Does xslt have functions for doing exponential calculations?

I'm not aware of a built-in function (either in XPath
or as an XSLT function). You can check the EXSLT math
module, which is supported by many processors, check
the list of extension functions your processor offers,
check whether you can call it from the underlying
system, or, if all else fails, get a book about numerical
calculations and implement it yourself in XSLT. It's not
all that hard unless you want to have high accuracy and/or
high performance.