Just learnt today that the popular command $$…$$ in LaTeX should not be used as it produces wrong spacing and is an obsolete command.
The correct command should be \[ … \].
Both have the same number of characters and thus are just as easy to type, so it is probably a good idea to switch to \[ … \].
See http://tex.stackexchange.com/questions/503/why-is-preferable-to
Mathtuition88 