An $L^\infty$-variational problem involving the Fractional Laplacian

For $s\in(0,1)$ and an open bounded set $\Omega\subset\mathbb R^n$, we prove existence and uniqueness of absolute minimisers of the supremal functional $$E_\infty(u)=\|(-\Delta)^s u\|_{L^\infty(\mathbb R^n)},$$ where $(-\Delta)^s$ is the Fractional Laplacian of order $s$ and $u$ has prescribed Dirichlet data in the complement of $\Omega$. We further show that the minimiser $u_\infty$ satisfies the (fractional) PDE $$(-\Delta)^s u_\infty=E_\infty(u_\infty)\,\mathrm{sgn}f_\infty \qquad\mbox{in }\Omega,$$ for some analytic function $f_\infty\in L^1(\Omega)$ obtained as the restriction of an $s$-harmonic measure $\mu$ in $\Omega$.