$W^{1,N}$ versus $C^1$ local minimizer for a singular functional with Neumann boundary condition

Abstract

Let $\Omega\subset\R^N,$ be a bounded domain with smooth boundary. Let $g:\R^+\to\R^+$ be a continuous on $(0,+\infty)$ non-increasing and satisfying $$c_1=\liminf_{t\to 0^+}g(t)t^{\delta}\leq\underset{t\to 0^+}{\limsup} g(t)t^{\delta}=c_2,$$ for some $c_1,c_2>0$ and $00$ is a constant. Consider the singular functional $I: W^{1,N}(\Omega)\to \R$ defined as \begin{eqnarray*} &&I(u) \eqdef\frac{1}{N}\|u\|^N_{W^{1,N}(\Omega)}-\int_{\Omega}G(u^+)\,{\rm d} x -\int_{\Omega}F(x,u^+) \,{\rm d} x\nonumber\\ && -\frac{1}{q+1}||u||^{q+1}_{L^{q+1}(\partial\Omega)} \nonumber \end{eqnarray*} where $F(x,u)= \int_0^sf(x,s)\,{\rm d}s$, $G(u)=\int_0^s g(s)\,{\rm d}s$. We show that if $u_0\in C^1(\overline{\Omega})$ satisfying $u_0\geq \eta \mbox{dist}(x,\partial\Omega)$, for some $0<\eta$, is a local minimum of $I$ in the $C^1(\overline{\Omega})\cap C_0(\overline{\Omega})$ topology, then it is also a local minimum in $W^{1,N}(\Omega)$ topology. This result is useful %for proving multiple solutions to the associated Euler-lagrange equation ${\rm (P)}$ defined below. to prove the multiplicity of positive solutions to critical growth problems with co-normal boundary conditions.

Keywords

• N-Laplace operator
• singular equations
• Neumann boundary condition
• Variational methods
• Local minimizers