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In the computational sciences, the Ackermann function (represented as A(m,n) is the simplest example of a total function (a function defined for all possible input values) which is computable but not primitive recursive, which was originally formulated to disprove the once-common belief that every computable function was also primitive recursive, but is now used to generate absurdly large numbers to horrify mathematicians.
Calling the Ackermann function with (4, 4) as its parameters is already too large to calculate for most computers.

xkcd's number calls the Ackermann function with Graham's Number as both of its parameters.
by Ἀπολλύων September 17, 2016
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