# On stable solutions for boundary reactions: a De Giorgi-type result in dimension 4+1

created by figalli on 27 Jun 2019

[BibTeX]

Accepted Paper

Inserted: 27 jun 2019
Last Updated: 27 jun 2019

Journal: Invent. Math.
Year: 2019

Abstract:

We prove that every bounded stable solution of $(-\Delta)^{1/2} u + f(u) =0 \qquad \mbox{in }\mathbb{R}^3$ is a 1D profile, i.e., $u(x)= \phi(e\cdot x)$ for some $e\in \mathbb S^2$, where $\phi:\mathbb{R}\to \mathbb{R}$ is a nondecreasing bounded stable solution in dimension one. Equivalently, stable critical points of boundary reaction problems in $\mathbb{R}^{d+1}_+=\mathbb{R}^{d+1}\cap \{x_{d+1}\geq 0\}$ of the form $\int_{\{x_{d+1\geq 0}\}} \frac{1}{2} {\nabla U} ^2 \,dx\, dx_{d+1} + \int_{\{x_{d+1}=0\}} F(U) \,dx$ are 1D when $d=3.$

These equations have been studied since the 1940’s in crystal dislocations. Also, as it happens for the Allen-Cahn equation, the associated energies enjoy a $\Gamma$-convergence result to the perimeter functional. In particular, when $f(u)=u^3-u$ (or equivalently when $F(U)=\frac14 (1-U^2)^2$), our result implies the analogue of the De Giorgi conjecture for the half-Laplacian in dimension $4$, namely that monotone solutions are 1D.

Note that our result is a PDE version of the fact that stable embedded minimal surfaces in $\mathbb{R}^3$ are planes. It is interesting to observe that the corresponding statement about stable solutions to the Allen-Cahn equation (namely, when the half-Laplacian is replaced by the classical Laplacian) is still unknown for $d=3$.