Reference: express
https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-referenceui
一、 絕對值和範數（Absolute values and norms）spa
The absolute value of some expression can be denoted as \lvert x\rvert or, more generally, as \left\lvert … \right\rvert. It renders as $\lvert x\rvert$ .code
The norm of a vector (or similar) can be denoted as \lVert v\rVert or, more generally, as \left\lVert … \right\rVert. It renders as $\lVert v\rVert$. (You may also write \left\|…\right\| instead.)orm
In both cases, the rendering is better than what you'd get from |x| or ||v|| , which render with bars that don't descend low enough and sub-optimal spacing. $|x|$ or $||v||$.ci
It was typeset asget
$$|x|, ||v|| \quad\longrightarrow\quad\lvert x\rvert, \lVert v \rVert$$it