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Why does \cal{M}_n give “M>” in parts of my LaTeX file and works correctly in other parts?

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  • 3
    Instead of posting a “Thank you” in your question, you should thank egreg by upvoting his answer (with the upward pointing arrow to the left of it) and accepting it (by clicking on the checkmark). Additionally, you can comment his answer, too. – Qrrbrbirlbel Nov 24 '12 at 15:52
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The correct syntax is

$\mathcal{C}_n$

The \cal command was used in LaTeX 2.09 and has been obsolete since the release of LaTeX2e (1992). It worked as a font change declaration rather than a command with argument, so the right way would have been

${\cal C}_n$

which is quite confusing.

  • I've heard that some obsolete commands are nice in latex like (over for fraction). Does this mean I should never use obsolete commands or is this (\cal) a special case? – Charlie Parker Nov 7 '16 at 18:37
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    @CharlieParker It's best to use the “official” commands. The usage of \cal in this case was wrong to begin with; the result would have been the same with LaTeX2.09. – egreg Nov 7 '16 at 18:42

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