While jsMath is amazingly helpful for displaying complex formulas, it does significantly slow down page loading for a lot of people.
Therefore, please refrain from using it for things which are perfectly easy to display using plain text and other bbcode tags.
If you just want to raise something to an exponent, use [sup].
If you just want a subscript, use [sub].
If you just want to make clear what letters are variables as opposed to commentary text, make them italic or bold instead of [math]y.
These characters and strings are perfectly usable in ASCII: { } | -> <- ... ! < > != and so on.

