Consider the eigenfunction $\varphi_R>0$ $$L\varphi_R=\lambda_R\varphi_R, \ \ \ in \ \ B_R,$$ and $$\varphi_R=0, \ \ \ in \ \ \partial B_R,$$ where $L$ is a elliptic operator and $\lambda_R$ is the corresponding principal eigenvalue. We can normalize $\varphi_R$ such that $\varphi_R(0)=1$. Then, for the "Harnack inequality" in the ball $B_{2R}$, exists a constant $\delta_R>0$ such that $$\delta_R\leq\varphi_R\leq\delta_R^{-1}, \ \ \ in \ \ \overline{B_{3R/2}}.$$
Where can I find this Harnack inequality?