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How does one write the two arrows that are piled on top of each other in an adjoin pair, where one is pointing to the left and the other to the right?

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Welcome to TeX.SE! Have a look at how-to-look-up-a-symbol – cmhughes Feb 23 '12 at 2:21
Not sure exactly what you mean, but is something like $\substack{\rightarrow \\[-0.7ex] \leftarrow}$ what you are looking for, assuming you have \usepackage{amsmath} in your preamble. Or you could just use $\rightleftarrows$ with \usepacakge{amssymb}. – Peter Grill Feb 23 '12 at 2:32

You want \leftrightarrows, but you have to say \usepackage{amssymb} for that to work.

If you're using AMS-LaTeX (i.e., either the amsart or amsbook documentclasses, or just \usepackage{amsmath}), then you can get the spacing around the colons right using the following:

\newcommand{\adj}[4]{#1\negmedspace: #2\rightleftarrows #3:\negmedspace #4}

You can then say

We have the adjoint pair $\adj{f}{C}{D}{g}$

and it will come out looking right.

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