math-mode
I'm working on math. Here I want to reduce the spacing around the colon (':'). By default spaces are inserted before and after the colon. Could you please advise me as to how this could be rectified?
MWE:
\documentclass{book}
\usepackage{amsmath}
\begin{document}
$$
DE:AB = 18:6 \quad AB:12 = CE:15
$$
\end{document}
For a detailed answer why this is happening you can read this answer of mine (shameless plug indeed): In short, the italic correction of the f has a great part in this. But the italic correction only explains the spacing after the f, not before. For this you have to look at the bounding boxes of the letters:
The first f is a text italic letter in its bounding box, the second one is math italic (in its bounding box together with its italic correction). As you can see, the text letter protrudes a bit to the left (and a lot to the right); the math letter has a tiny bit of white space in the left (and also in the right, because of the italic correction). For a bit more about the bounding boxes see this question of mine (another shameless plug :-)).
I first noticed the problem when typing $Vf$, which doesn't yield a nice output. My resort is using $V\hspace{-0.1em}f$ instead (in a macro, of course), which I like much better. You could even use $V\!f$, but this I find too narrow. Compare these three:
I would not encourage you to follow Caramdir's (now removed) suggestion to use $\mathit{Vf}$ since this uses a different font (text italic, not math italic). You can see quite clearly that the V is narrower (in other words, the angle at the bottom of the V is more acute):
If you use a different math font (like Euler), then the difference is even more noticeable.
(For a case where \textit could be a good solution, see this answer of TH.)
\documentclass{article}
\begin{document}
Here is text which starts off the line.
We can write $A^n$ as
\[
\bigcup_{a^{n-1} \in A^{n-1}} \{(a^{n-1}, a) : a \in A \}.
\]
More text, then I have something like the following
$(-3, 3)^{C}$ in $A = (-\infty, -3] \cup [3, \infty)$.
\end{document}
The complicated formula is best set in display mode; no added spaces around the colon, possibly instead after \{ and before \}.
The second case has a big flaw: the word "in" must go outside math mode because it's text. If you want to avoid a break at \cup, write
\cup\nobreak
but leave these decisions for when the document is complete.
Best Answer
You can redeclare
:to be of class\mathord. If have to restore the original behaviour, use\mathrel{:}.