# \colon for maps in opposite direction

Using \colon gives correct spacing for a map $F\colon C\to D$ in one direction. But writing $L\colon C\rightleftarrows D\colon R$ for a couple of maps in both directions (e.g. for an adjunction) gives disturbingly asymmetric spaces near first and second colons (like if D were a map from R, not the other way round). Probably, it should be fixed (e.g. some other command should be used) — but how?

• Don't use \colon then. It is defined to have small spacing on the left and big spacing on the right. Maybe you should dig into the code and find the definition for \colon and define a command like \rcolon that swaps the spacing before and after and use that. Commented Aug 25, 2010 at 10:15
• Surely not \cocolon! It should be just \lon (and the opposite of the coefficient ring should be ...). Commented Aug 25, 2010 at 11:18
• Why is \colon preferred over :? I just checked a random sample of books in my office and the overwhelming majority uses the symmetrical spacing of :. The exceptions (among the 15 or so books I checked) are Atiyah-MacDonald, Besse and Kobayashi-Nomizu. I have to admit that I never sat down to think about this before. Commented Aug 25, 2010 at 12:01
• I think it is also something semantical in addition to spacing. : is interpreted as a relation (I guess the ratio). At least that's what the AMS Short Math Guide ftp.ams.org/pub/tex/doc/amsmath/short-math-guide.pdf suggests to me. Commented Aug 25, 2010 at 12:10
• @José Figueroa-O'Farrill: you could try defining a three-argument command \fcn{f}{A}{B} that expands to f \colon A \to B. I bet there are people who would say this is easier to parse in the source than f:A\to B, though I don't know that I'm one of them. Commented Aug 25, 2010 at 15:50

amsmath defines \colon as \nobreak \muskip 2mu\mathpunct{}\nonscript\mkern-\thinmuskip{:}\mskip 6mu plus1mu\relax. So how about the following?

\newcommand*\cocolon{%
\nobreak
\mskip6mu plus1mu
\mathpunct{}%
\nonscript
\mkern-\thinmuskip
{:}%
\mskip2mu
\relax
}

• works in simple cases (so I've accepted the answer), but try e.g. $\sin\colon X\rightleftarrows Y\cocolon\cos$ — the space between \cocolon and mathop is too big Commented Aug 25, 2010 at 18:01
• @Grigory M I found we can easily obtain what you want, by adding \!, to be ...{:}\!... Commented Apr 16, 2023 at 8:30
• missed opportunity to simply call it \lon. Commented Jun 29, 2023 at 11:55