Really Big Ass Number

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). 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.