Harnack's inequality of elliptic PDE This problem is on page 334 of Evan's PDE book, where he states the Harnack's
inequality and refers the reader to Gilbarg-Trudinger [G-T] for the general situation. But in [G-T], that is, Elliptic Partial Differential Equations of Second Order, I can only find the proof when the underlying space is $\mathbb{R}^2$. For the higher dimension counterparts, this proof is invalid and I failed to adapt it to this case. I want to know where I can find the proof of the
Harnack's inequality.

It is in Gilbarg-Trudinger on page 199.
