Asymptotic one-dimensional symmetry for the Fisher-KPP equation

TL;DR

This paper investigates the long-term geometric behavior of solutions to the Fisher-KPP equation, establishing conditions under which solutions become asymptotically one-dimensional and characterizing the directions of this asymptotic symmetry.

Contribution

It extends previous results by proving asymptotic one-dimensional symmetry for solutions with convex or near-convex initial supports, and characterizes the directions and profiles of this symmetry.

Findings

Solutions become locally planar when initial support is convex or close to convex.

The set of asymptotic directions is characterized and shown to be monotone.

Results recover and extend known cases with bounded or half-space initial supports.

Abstract

Let $u$ be a solution of the Fisher-KPP equation $$ \partial_t u=\Delta u+f(u),\quad t>0,\ x\in\mathbb{R}^N. $$ We address the following question: does $u$ become locally planar as $t\to+\infty$ ? Namely, does $u(t_n,x_n+\cdot)$ converge locally uniformly, up to subsequences, towards a one-dimensional function, for any sequence $((t_n,x_n))_{n\in\mathbb{N}}$ in $(0,+\infty)\times\mathbb{R}^N$ such that $t_n\to+\infty$ as $n\to+\infty$ ? This question is in the spirit of a conjecture of De Giorgi for stationary solutions of Allen-Cahn equations. The answer depends on the initial datum $u_0$ of $u$. It is known to be affirmative when the support of $u_0$ is bounded or when it lies between two parallel half-spaces. Instead, the answer is negative when the support of $u_0$ is "V-shaped". We prove here that $u$ is asymptotically locally planar when the support of $u_0$ is a convex set (satisfying in addition a uniform interior ball condition), or, more generally, when it is at finite Hausdorff distance from a convex set. We actually derive the result under an even more general geometric hypothesis on the support of $u_0$. We recover in particular the aforementioned results known in the literature. We further characterize the set of directions in which $u$ is asymptotically locally planar, and we show that the asymptotic profiles are monotone. Our results apply in particular when the support of $u_0$ is the subgraph of a function with vanishing global mean.