partial differential operators

We consider nonnegative solutions $u:\mathrm{\Omega }⟶\mathbb{R}$ of second order hypoelliptic equations \begin{equation*} \mathscr{L} u(x) =\sum_{i,j=1}^n \partial_{x_i} \left(a_{ij}(x)\partial_{x_j} u(x) \right) + \sum_{i=1}^n b_i(x) \partial_{x_i} u(x) =0, \end{equation*} where $\mathrm{\Omega }$ is a bounded open subset of ${\mathbb{R}}^n$ and $x$ denotes the point of $\mathrm{\Omega }$. For any fixed $x_0\in \mathrm{\Omega }$, we prove a Harnack inequality of this type $$\underset{K}{\sup }u\le C_{K}u(x_0)\mathrm{\forall }us\mathrm{.}t\mathrm{.}\mathcal{L}u=0,u\ge 0,$$ where $K$ is any compact subset of the interior of the $\mathcal{L}$-propagation set of $x_0$ and the constant $C_K$ does not depend on $u$.