Regularity of Singular Solutions to $p$-Poisson Equations

This work showcases level set estimates for weak solutions to the $p$-Poisson equation on a bounded domain, which we use to establish Lebesgue space inclusions for weak solutions. In particular we show that if $\Omega\subset\mathbb{R}^n$ is a bounded domain and $u$ is a weak solution to the Dirichlet problem for Poisson's equation $$-\Delta u=f\textrm{ in }\Omega$$ $$\quad\;\; u=0\textrm{ on }\partial\Omega$$ for $f\in L^q(\Omega)$ with $q<\frac{n}{2}$, then $u\in L^r(\Omega)$ for every $r<\frac{qn}{n-2q}$ and indeed $\|u\|_r\leq C\|f\|_q$. This result is shown to be sharp, and similar regularity is established for solutions to the $p$-Poisson equation including in the edge case $q=\frac{n}{p}$.