\Vvert / triple norm

[jd-develop]

Hello :)

Is your feature request related to a problem? Please describe.
I need to type a triple norm, like this:

But I didn’t find the symbol in MathJax. I tried \vvvert, \vVert, \rVvert, …

Describe the solution you'd like
A macro such as \rVvert (and \lVvert) would be really appreciated, and would be consistent with the already-existing \rVert and \lVert.

Describe alternatives you've considered
My current workaround is |\hspace{-0.1em}|\hspace{-0.1em}|.

[dpvc]

If you are using MathJax v4, then the default font includes the character U+2980, which is the triple vertical bar delimiter. You can access that via

\( \mathopen{\U{2980}} f \mathclose{\U{2980}} \)

Of course, you can make your own macros for these using

\newcommand{\lVvert}{\mathopen{\U{2980}}}
\newcommand{\rVvert}{\mathclose{\U{2980}}}

and you can pre-define them by adding these to the tex block of your MathJax configuration:

MathJax = {
  tex: {
    macros: {
      lVvert: '\\mathopen{\\U{2980}}',
      rVvert: '\\mathclose{\\U{2980}}',
    }
  }
};

Note, however, that the mathjax-newcm doesn't include the data for stretching U+2980, so it can't be used with \left and \right with the definitions above. If you need to use these as stretchy delimiters, then you have to use a bit more complicated configuration that adds the needed data, and defines \lVvert and \rVvert as stretchy delimiters.

For example, this configuration

MathJax = {
  addU2980(jax) {
    const {MathJaxNewcmFont} = MathJax._.output.fonts['mathjax-newcm'][`${jax}_ts`];
    MathJaxNewcmFont.defaultDelimiters[0x2980] = {
      dir: 'v', sizes: [1], stretch: [0, 0x2980], HDW: [.75, .25, .694],
    };
  },
  loader: {
    '[mathjax-newcm]/chtml': {ready() {MathJax.config.addU2980('chtml')}},
    '[mathjax-newcm]/svg': {ready() {MathJax.config.addU2980('svg')}},
  },
  startup: {
    ready() {
      MathJax.startup.defaultReady();
      const {TEXCLASS} = MathJax._.core.MmlTree.MmlNode;
      const {Token} = MathJax._.input.tex.Token;
      const {MapHandler} = MathJax._.input.tex.MapHandler;
      const delimiter = MapHandler.getMap('delimiter');
      delimiter.add('\\lVvert', new Token('\\lVvert', '\u2980', {texClass: TEXCLASS.OPEN}));
      delimiter.add('\\rVvert', new Token('\\rVvert', '\u2980', {texClass: TEXCLASS.CLOSE}));
    }
  },
};

would do that.

[jd-develop]

Thank you for your detailed answer! Sadly, I'm not using MathJax 4 (I'm using whatever vesion is bundled with Anki), but once it is updated I will try your suggestions :)

[dpvc]

Well, in versions 2 and 3, you can use \mathopen{\unicode{2980}} and \mathclose{\unicode{2980}} for the normal size of these characters, but won't b able to get the stretchy versions. Note, however, that this will rely on the system fonts installed on your reader's computer, and the character may not look the same for everyone, and it may not fit well with the other mathematical notation.