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I've used \cal throughout my document and now when I go to switch the document class the \cal is giving me a "Undefined control sequence." error. Any thoughts on why the document class would make a difference?

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Use \mathcal{...} instead. It should be possible to perform a global search-and-replace to achieve this. Does this solve your problem? – Werner Oct 27 '11 at 20:58
What documentclass did you use originally and which one did you switch to? – Werner Oct 27 '11 at 21:16
up vote 8 down vote accepted

You could define \cal by yourself, such as for example the article class does. In the minimal class it's undefined, and this way you can fix it:

$\cal ABC$

enter image description here

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