Sublinear elliptic problems under radiality. Harmonic $NA$ groups and Euclidean spaces

Abstract

Let $\L $ be the Laplace operator on $\R ^d$, $d\geq 3$ or the Laplace Beltrami operator on the harmonic $NA$ group (in particular on a rank one noncompact symmetric space). For the equation $ \L u - \varphi(\cdot,u)=0$ we give necessary and sufficient conditions for the existence of entire bounded or large solutions under the hypothesis of radiality of $\varphi$ with respect to the first variable. A Harnack-type inequality for positive continuous solutions is also proved.