We will show a basic argument why Harnack inequality implies Holder continuity. Assume is a weak solution to a uniform elliptic equation

in . Assume we have Harnack inequality (Indeed we do have due to Moser), i.e. for any positive weak solution we will have

where is a constant independent of .

First one will know , see this note.

If is a weak solution, so are and . And they are strictly positive in due to the strong maximum principle, otherwise will be a constant. By Harnack inequality, we will have

Adding these two inequalities up we will have

hence

Denote , and choose such that , by interation we know

Then for any , choose such that , we will have

So Holder continuity follows.

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