  This paper is concerned with semilinear equations in divergence form \[
\diver(A(x)Du) = f(u) \] where $f :\R \to [0,\infty)$ is nondecreasing. We
prove a sharp Harnack type inequality for nonnegative solutions which is
closely connected to the classical Keller-Osserman condition for the existence
of entire solutions.
