Ancient solutions to the Allen Cahn equation in catenoids

Abstract

Let $N\ge 2$ and $F:{\mathbb{R}}^N\to \mathbb{R}$ be the unique increasing radially symmetric function satisfying the minimal surface equation for graphs with the initial conditions $F(1)=0$ and ${\lim }_{r\to 1}{F}_r(r)=\infty ;$ $r=|x|.$ We construct an ancient solution to Allen-Cahn equation ${\tilde{u}}_t={\Delta }_M\tilde{u}+(1-{\tilde{u}}^2)\tilde{u}$ in $M\times (-\infty ,0),$ where $M=\{(x,\pm F(|x|)):x\in {\mathbb{R}}^N,|x|\ge 1\}$ is a $N$-dimensional catenoid in ${\mathbb{R}}^{N+1}$ and ${\Delta }_M$ is the Laplace Beltrami operator of $M.$ In particular, we construct a solution of the form $u(t,r,F(r))=u(t,r,-F(r))$ such that $$u(t,r,F(r))\approx \sum _{j=1}^k(-1{)}^{j-1}w(r-{\rho }_j(t))-\frac{1}{2}(1+(-1{)}^k)ast\to -\infty ,$$ where $w(s)$ is a solution of $w^{\prime \prime }+(1-w^2)w=0$ with $w(\pm \infty )=\pm 1,$ given by $w(s)=\tanh \left(\frac{s}{\sqrt{2}}\right),$ and $${\rho }_j(t)=\sqrt{-2(n-1)t}+\frac{1}{\sqrt{2}}\left(j-\frac{k+1}{2}\right)\log \left(\frac{|t|}{\log |t|}\right)+O(1),j=1,\dots ,k.$$