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Possible Duplicate:
Blackboard bold characters

I want to make a blackboard "1" character to show an indicator variable, but $\mathbb{1}$ doesn't work. How do I do it?

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symbols (see “How to look up a math symbol?”) lists the following packages: bbold, mathbbol, mbboard, bbm (with \mathbbm, \mathbbmss or \mathbbmtt) and dsfont (with or without the sans option).

Alternatively, XeTeX+unicode math supports it directly.

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