Existence and nonexistence of positive solutions of quasi-linear elliptic equations with gradient terms

We study the existence and nonexistence of positive solutions in the whole Euclidean space of coercive quasi-linear elliptic equations such as $$\Delta_p u = f(u)\pm g(\left|\nabla u\right|)$$ where $f\in C([0,\infty))$ and $g\in C^{0,1}([0,\infty))$ are strictly increasing with $f(0)=g(0)=0$. Among other things we obtain generalized integral conditions of Keller-Osserman type. In the particular case of plus sign on the right-hand side we obtain that different conditions are needed when $p\geq 2$ or $p\leq 2$, due to the degeneracy of the operator.