Can I use \clap, \rlap and \llap in math mode?

The mathtools package contains (among other useful things) the implementations of \mathclap and relatives from Andrew's answer.

I don't know if they are implemented in a specific package (search CTAN), but there's an article in TUGBoat, (TUGboat 22 (2001), 350–352.) that gives an implementation. Here's the article on the author's website:

http://math.arizona.edu/~aprl/publications/mathclap/

I suspect that anyone putting this in a package would have used the same names, or at least referred to this article, so searching on CTAN for the command names used there would be a reasonable thing to do.

(Breadcrumb trail: I first learnt of this via this blog post about these commands in iTeX.)

(I'm feeling generous: I just tried the obvious searches on CTAN and nothing came up; so based on that, you should copy the definitions from that article.)

FWIW, in ConTeXt \rlap etc do the right thing in math mode as well. They are defined as

\unexpanded\def\rlap{\mathortext\domathrlap\dorlap}
\unexpanded\def\llap{\mathortext\domathllap\dollap}
\unexpanded\def\clap{\mathortext\domathclap\doclap}

where \domathrlap and \dorlap are roughly equivalent to LaTeX's \mathrlap and \rlap.