Arrow notation


 * For other arrow notations, see down-arrow notation, mixed arrow notation, chained arrow notation, irrational arrow notation.

Arrow notation or up-arrow notation is a widely used notation for the hyper operators, devised by Donald Knuth in 1976 to represent large numbers. Knuth roughly introduced \(a \uparrow b\) as \(a^b\) and \(a \underbrace{\uparrow \cdots \uparrow}_{n \textrm{ times}} b\) as \(\underbrace{a \underbrace{\uparrow \cdots \uparrow}_{n-1 \textrm{ times}} \cdots a \underbrace{\uparrow \cdots \uparrow}_{n-1 \textrm{ times}} a}_{b \textrm{ times}}\).

Definition
We explain the precise intention of the original definition by Knuth. For any positive integers \(a\), \(b\), and \(n\), \(a \uparrow^n b\) is defined by the following recursive way:

\begin{eqnarray*} a \uparrow^n b = \left\{ \begin{array}{ll} a^b & (n = 1) \\ a & (n > 1, b = 1) \\ a \uparrow^{n-1} (a \uparrow^n (b-1)) & (n > 1, b > 1) \end{array} \right. \end{eqnarray*}

This notation gives a total computable function \begin{eqnarray*} \uparrow \colon \mathbb{N}_{>0}^3 & \to & \mathbb{N}_{>0} \\ (a,b,n) & \mapsto & a \uparrow^n b, \end{eqnarray*} where \(\mathbb{N}_{>0}\) denotes the set of positive integers. Readers should be careful that some authors implicitly extend it so that \(a \uparrow^n 0 = 1\) holds for any positive integers \(a\) and \(n\).

It satisfies the following relations, which also characterise the notation, for any positive integers \(a\), \(b\), and \(n\):

\begin{eqnarray*} a \uparrow^1 b &=& a^b \\ a \uparrow^n 1 &=& a \\ a \uparrow^{n + 1} (b + 1) &=& a \uparrow^n (a \uparrow^{n + 1} b) \\ \end{eqnarray*}

Here, \(a \uparrow^{n} b\) is regarded as a shorthand for \(a \uparrow\uparrow\cdots\uparrow\uparrow b\) with \(n\) arrows. So, for example, we have \(a \uparrow^2 b = a \uparrow\uparrow b\). Arrow notation operators are right-associative; \(a \uparrow b \uparrow c\) always means \(a \uparrow (b \uparrow c)\).

Specifically, \(a \uparrow b\) is exponentiation, \(a \uparrow\uparrow b\) is tetration, \(a \uparrow\uparrow\uparrow b\) is pentation, and in general \(a \uparrow^n b\) is the (n+2)th hyper-operation. In ASCII, these are often written a^b, a^^b, a^^^b, ... or a**b, a***b, a****b as consistent with some programming languages.

Application to googology
The function \(f(n) = n \uparrow^n n\) is a fast-growing function that eventually dominates all primitive recursive functions, and can be approximated using the fast-growing hierarchy as \(f_\omega(n)\). This is the limit of the non-extended hyper operators, and by extension, arrow notation.

Arrow notation has been generalized to other notations. A few notable ones are chained arrow notation, BEAF, and BAN. It has also been compared exactly with, among others, Notation Array Notation using the function (a{2, number of arrows}b).

Nathan Ho and Wojowu showed that arrow notation terminates — that is, \(a \uparrow^n b\) exists for all \(a,b,n\).

Examples

 * \(2 \uparrow 3 = 2^3 = 8\)
 * \(5 \uparrow 6 = 5^6 = 15,625\)
 * \(10 \uparrow 100 = 10^{100} =\) googol
 * \(3 \uparrow\uparrow 4 = 3 \uparrow 3 \uparrow 3 \uparrow 3 = 3 \uparrow 3 \uparrow 27 = 3^{7,625,597,484,987} \approx 1.2580143*10^{3638334640024}\)
 * \(5 \uparrow\uparrow 3 = 5 \uparrow 5 \uparrow 5 = 5^{5^5} \approx 1.9110126*10^{2184}\)
 * \(2 \uparrow\uparrow\uparrow 2 = 2 \uparrow\uparrow 2 = 2 \uparrow 2 = 2^2 = 4\)
 * \(2 \uparrow\uparrow\uparrow\uparrow 2 = 2 \uparrow\uparrow\uparrow 2 = 2 \uparrow\uparrow 2 = 2 \uparrow 2 = 2^2 = 4\)
 * \(3 \uparrow\uparrow\uparrow 2 = 3 \uparrow\uparrow 3 = 3 \uparrow 3 \uparrow 3 = 3^{3^3} = 3^{27} = 7,625,597,484,987\)
 * \(2 \uparrow\uparrow\uparrow 3 = 2 \uparrow\uparrow 2 \uparrow\uparrow 2 = 2 \uparrow\uparrow 4 = 2 \uparrow 2 \uparrow 2 \uparrow 2 = 2 \uparrow 2 \uparrow 4 = 2 \uparrow 16 = 65,536\)
 * \(3 \uparrow\uparrow\uparrow 3 = 3 \uparrow\uparrow 3 \uparrow\uparrow 3 = 3 \uparrow\uparrow 7,625,597,484,987 =\) Tritri
 * \(a \uparrow^{n+1} 2 = a \uparrow^{n} a\)

Turing machine code
Input form: to represent \(a \uparrow^{c} b\), place a 1's, then c+2 ^'s, then b 1's. For example, 111^^^^^111 computes tritri.

Turing machine code

0 * * r 0 0 _ _ l 1 1 1 _ l 2 2 ^ _ l 3 2 1 1 l 4 3 ^ _ l 3 3 1 _ l 2 4 1 1 l 4 4 ^ 1 l 4' 4 _ 1 l halt 4' ^ ^ l 5 4' 1 1 r 0 5 ^ ^ l 5 5 1 1 r 6 6 ^ x r 7 6 1 y r 9 6 | ^ l 12 7 * * r 7 7 _ | r 8 7 | | r 8 8 * * r 8 8 _ ^ l 11 9 * * r 9 9 | | r 10 10 * * r 10 10 _ 1 l 11 11 * * l 11 11 x ^ r 6 11 y 1 r 6 12 * * l 12 12 ^ ^ l 12' 12' ^ ^ l 12' 12' * * l 13 12' 1 x r 14 13 * * l 13 13 ^ ^ r 20 13 _ _ r 20 13 1 x r 14 14 * * r 14 14 ^ ^ r 15 15 ^ ^ r 15 15 x x r 16 15 1 x l 12 16 x x r 16 16 1 x l 12 16 ^ x r 17 17 ^ ^ r 17 17 1 ^ r 18 18 ^ 1 r 17 18 1 1 r 18 18 _ 1 l 19 19 * * l 19 19 x x l 12 20 x 1 r 20 20 ^ ^ r 21 21 ^ ^ r 21 21 x x r 22 22 x x r 22 22 1 _ r 23 22 ^ ^ l 30 23 1 _ r 23 23 ^ ^ l 24 24 _ ^ r 25 25 ^ ^ r 25 25 1 1 l 26 26 ^ 1 r 27 27 1 1 r 27 27 _ _ l 28 28 1 _ l 29 29 * * l 29 29 _ ^ r 25 29 x 1 l 30 30 x 1 l 30 30 ^ ^ r 31 31 * * r 31 31 _ _ l 32 32 1 _ l 1