On the failure of multilinear multiplier theorem with endpoint smoothness conditions (1912.03873v3)

Abstract: We study a multilinear version of H\"ormander multiplier theorem, namely \begin{equation*} \Vert T_{\sigma}(f_1,\dots,f_n)\Vert_{L^p}\lesssim \sup_{k\in\mathbb{Z}}{\Vert \sigma(2^k\cdot,\dots,2^k\cdot)\widehat{\phi^{(n)}}\Vert_{L^{2}_{(s_1,\dots,s_n)}}}\Vert f_1\Vert_{H^{p_1}}\cdots\Vert f_n\Vert_{H^{p_n}}. \end{equation*} We show that the estimate does not hold in the limiting case $\min{(s_1,\dots,s_n)}=d/2$ or $\sum_{k\in J}{({s_k}/{d}-{1}/{p_k})}=-{1}/{2}$ for some $J \subset {1,\dots,n}$. This provides the necessary and sufficient condition on $(s_1,\dots,s_n)$ for the boundedness of $T_{\sigma}$.