[再寄小讀者之數學篇](2014-06-23 Bernstein's inequality)

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$$\bex \supp \hat u\subset \sed{2^{j-2}\leq |\xi|\leq 2^j} \ra \cfrac{1}{C}2^{jk}\sen{f}_{L^p} \leq \sen{D^k f}_{L^p}\leq C2^{jk} \sen{f}_{L^p}; \eex$$ $$\bex \supp \hat u\subset \sed{|\xi|\leq 2^j} \ra \sen{f}_{L^q}\leq C2^{jn\sex{\frac{1}{p}-\frac{1}{q}}} \sen{f}_{L^p}\quad\sex{1\leq p\leq q\leq \infty}. \eex$$  see [D. Chae, J. Lee, On the blow-up criterion and small data global existence for the Hall-magnetohydrodynamics, J. Differential Equations, 256 (2014), 3835--3858].