Questions on the special mathematical functions implemented in Mathematica.
See Special Functions for a listing of the named mathematical functions currently supported by Mathematica, and The Wolfram Functions Site for an overview of the properties and identities satisfied by these functions. For additional information on special functions, you might be interested in the NIST Digital Library of Mathematical Functions.
Questions on how to use functions as a programming device (i.e. not about mathematical functions) should use the functions tag.