I would like to declare a paired delimiter for a "triple norm".
A answer to this question already exists (cf.
Spacing between triple vertical lines), however, as I'm already using mathtools
' DeclarePairedDelimiter*
macros a lot, I would like to stay consistent and use them.
I tried to declare "naively" my norm like that:
\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{etoolbox}
\DeclarePairedDelimiterX{\normi}[1]
{|||}
{|||}
{\ifblank{#1}{\:\cdot\:}{#1}}
\begin{document}
\(\normiii{x}\)
\(\normiii{}\)
\[
\normiii*{\sum_{k \geq 0} a_k}
\]
\end{document}
However, only the first bar is stretched. Is there a way to group the 3 bars such that they all stretch?
PS: I would prefer not to use a custom font.