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How can I write |||A||| with three bars in equal distances that scale? I read through the exiting entries and none of the solutions worked for me, the closest one was with "vertiii" but this put out bars in different sizes.

2 Answers 2

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Using barbara's approach at Extensible double angle, etc, which essentially uses triple nested \left| and likewise \right|, with kerning added.

\documentclass{article}

\usepackage{mathtools}

\makeatletter
\def\@tvsp{\mathchoice{{}\mkern-4.5mu}{{}\mkern-4.5mu}{{}\mkern-2.5mu}{}}
\def\ltrivert{\left|\@tvsp\left|\@tvsp\left|}
\def\rtrivert{\right|\@tvsp\right|\@tvsp\right|}
\makeatother
\newcommand\tnorm[1]{\ltrivert#1\rtrivert}


\begin{document}

\[
\tnorm{x}  \tnorm{ \frac{A}{B} }
\]
\end{document}

enter image description here

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  • Thank you for your answer. Sadly the same problem occurcs. All three lines are in different sizes...
    – milenteor
    May 30, 2021 at 13:46
  • @milenteor You haven't provided enough information in your question to even attempt to explain why. May 30, 2021 at 15:36
  • Hello. What additional information do you wish for? It does not work in overleaf, I do get 3 bars but they cascade in size.
    – milenteor
    Jun 2, 2021 at 9:05
  • @milenteor So that I understand, you say that my MWE, run in overleaf, exhibits this behavior you describe? (FYI, I am not a user of Overleaf) Jun 2, 2021 at 9:10
  • Yes, exactly. Thank you for pointin this out.
    – milenteor
    Jun 2, 2021 at 9:14
2

The mathabx package defines a \vvvert delimiter. Here is a code to use it without loading the package, based on \DeclarePairedDelimiterX from mathtools:

\documentclass{article}
\usepackage{mathtools}
\usepackage{etoolbox} 

\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
<-6> matha5 <6-7> matha6 <7-8> matha7
<8-9> matha8 <9-10> matha9
<10-12> matha10 <12-> matha12
}{}
\DeclareSymbolFont{matha}{U}{matha}{m}{n}

\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{
<-6> mathx5 <6-7> mathx6 <7-8> mathx7
<8-9> mathx8 <9-10> mathx9
<10-12> mathx10 <12-> mathx12
}{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}

\DeclareMathDelimiter{\vvvert} {0}{matha}{"7E}{mathx}{"17}%

\DeclarePairedDelimiterX{\normiii}[1]
{\vvvert}
{\vvvert}
{\ifblank{#1}{\: · \:}{#1}}

\begin{document}

\[ \normiii[\Big]{A^k } \qquad \normiii{} \]

\[ \normiii*{∑_{k \geq 0}^∞ A_k} \]

\end{document} 

enter image description here

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