Stable solutions to the fractional Allen-Cahn equation in the nonlocal perimeter regime

We study stable solutions to the fractional Allen-Cahn equation \linebreak $(-\mathrm{\Delta }{)}^{s\mathrm{/}2}u=u-u^3$, $\mathrm{\mid }u\mathrm{\mid }<1$ in ${\mathbb{R}}^n$. For every $s\in (0,1)$ and dimension $n\ge 2$, we establish sharp energy estimates, density estimates, and the convergence of blow-downs to stable nonlocal $s$-minimal cones. As a consequence, we obtain a new classification result: if for some pair $(n,s)$, with $n\ge 3$, hyperplanes are the only stable nonlocal $s$-minimal cones in ${\mathbb{R}}^n\setminus \{0\}$, then every stable solution to the fractional Allen-Cahn equation in ${\mathbb{R}}^n$ is 1D, namely, its level sets are parallel hyperplanes. Combining this result with the classification of stable $s$-minimal cones in ${\mathbb{R}}^3\setminus \{0\}$ for $s\sim 1$ obtained by the authors in a recent paper, we give positive answers to the “stability conjecture” in ${\mathbb{R}}^3$ and to the “De Giorgi conjecture” in ${\mathbb{R}}^4$ for the fractional Allen-Cahn equation when the order $s\in (0,1)$ of the operator is sufficiently close to $1$.