Harnack's inequality

Let $D=D(z_{0},R)$ be an open disk and let f be a harmonic function on D such that f(z) is non-negative for all $z\in D$.

Then the following inequality holds for all $z\in D$:

$0\leq f(z)\leq \left({\frac {R}{R-\left|z-z_{0}\right|}}\right)^{2}f(z_{0})$