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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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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
up vote 26 down vote accepted

The correct syntax is


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.

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