Graham's number
Graham's number is an unimaginablylarge number that is a proven upper bound to the solution of a certain problem in Ramsey theory. It is named after mathematician Ronald Graham who used the number as a simplified explanation of the upper bounds of the problem he was working on in conversations with popular science writer Martin Gardner. Gardner later described the number in Scientific American in 1977, introducing it to the general public. The number was published in the 1980 Guinness Book of World Records which added to the popular interest in the number.
Graham's number, although smaller than TREE(3), is much larger than many other large numbers such as Skewes' number and Moser's number, both of which are in turn much larger than a googolplex. As with these, it is so large that the observable universe is far too small to contain an ordinary digital representation of Graham's number, assuming that each digit occupies one Planck volume, possibly the smallest measurable space. Even power towers of the form are insufficient for this purpose, although it can be described by recursive formulas using Knuth's up-arrow notation or equivalent, as was done by Graham. Though too large to be computed in full, many of the last digits of Graham's number can be derived through simple algorithms. The last 12 digits are: 262464195387.
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