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The wikepedia page Folge has this formula (under Formale Definition)

\begin{matrix} a:&\N &\to &X\\ & i &\mapsto &a_i, \end{matrix}

where \N is used for the set of natural numbers. I am able to create its symbol with a \mathbb{N}. But when I use \N, I get the error ! Undefined control sequence..

I assume I need to use a specific package in order to have \N defined. Is this so and if so, which one is it?

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no just define \N yourself: \usepackage{amsfonts} \newcommand\N{\mathbb{N}} – David Carlisle Dec 21 '12 at 12:53
I see. Does wikipedia do this definition "behind the scenes"? – René Nyffenegger Dec 21 '12 at 12:57
Instead of that font I use dsfonts package, which defines \mathds{} instead of \mathbb{} and gives you better output (in my opinion). – Manuel Dec 21 '12 at 14:38

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up vote 2 down vote accepted

You could also define that yourself:

\newcommand{\N}{\mathbb{N}}

I am not aware of any package that will define \N for you, but if it does, it will probably look like the definition I gave.

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Isn't necessary to load amsfonts first, like in David Carlisle's comment to the question? – mbork Dec 21 '12 at 12:54
1  
@mbork Yes and no. Any package that defines \mathbb will do: amsfonts, mbboard, mathbbol, bbold ... Chose what you like more. – mafp Dec 21 '12 at 13:00
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ah well you need to load some package to define \mathbb (nut same is true of matrix) . It is the amsfonts package (if you want the ams version of mathbb) that actually defines \mathbb although some of the other ams packages load this automatically of course/ – David Carlisle Dec 21 '12 at 13:01

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