Ancient solutions to the Allen Cahn equation in catenoids

Abstract

Let $N\geq 2$ 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 $M=\{(x, \pm F(|x|)):\;x\in\mathbb{R}^N,\;|x|\geq1\}$ 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 $$ u(t,r,F(r)) \approx \sum_{j=1}^k (-1)^{j-1}w(r-\rho_j(t)) - \frac 12 (1+ (-1)^{k}) \quad \hbox{ as } t\to -\infty, $$ where $w(s)$ is a solution of $w'' + (1-w^2)w=0$ with $w(\pm \infty)= \pm 1,$ given by $w(s) = \tanh \left(\frac s{\sqrt{2}} \right),$ and $$\rho_j(t)=\sqrt{-2(n-1)t}+\frac{1}{\sqrt{2}}\left(j-\frac{k+1}{2}\right)\log\left(\frac {|t|}{\log |t| }\right)+ O(1),\quad j=1,\ldots ,k.$$