Arrow notation

Arrow Notation is a famous notation invented by Knuth in 1976 to represent large numbers.

Formula
$$a \uparrow b = a^b$$

$$a \uparrow\uparrow b = \underbrace{a \uparrow a \uparrow a \uparrow \ldots}_{b}$$

$$a \uparrow\uparrow\uparrow b = \underbrace{a \uparrow\uparrow a \uparrow\uparrow a \uparrow\uparrow \ldots}_{b}$$

$$a \uparrow\uparrow\uparrow\uparrow b = \underbrace{a \uparrow\uparrow\uparrow a \uparrow\uparrow\uparrow a \uparrow\uparrow\uparrow \ldots}_{b}$$