How to use \mathcal with subscript? Undefined control sequence..

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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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}}.
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