A new look at the fractional Poisson problem via the Logarithmic Laplacian

Abstract: We analyze the $s$-dependence of solutions $u_s$ to the family of fractional Poisson problems $(-\Delta)^s u =f$ in $\Omega$, $u \equiv 0$ on $\mathbb{R}^N\setminus \Omega$ in an open bounded set $\Omega \subset \mathbb{R}^N$, $s \in (0,1)$. In the case where $\Omega$ is of class $C^2$ and $f \in C^{\alpha}(\bar{\Omega})$ for some $\alpha>0$, we show that the map $(0,1) \to L^\infty(\Omega)$, $s\mapsto u_s$ is of class $C^1$, and we characterize the derivative $\partial_s u_s$ in terms of the logarithmic Laplacian of $f$. As a corollary, we derive pointwise monotonicity properties of the solution map $s \mapsto u_s$ under suitable assumptions on $f$ and $\Omega$. Moreover, we derive explicit bounds for the corresponding Green operator on arbitrary bounded domains which are new even for the case $s=1$, i.e., for the local Dirichlet problem $-\Delta u = f$ in $\Omega$, $u \equiv 0$ on $\partial \Omega$.