Tetration is used to express Graham's number (http://en.wikipedia.org/wiki/Grahams_number http://en.wikipedia.org/wiki/Grahams_number) which appeared in the Gui

Tetration is used to express Graham's number (http://en.wikipedia.org/wiki/Grahams_number http://en.wikipedia.org/wiki/Grahams_number) which appeared in the Guinness book of records as the largest number ever used in a serious mathematical proof.