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My latex code is $(X_i)_{i \in \N}$ and I get this error:

! Undefined control sequence. of $(X_i)_{i \in \N

Can anybody tell me why?

I have included the amssymb package:

\usepackage{amssymb}
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Welcome to TeX.SE. I do not think that \N is defined. You have to define your own symbol for that. –  Peter Grill Feb 14 '13 at 22:31
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1 Answer

You need to define what symbol you want to use. One option would be something like this:

enter image description here

Code:

\documentclass{article}
\usepackage{amssymb}

\newcommand*{\field}[1]{\mathbb{#1}}%

\begin{document}
    $(X_i)_{i \in \field{N}}$ 
\end{document}
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Hey Peter, thank you very much! :) –  yoanab Feb 14 '13 at 22:36
1  
Using \field for the natural numbers seems a bit odd:) –  Marc van Dongen Feb 15 '13 at 7:45
    
@MarcvanDongen: Well I use it for all the number sets: Q, R, C, Z, etc. Do you have a better name in mind? –  Peter Grill Feb 15 '13 at 8:08
    
Perhaps \AsField of \blackboard? –  Marc van Dongen Feb 15 '13 at 8:10
    
When you see a good answer to your question the best way to thank the author is to accept it. That adds to his or her reputation (and yours). You can vote it up, too. And you can vote up other good answers besides the one you accept. –  Ethan Bolker Feb 15 '13 at 22:44
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