The Allen-Cahn equation on the complete Riemannian manifolds of finite volume

Abstract: The semi-linear, elliptic PDE $AC_{\varepsilon}(u):=-\varepsilon^2\Delta u+W'(u)=0$ is called the Allen-Cahn equation. In this article we will prove the existence of finite energy solution to the Allen-Cahn equation on certain complete, non-compact manifolds. More precisely, suppose $M^{n+1}$ (with $n+1\geq 3$) is a complete Riemannian manifold of finite volume. Then there exists $\varepsilon_0>0$, depending on the ambient Riemannian metric, such that for all $0<\varepsilon\leq\varepsilon_0$, there exists $\mathfrak{u}{\varepsilon}:M\rightarrow (-1,1)$ satisfying $AC{\varepsilon}(\mathfrak{u}{\varepsilon})=0$ with the energy $E{\varepsilon}(\mathfrak{u}{\varepsilon})<\infty$ and the Morse index $\text{Ind}(\mathfrak{u}{\varepsilon})\leq 1$. Moreover, $0<\liminf_{\varepsilon\rightarrow 0}E_{\varepsilon}(\mathfrak{u}{\varepsilon})\leq\limsup{\varepsilon\rightarrow 0}E_{\varepsilon}(\mathfrak{u}_{\varepsilon})<\infty.$ Our result is motivated by the theorem of Chambers-Liokumovich and Song, which says that $M$ contains a complete minimal hypersurface $\Sigma$ with $0<\mathcal{H}^n(\Sigma)<\infty.$ This theorem can be recovered from our result.