I have seen that in Category Theory there is a symbol which seems like two parallel lines with text above and text below. This means that the thing above corresponds in an 1-1 way with the thing below. I have no idea how to write it in a clever way in latex. I, therefore, want to write something like:

\underset{x\in F(C)}{\overset{y_{C}\to F}{=}}

enter image description here

but I want the two parallel line to behave as \xrightarrow.

I also want to stack more than two "=", like:

enter image description here

  • 1
    Welcome to TeX.SE! Could you perhaps post an image of the desired result, i.e. an example of how these two parallel lines are exactly used? – Jasper Habicht Nov 21 '19 at 21:22

I add my humble proposal (off-topic) (I think) using the simple tikz-cd package to create one of the images made so excellent user @Schrödinger's cat. You could put more close A and B using the row sep and column sep parameters used within the environment \begin{tikzcd} ... \end{tikzcd}.

A \arrow[r,dash, yshift=0.2ex, "y_{C}\to F"] \arrow[r,dash, yshift=-0.2ex, "x\in F(C)"']
& B 

Here there is the screenshot of the output. enter image description here

  • @down-voter(s) Always thank you. Is there some mistakes into my code or is it the problem the tag? – Sebastiano Nov 21 '19 at 23:57

Welcome! This is just copied from amsmath.sty and mathtools.sty, and adjusted. EDIT: Used GUM's kind suggestion, which is less likely to cause viewer-dependent issues, and is also simpler.

  \ext@arrow 0055{\DoubleFill@}{#1}{#2}}
\[ A\xDouble[x\in F(C)]{y_{C}\to F}B\]
\[ \scriptstyle A\xDouble[x\in F(C)]{y_{C}\to F}B\]
\[ A\xDouble[x\in F(C)]{y_{C}\to F}B\]
\[ \xDouble[{\xDouble[\mathrm{presheaf}]{x\in F(C)}}]{y_{C}\to F}\]

enter image description here

  • Thank you, but how can I add more? – richarddedekind Nov 21 '19 at 22:49
  • @richarddedekind I do not quite understand what you mean by that. What do you want to add? – user194703 Nov 21 '19 at 23:33
  • 1
    Not really important, but IMHO \def\DoubleFill@{\arrowfill@ \std@equal\Relbar\std@equal}, or—better—\newcommand*\DoubleFill@{\arrowfill@ \Relbar\Relbar\Relbar}, would be more elegant. See also this question (of mine), – GuM Nov 22 '19 at 20:19
  • @GuM Thanks for the heads up, very much appreciated. I will add these options once it is clear what the OP means by adding more. – user194703 Nov 22 '19 at 20:21
  • By more I mean have more double dashes like the second image in the post. – richarddedekind Nov 30 '19 at 22:27

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