We study the regularity of stable solutions to the problem
$\begin{align}\left\{ \begin{gathered} {\left( { - \Delta } \right)^s}&u = f\left( u \right)&{\text{in}}\;\;{B_1}, \hfill \\ &u \equiv 0&{\text{in}}\;\;{{\mathbb{R}}^n}\backslash {B_1}, \hfill \\ \end{gathered} \right.\end{align}$
where $s∈(0,1)$ . Our main result establishes an $L^∞$ bound for stable and radially decreasing $H^s$ solutions to this problem in dimensions $2 ≤ n < 2(s+2+\sqrt{2(s+1)})$ . In particular, this estimate holds for all $s∈(0,1)$ in dimensions $2 ≤ n≤ 6$ . It applies to all nonlinearities $f∈ C^2$ .
For such parameters $s$ and $n$ , our result leads to the regularity of the extremal solution when $f$ is replaced by $λ f$ with $λ > 0$ . This is a widely studied question for $s = 1$ , which is still largely open in the nonradial case both for $s = 1$ and $s < 1$ .
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