# Spacing around a character in math mode

I'd like to use a character that looks like $\sim$, but has spacing like $\neg$ (in particular, which doesn't leave a huge gap to its right). I know I could do this by defining a symbol which expands to $\sim\hspace{-1pt}$, with 1pt replaced by some amount that looks about right to me, but is there a better way to do this (for instance, which doesn't rely on my guess about what spacing looks about right)?

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I am not too sure about your question but you can try one of these: $\sim\!a$ or $\sim\mkern-4.5mu a$. –  Yiannis Lazarides Jan 27 '11 at 18:46
But why -4.5mu rather than, say -4mu? Or -4.2mu? (Indeed, -4.5mu seems to be running slightly into other characters.) –  Henry Jan 27 '11 at 18:54
That was my guess to get it as close as possible to what you wanted. There are no rules as far as I know. –  Yiannis Lazarides Jan 27 '11 at 18:55
@Yiannis: It's a bad idea to use a negative space here. The space after \sim is strechable, so you won't get good results with \! or similar kerning. –  Hendrik Vogt Jan 27 '11 at 20:34

To avoid this spacing you could define a symbol using \sym but as an ordinary math symbol instead of a relation symbol, such as
\newcommand*{\mysim}{\mathord{\sim}}

As is explained in TeXbook, \mathord is not necessary. {\sim} is enough. –  Leo Liu Jan 27 '11 at 19:10
@Leo: That's right, though I use \mathord because it's clear what it's been done. We had this discussion here. –  Stefan Kottwitz Jan 27 '11 at 19:35