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How to look up a math symbol?

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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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4  
Detexify makes it really easy to find such symbols. –  Jake Aug 5 '11 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. –  Andrew Stacey Aug 5 '11 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 Aug 5 '11 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. –  Andrew Stacey Aug 5 '11 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! –  Artem Kaznatcheev Aug 7 '11 at 18:53
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marked as duplicate by Jake, Andrew Stacey, Caramdir, Alan Munn, Lev Bishop Aug 7 '11 at 16:02

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3 Answers

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

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Wow, didn't know about that one. –  Peter Grill Aug 5 '11 at 5:59
5  
+1 Monty Python reference –  Federico Poloni Aug 5 '11 at 11:57
    
UNfortunately, neither of \niton or \notni are defined for a backwards \notin... –  Seamus Aug 5 '11 at 16:05
4  
"\not\ni" works well. –  Artem Kaznatcheev Aug 7 '11 at 18:54
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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 –  Herbert Aug 5 '11 at 7:40
    
Didn't know \reflectbox either. Thanks for point that out. –  Peter Grill Aug 5 '11 at 7:44
4  
@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. –  Philippe Goutet Aug 5 '11 at 10:42
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