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The second fundamental form of a surface (wikipedia link, see heading 'Classical notation' or 'Physicists notation' in the first section of the article) is denoted by something resembling II, and I would like to know the exact syntax to get it.

Thank you.

2 Answers 2

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The software behind Wikipedia, MediaWiki, uses MathJax which has essentially the same syntax as LaTeX for mathematics to typeset its mathematics.

If you click the Edit Source link at the top of any page, you can see the source and there it's revealed that the symbol typeset as

II

is typeset using \mathrm{I\!I}

I would suggest creating a new command for typesetting it:

\NewDocumentCommand{\sff}{}{\mathrm{I\!I}}
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  • Thank you. I dug around the symbols list and came up with \usepackage{bbm} and them \mathbbb{I}. But I'll stick to wikipedia one.
    – duality
    Aug 7, 2021 at 1:34
  • @duality Note that sometimes \mathrm{I\!<capital letter>} is used as a cheap replacement for blackboard bold (i.e. for \mathbb{<capital letter>} with \usepackage{amssymb} or \mathds{<capital letter>} with \usepackage{dsfont}). I have seen this happen with \mathrm{I\!N} for \mathbb{N} and \mathrm{I\!R} for \mathbb{R}, so it could be that is what happened here as well. In that case \mathbb{I} or \mathds{I} might be a slightly more harmonic choice. On the other hand, the replacement might be commonplace in texts so that people expect it and find the actual character odd.
    – moewe
    Aug 7, 2021 at 6:42
  • That said, there is a case to be made here that this is not an I and that using two Is actually comes closer to the meaning (second form).
    – moewe
    Aug 7, 2021 at 6:44
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If I compile this:

\documentclass{article}
\begin{document}
  \[\mathrm{I\!I}\]
\end{document}

then I get this:

enter image description here

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  • Yes. Thank you.
    – duality
    Aug 7, 2021 at 1:35

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