Overdetermined problems with fractional laplacian

Fall, Mouhamed Moustapha¹ ; Jarohs, Sven²

¹ African Institute for Mathematical Sciences of Senegal. Km 2, Route de Joal. BP 1418 Mbour, Senegal
² Goethe-Universität Frankfurt, Institut für Mathematik. Robert-Mayer-Str. 10, 60054 Frankfurt, Germany

ESAIM: Control, Optimisation and Calculus of Variations, Tome 21 (2015) no. 4, pp. 924-938

Résumé

Let $N\ge 1$ and $s\in (0,1)$. In the present work we characterize bounded open sets $\Omega$ with $C^2$ boundary (not necessarily connected) for which the following overdetermined problem

$$\begin{array}{c}\hfill \left\{\begin{array}{cccc}\hfill {(-\Delta )}^{s}u& =f\left(u\right)\hfill & & \text{in}\Omega ;\hfill \\ \hfill u& =0\hfill & & \text{in}{ℝ}^N\setminus \Omega ;\hfill \\ \hfill {\left({\partial }_{\eta }\right)}_{s}u& =\mathrm{Const}.\hfill & & \text{on}\partial \Omega \hfill \end{array}\right.\end{array}$$

has a nonnegative and nontrivial solution, where $\eta$ is the outer unit normal vectorfield along $\partial \Omega$ and for $x_0\in \partial \Omega$

$$\begin{array}{c}\hfill {\left({\partial }_{\eta }\right)}_{s}u\left(x_0\right)=-\underset{t\to 0}{lim}\frac{u(x_0-t\eta \left(x_0\right))}{t^s}.\end{array}$$

Under mild assumptions on $f$, we prove that $\Omega$ must be a ball. In the special case $f\equiv 1$, we obtain an extension of Serrin’s result in 1971. The fact that $\Omega$ is not assumed to be connected is related to the nonlocal property of the fractional Laplacian. The main ingredients in our proof are maximum principles and the method of moving planes.

Détail

Reçu le : 2014-05-12
DOI : 10.1051/cocv/2014048

Classification : 35B50, 35N25
Keywords: Fractional Laplacian, maximum principles, Hopf’s Lemma, overdetermined problems