# How do I insert a backslash inside a mathematical statement?

I'm trying to write a set subtraction operation in an AMS formatted template. In the end, what I want should look like A\B. But I'm doing this in math-mode $A\B$ and this gives me errors. I've also tried using the \diagdown symbol, but it doesn't work either. Is there another way to do this?

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Remember that \ is TeX escape character, so TeX is seeing \B as a macro name (probably undefined). What about $A\backslash B$? –  JLDiaz Dec 9 '12 at 18:33
For your special Problem, @corentin already gave the answer below. For a more geneal answer, see tex.stackexchange.com/q/9363/18674 –  Benedikt Bauer Dec 9 '12 at 18:33

\setminus should be what you are after.

\documentclass{article}
\usepackage{amssymb} % for \smallsetminus

\begin{document}
$\mbox{setminus: } A \setminus B,\quad \mbox{smallsetminus: } A \smallsetminus B, \quad \mbox{backslash: } A \backslash B$
\end{document}


Note that you can find more information here on How to look up a symbol?. In particular, you have the Comprehensive LaTeX symbols and the website Detexify. There is also this post which was mentioned earlier in a comment

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You can include also $\backslash$, which produces a different spacing, for comparison. –  JLDiaz Dec 9 '12 at 18:35
@JLDiaz Thanks for your comment, you are right, I will update my post. –  Corentin Dec 9 '12 at 18:37
there is also \smallsetminus, available with \usepackage{amssymb}, for those who prefer a smaller symbol (with the same meaning). –  barbara beeton Dec 9 '12 at 18:38
@barbarabeeton Thanks, answer updated! –  Corentin Dec 9 '12 at 18:46
This was exactly what I was looking for. Thanks! :) –  Paul Dec 9 '12 at 19:06