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Reversed Hardy-Littewood-Sobolev inequality

The classical sharp Hardy-Littlewood-Sobolev inequality states that, for $1<p, t<\infty$ and $0<λ=n-α<n$ with $ 1/p +1 /t+ λ/n=2$, there is a best constant $N(n,λ,p)>0$, such that $$ |\int_{\mathbb{R}^n} \int_{\mathbb{R}^n} f(x)|x-y|^{-λ} g(y) dx dy|\le N(n,λ,p)||f||_{L^p(\mathbb{R}^n)}||g||_{L^t(\mathbb{R}^n)} $$ holds for all $f\in L^p(\mathbb{R}^n), g\in L^t(\mathbb{R}^n).$ The sharp form is due to Lieb, who proved the existence of the extremal functions to the inequality with sharp constant, and computed the best constant in the case of $p=t$ (or one of them is 2). Except that the case for $p\in ((n-1)/n, n/α)$ (thus $α$ may be greater than $n$) was considered by Stein and Weiss in 1960, there is no other result for $α>n$. In this paper, we prove that the reversed Hardy-Littlewood-Sobolev inequality for $0<p, t<1$, $λ<0$ holds for all nonnegative $f\in L^p(\mathbb{R}^n), g\in L^t(\mathbb{R}^n).$ For $p=t$, the existence of extremal functions is proved, all extremal functions are classified via the method of moving sphere, and the best constant is computed.