\mod operator is padded with too much space: $a \mod b$. Am I missing something in the TeX, or is something configured wrong?
I've tried with Firefox and Safari on OS X.
As a workaround with
\mod (being a little forgetful and more than a little obstinate), I sometimes use the thin negative space modifier
\! to remove the excess whitespace. Here are the results without the modifier, followed by using it once and then twice:
$$ 5 \equiv 8 \mod 3 $$
$$ 5 \equiv 8 \! \mod 3 $$
$$ 5 \equiv 8 \!\! \mod 3 $$
I generally find the latter to be about what I want for inline "modulo" equations.