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Possible Duplicate:
How to look up a math symbol?

Hopefully this question has a one word answer!

I want to be able to write a backwards epsilon of the sort used for set membership in mathmode. Lets say this command was called "\backin", then here would be an example use:

\Theta(f(n)) \backin g(n) \in O(h(n))

To say that g(n) is in both \Theta(f(n)) and O(h(n)), but with an ordering that kind of lays them out nicely in one line in a sort of greater-than-less-than analogy.

Do you know an easy way to do this? I searched around but could not find an example.

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    Detexify makes it really easy to find such symbols.
    – Jake
    Commented Aug 5, 2011 at 6:01
  • I'm voting to close this as a duplicate of "how to look up a math(s) symbol". My reason being that things like Detexify are so much better for this sort of thing than a Q&A set up so I want to be sure that anyone happening on this question who doesn't already know about these resources gets clearly directed there. Closing as a duplicate seems a clear way to lead them there. Commented Aug 5, 2011 at 8:42
  • @Andrew, I am not sure I agree with you. Someone using google might be directed to this website, which not only would promote Tex.StackExchange, but would also help them find detexify.
    – Vivi
    Commented Aug 5, 2011 at 9:17
  • @Vivi: "Close as duplicate" does not say "This question shouldn't be here". I certainly hope that Google does direct that person here, but then I hope that that person can easily find detexify. "Close as duplicate" is, I think, merely a way to harden the trail from this question to the more general "How do I find" question. This is not a bad question. I'm merely thinking about "housekeeping" and what's best for others coming along later. Commented Aug 5, 2011 at 9:20
  • I also don't mind the question being closed. I searched this site for what I would expect to be search terms associated with my question, but did not come across detexify. This was mostly because I did not search for a general "look up a symbol" question because I had already checked the standard references I usually use for symbols and could not see "\ni" there. Thanks! Commented Aug 7, 2011 at 18:53

3 Answers 3

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Did you try \ni (\in spelled backwards)?

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  • UNfortunately, neither of \niton or \notni are defined for a backwards \notin...
    – Seamus
    Commented Aug 5, 2011 at 16:05
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    "\not\ni" works well. Commented Aug 7, 2011 at 18:54
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    so does "\not\owns"
    – user 85795
    Commented Sep 10, 2015 at 9:16
  • @FedericoPoloni What's the Monty Python reference, again?
    – taninamdar
    Commented Feb 26, 2016 at 18:26
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    @taninamdar Only a weak one... en.wikipedia.org/wiki/Knights_who_say_Ni Commented Feb 26, 2016 at 19:26
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LaTeX has built-in support for that in the form of \ni:

$\Theta(f(n)) \ni g(n) \in O(h(n))$

Backwards \in=\ni

As a resource, Scott Pakin's Comprehensive LaTeX Symbol List is always a good place to start for a vast array of mathematical symbols.

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\documentclass{article}
\usepackage{graphicx}
\newcommand*{\backin}{\rotatebox[origin=c]{-180}{$\in$}}%
\begin{document}
\[
   \backin
\]
\end{document}
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  • 6
    in such caeses it makes more sense to use \reflectbox{$\in$} instead
    – user2478
    Commented Aug 5, 2011 at 7:40
  • Didn't know \reflectbox either. Thanks for point that out. Commented Aug 5, 2011 at 7:44
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    @Peter: you're missing a \mathrel around the whole content of the macro to get correct spacing. Also, this kind of solution won't work correctly in subscripts unless you use \mathchoice. Commented Aug 5, 2011 at 10:42

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