Sharp reversed Hardy--Littlewood--Sobolev inequality on the half space $\mathbb R_+^n$

Abstract: This is the second in our series of papers concerning some reversed Hardy--Littlewood--Sobolev inequalities. In the present work, we establish the following sharp reversed Hardy--Littlewood--Sobolev inequality on the half space $\mathbb R_+^n$ [ \int_{\mathbb R_+^n} \int_{\partial \mathbb R_+^n} f(x) |x-y|^\lambda g(y) dx dy \geqslant \mathscr C_{n,p,r} |f|{L^p(\partial \mathbb R+^n)} \, |g|{L^r(\mathbb R+^n)} ] for any nonnegative functions $f\in L^p(\partial \mathbb R_+^n)$, $g\in L^r(\mathbb R_+^n)$, and $p,r\in (0,1)$, $\lambda > 0$ such that $(1-1/n)1/p + 1/r -(\lambda-1) /n =2$. Some estimates for $\mathscr C_{n,p,r}$ as well as the existence of extrema functions for this inequality are also considered. New ideas are also introduced in this paper.