Superharmonic functions in the upper half space with a nonlocal boundary condition

Abstract

We discuss the existence of positive superharmonic functions $u$ in ${\mathbb{R}}_{+}^N={\mathbb{R}}^{N-1}\times (0,\infty )$, $N\ge 3$, in the sense $-\hat{I}”u=\hat{I}¼$ for some Radon measure $\hat{I}¼$, so that $u$ satisfies the nonlocal boundary condition $$ \frac{\partial u}{\partial n}(x',0)=λ\int\limits_{\mathbb{R}^{N-1}}\frac{u(y',0)^p}{|x'-y'|^k}dy' \quad\mbox{ on }\partial \mathbb{R}^N_+, $$ where $p,\hat{I}»>0$ and $k\in (0,N-1)$. First, we show that no solutions exist if $0<k\le 1$. Next, if $1<k<N-1$, we obtain a new critical exponent given by $p^{\ast }=\frac{N-1}{k-1}$ for the existence of such solutions. If $\hat{I}¼\equiv 0$ we construct an exact solution for $p>p^{\ast }$ and discuss the existence of regular solutions, case in which we identify a second critical exponent given by $p^{\ast \ast }=2\cdot \frac{N-1}{k-1}-1$. Our approach combines various integral estimates with the properties of the newly introduced $\hat{I}\pm$-lifting operator and fixed point theorems.