[再寄小读者之数学篇](2014-06-21 向量公式)

$$\bex (\n\times{\bf b})\times{\bf b}=-\n\cfrac{|{\bf b}|^2}{2}+({\bf b}\cdot\n){\bf b}. \eex$$ see [D. Chae, P. Degond, J.G. Liu, Well-posedness for Hall-magnetohydrodynamics, Ann. I. H. Poincar\'e-AN, 31 (2014),555--565].