The Really Big Ass Number function is defined as \(\mathrm{rban}(n) = n^{n \uparrow\uparrow\uparrow n}\) (n to the power of n pentated to n).[1] It was invented by Matt Leach along with the Big Ass Number function in an attempt to create an uncomputable function. In reality, the function is only on par with \(f_4(n)\) in the fast-growing hierarchy, and is definitely computable. In fact, it's primitive recursive.

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  1. Really Big Numbers