# reference request: Riesz/Newton potential and HLS inequality in L1.logL1

Let's consider in dimension $d\geq 3$ the Newton/riesz potential $f=I_2[g]$ $$f(x)=\int_{R^d}\frac{1}{|x-y|^{d-2}}g(y)dy,$$ which solves $-\Delta f=g$ (up to positive normalizing constants, which I shall ignore).

The classical Hardy-Littlewood-Sobolev inequality guarantees that $$|f|_{L^q(R^d)}\leq C|g|_{L^p(R^d)},\qquad q=\frac{dp}{d-2p}$$ as soon as $g\in L^p(R^d)$ for some $p\in (1,d/2)$.

If $g$ is only in $L^1$ the naive guess $|f|_{L^{d/(d-2)}}\leq C|g|_{L^1}$ fails, but we know that the singular integral converges for almost every $x\in R^d$ and that there is a weak type estimate $$\forall \lambda\geq 0, \qquad\mathcal{L}(\{x:\,|f|(x)\geq \lambda\})\leq C\left(\frac{|g|_{L^1}}{\lambda}\right)^{d/(d-2)}$$ ($\mathcal{L}$ denoting the usual Lebesgue measure on $R^d$).

My question is the following: what can we say if $g$ is in the slightly better space $g\in L^1\log L^1$, i-e if $|g|.|\log(|g|)|\in L^1(R^d)$? I don't think we can expect $f\in L^{d/(d-2)+\varepsilon}$ for any $\varepsilon>0$, but maybe $f\in L^{d/(d-2)}\log L^1$ or something like that? I would expect at least some improvement and in particular $f\in L^{d/(d-2)}$, but maybe not.

PS: I wrote my question in dimension $d\geq 3$ for simplicity, but I'm also interested in the borderline case $d=2$ when the Poisson kernel $1/|x-y|^{d-2}$ is replaced by $-\log|x-y|$.
• Also, could anyone please recommend a standard reference for the related problem of (fractional) integration with measure data, typically $-\Delta u=\mu$ in the whole space? – leo monsaingeon Jun 6 '14 at 8:17
This is just a guess, I'm writing it since nobody commented on your question. You might want to check Yano's extrapolation theorem (see here for a nice exposition) which states that: if an operator $T$ is bounded on all $L^p$ for $p>1$ close to 1, and the norm grows at most like $(p-1)^{-1}$, then $T$ extends to a bounded operator on $Llog L$. Not exactly what you are asking, but this might give you some ideas to get started.