Reference:
https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference
1、 绝对值和范数（Absolute values and norms）
The absolute value of some expression can be denoted as lvert x
vert or, more generally, as leftlvert …
ight
vert. It renders as $lvert x
vert$ .
The norm of a vector (or similar) can be denoted as lVert v
Vert or, more generally, as leftlVert …
ight
Vert. It renders as $lVert v
Vert$. (You may also write left|…
ight| instead.)
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 $||v||$.
It was typeset as
$$|x|, ||v|| quadlongrightarrowquadlvert x vert, lVert v Vert$$