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I am trying to have a cursively written l (L) with a subscript A. I tried this:

If A has a straight line $\matchcal{l\subscript{A}}$

But I'm getting undefined control sequence. Why is this?

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See also “Lowercase \mathcal”. – Caramdir Oct 8 '10 at 19:38
up vote 3 down vote accepted

You realize you're using \matchcal instead of \mathcal? If your code snippet is wrong, please provide the correct piece of code, otherwise we just have to guess.

But I suppose you're looking for \mathcal{l}_A or \ell_A.

Undefined control sequences occur when LaTeX encounters something it doesn't know, such as a typo.

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I'm sorry, in my rush to post this I described the issue wrong. I get an arrow instead of a cursive l – whatnow Oct 8 '10 at 18:58
But the \ell was the better answer, I was looking for. Thanks – whatnow Oct 8 '10 at 18:59
@whatnow: I disagree that \ell is good. It seems that some areas of math actually use it, but unless it is common for your particular use case, you should use l as normal. \ell is a way of writing on the blackboard to differentiate an l from a 1 or an I. In most fonts, this is not an issue in printed text. – TH. Oct 8 '10 at 22:58

Since only the letter is in mathcal mode, you want \mathcal{l}_{A}

If you wanted an ell with a mathcal A subscript, it would be \ell_{\mathcal{A}} for a "cursive" ell, or l_{\mathcal{A}} for a regular math-italic ell with a caligraphic subscript.

For both, you would want to specify the caligraphic font separately in the "body" and "subscript". Thus, for example, \mathcal{A}_{\mathcal{B}}.

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There is no need to specify 'to specify the caligraphic font separately in the "body" and "subscript"'. \mathcal{A_B} has the same effect as \mathcal{A}_{\mathcal{B}} – Lev Bishop Oct 8 '10 at 22:39

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