TREE is an extremely fast-growing function in set theory. TREE(1) equals 1, TREE(2) equals 3, and TREE(3) equals a number so large that its lower bound easily dwarfs Graham's Number.
man hes gone have to do a lot of walking again... Offtopic: The funny thing is TREE(3) is an absurdly long distance, independent of the physical unit. Even TREE(3) times the planck length is unimaginable long, as the ratio between a meter and the planck length is absolutely neglectable against such super large numbers.
It's so large that the number of digits it has is too large to represent, and the number of digits in that number is too large to represent, and so on for a number of times that is also far too large to represent.