Solutions in Lebesgue spaces to nonlinear elliptic equations with sub-natural growth terms

We study the existence problem for positive solutions $u \in L^{r}(\mathbb{R}^{n})$, $0<r<\infty$, to the quasilinear elliptic equation $$-\Delta_{p} u = \sigma u^{q} \quad \text{in} \;\; \mathbb{R}^n$$ in the sub-natural growth case $0<q< p-1$, where $\Delta_{p}u = \text{div}( |\nabla u|^{p-2} \nabla u )$ is the $p$-Laplacian with $1<p<\infty$, and $\sigma$ is a nonnegative measurable function (or measure) on $\mathbb{R}^n$. Our techniques rely on a study of general integral equations involving nonlinear potentials and related weighted norm inequalities. They are applicable to more general quasilinear elliptic operators such as the $\mathcal{A}$-Laplacian $\text{div} \mathcal{A}(x,\nabla u)$, and the fractional Laplacian $(-\Delta)^{\alpha}$ on $\mathbb{R}^n$, as well as linear uniformly elliptic operators with bounded measurable coefficients $\text{div}(\mathcal{A} \nabla u)$ on an arbitrary domain $\Omega \subseteq \mathbb{R}^n$ with a positive Green function.