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I came across the following error message:

! Undefined control sequence.
l.206 ...quad a_k > \displaystyle\sum_{{\substrack
                                              {j=1 \\ j \neq k}}}^N\disp...

Pardon my ignorance.

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In the spirit of this Q&A site, please consider reviewing your existing questions and accept those answers you consider most helpful. This will both improve your current 0% accept rate, as well as make community members more willing to support and answer your questions. There is a reward for accepting answers as well. –  Werner Jan 24 '12 at 6:38
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Read the FAQ entry How do I ask questions here? for guidance on voting and accepting answers. –  Werner Jan 24 '12 at 6:59
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It would be helpful if, in your posting, you only included the error message and left out the (at this point) meaningless LaTeX warnings. –  Mico Jan 24 '12 at 7:11
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closed as too localized by egreg, topskip, Seamus, Joseph Wright Jan 24 '12 at 18:47

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2 Answers

You are looking for the command \substack not \substrack.

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Thak's for everyone and specially for Werner. After correction, I still have this error message: ! Undefined control sequence. l.206 ...\quad a_k > \displaystyle\sum_{{\substack {j=1 \\ j \neq k}}}^N\disp.. –  Zbigniew Jan 24 '12 at 11:42
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@Labaïr If you have an error message, why not edit the question showing what you did and the error message? –  egreg Jan 24 '12 at 15:03
    
Now i have this message: ! LaTeX Error: Environment remarques undefined. See the LaTeX manual or LaTeX Companion for explanation. Type H <return> for immediate help. ... l.396 \begin{remarques} –  Zbigniew Jan 24 '12 at 15:04
    
@Labaïr: The remarques environment is not defined. But this could be a host of things. For example, if it should be a "theorem" or a regular environment. You need to post more information. –  Werner Jan 24 '12 at 15:20
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\substack is defined in the amsmath package, you need to load that package (or one of the ams classes that includes it)

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Now I have this message:! LaTeX Error: Environment remarques undefined. See the LaTeX manual or LaTeX Companion for explanation. Type H <return> for immediate help. ... l.396 \begin{remarques} –  Zbigniew Jan 24 '12 at 15:06
    
so clearly you need to load whatever package defines that. i have never used it but lmgtfy.com/?q=latex+remarques shows some hits –  David Carlisle Jan 24 '12 at 15:30
    
I write \newenvironment{remark}[1][\textbf{Remarques:}]{\begin{trivlist} \item[\hskip \labelsep {\bfseries #1}]}{\end{trivlist}} with the package ntheorem –  Zbigniew Jan 24 '12 at 15:43
    
I'm really surprised that after you are able to ignore the crowd of people who ask you to be fair and accept the answers to your questions. To your issue: Your environment is obviously named remark and not remarque. –  tohecz Jan 24 '12 at 16:00
    
I would like to extend my apologies to all people in this post. –  Zbigniew Jan 24 '12 at 17:06
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