Involution : function which is its own inverse

(e.g. If f(x) = x / (x-1), then f ^-1(x) = x / (x-1) = f(x).)

Contronym : word which is its own opposite

(e.g. fast, which means moving quickly, as in “He ran fast.”, as well as not

moving at all, as in “He held fast.”)

http://en.wikipedia.org/wiki/Involution

http://mathworld.wolfram.com/Involution.html

http://en.wikipedia.org/wiki/Auto-antonym

http://www.rinkworks.com/words/contronyms.shtml

(nice list !!)