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Sep 17, 2020 at 15:38 comment added Slm2004 I think you are right. Now I believe the claim of the paper should be if $(-\Delta)^{\frac14}u\in L^{2,\infty}\cap L^p$ for some $1\leq p<\infty$ then $u\in BMO$. The claim implicitly uses $I_{1/2}v$ is a locally integrable function for $v=(-\Delta)^{1/4}u$.
Sep 17, 2020 at 4:33 comment added Piero D'Ancona Let $v=(-\Delta)^{1/4}u$. The claim of the paper states that if $v\in L^{2,\infty}$ then $(-\Delta)^{-1/4}v\in BMO$. Is this the claim or not?
Sep 16, 2020 at 13:14 comment added Slm2004 Thank you for your answer. However, I believe the claim is right. It says about if $u\in \mathcal{H}^1$ then $I_{1/2}u\in L^{2,1}$. Only those $v\in L^{2,\infty}$ can be written as $(-\Delta)^{\frac14}u$ can have $I_{1/2}v\in BMO$. Do you have counter example of $u\in \mathcal{H}^1$?
Sep 16, 2020 at 11:37 history answered Piero D'Ancona CC BY-SA 4.0