Nonlinear equations for fractional Laplacians, I: Regularity, maximum principles, and Hamiltonian estimates
Annales de l'I.H.P. Analyse non linéaire, Volume 31 (2014) no. 1, pp. 23-53.

This is the first of two articles dealing with the equation ${\left(-\Delta \right)}^{s}v=f\left(v\right)$ in ${ℝ}^{n}$, with $s\in \left(0,1\right)$, where ${\left(-\Delta \right)}^{s}$ stands for the fractional Laplacian — the infinitesimal generator of a Lévy process. This equation can be realized as a local linear degenerate elliptic equation in ${ℝ}_{+}^{n+1}$ together with a nonlinear Neumann boundary condition on $\partial {ℝ}_{+}^{n+1}={ℝ}^{n}$.In this first article, we establish necessary conditions on the nonlinearity f to admit certain type of solutions, with special interest in bounded increasing solutions in all of $ℝ$. These necessary conditions (which will be proven in a follow-up paper to be also sufficient for the existence of a bounded increasing solution) are derived from an equality and an estimate involving a Hamiltonian — in the spirit of a result of Modica for the Laplacian. Our proofs are uniform as $s↑1$, establishing in the limit the corresponding known results for the Laplacian.In addition, we study regularity issues, as well as maximum and Harnack principles associated to the equation.

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Cabré, Xavier; Sire, Yannick. Nonlinear equations for fractional Laplacians, I: Regularity, maximum principles, and Hamiltonian estimates. Annales de l'I.H.P. Analyse non linéaire, Volume 31 (2014) no. 1, pp. 23-53. doi : 10.1016/j.anihpc.2013.02.001. http://archive.numdam.org/articles/10.1016/j.anihpc.2013.02.001/

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