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Knuth's up-arrow notation : ウィキペディア英語版
Knuth's up-arrow notation

In mathematics, Knuth's up-arrow notation is a method of notation for very large integers, introduced by Donald Knuth in 1976. It is closely related to the Ackermann function and especially to the hyperoperation sequence. The idea is based on the fact that multiplication can be viewed as iterated addition and exponentiation as iterated multiplication. Continuing in this manner leads to iterated exponentiation (tetration) and to the remainder of the hyperoperation sequence, which is commonly denoted using Knuth arrow notation.
==Introduction==

The ordinary arithmetical operations of addition, multiplication and exponentiation are naturally extended into a sequence of hyperoperations as follows.

Multiplication by a natural number is defined as iterated addition:
:
\begin
a\times b & = & \underbrace \\
& & b\mboxa
\end

For example,
:
\begin 4\times 3 & = & \underbrace & = & 12\\
& & 3\mbox4
\end

Exponentiation for a natural power b is defined as iterated multiplication, which Knuth denoted by a single up-arrow:
:
\begin
a\uparrow b= a^b = & \underbrace\\
& b\mboxa
\end

For example,
:
\begin
4\uparrow 3= 4^3 = & \underbrace & = & 64\\
& 3\mbox4
\end

To extend the sequence of operations beyond exponentiation, Knuth defined a “double arrow” operator to denote iterated exponentiation (tetration):
:
\begin
a\uparrow\uparrow b & = = & \underbrace} &
= & \underbrace
\\
& & b\mboxa
& & b\mboxa
\end

For example,
:
\begin
4\uparrow\uparrow 3 & = = & \underbrace & = & 4^ & \approx & 1.340~780~79\times 10^&
\\
& & 3\mbox4
& & 3\mbox4
\end

Here and below evaluation is to take place from right to left, as Knuth's arrow operators (just like exponentiation) are defined to be right-associative.
According to this definition,
:3\uparrow\uparrow 2=3^3=27
:3\uparrow\uparrow 3=3^=3^=7~625~597~484~987
:3\uparrow\uparrow 4=3^}=3^\approx 1.258~014~3\times 10^
:3\uparrow\uparrow 5=3^=3^=3^
a\uparrow\uparrow\uparrow b= &
\underbrace\\
& b\mboxa
\end

followed by a “quadruple arrow“ operator for iterated pentation (hexation):
:
\begin
a\uparrow\uparrow\uparrow\uparrow b= &
\underbrace\\
& b\mboxa
\end

and so on. The general rule is that an n-arrow operator expands into a right-associative series of (n - 1)-arrow operators. Symbolically,
:
\begin
a\ \underbrace_\ b=
\underbrace
\ (a\ \underbrace_
\ (\dots
\ \underbrace_
\ a))}_
\end

Examples:
:3\uparrow\uparrow\uparrow2 = 3\uparrow\uparrow3 = 3^ = 3^=7~625~597~484~987
:
\begin
3\uparrow\uparrow\uparrow3 = 3\uparrow\uparrow(3\uparrow\uparrow3) = 3\uparrow\uparrow(3\uparrow3\uparrow3) = &
\underbrace \\
& 3\uparrow3\uparrow3\mbox3
\end
\begin
= & \underbrace \\
& \mbox
\end
\begin
= & \underbrace}}}}}} \\
& \mbox
\end

The notation a \uparrow^n b is commonly used to denote a \uparrow\uparrow \dots \uparrow b with ''n'' arrows. In fact, a \uparrow^n b is ''a'' () ''b'' with hyperoperation. For example,39\uparrow\uparrow14 can also be written as 39 () 14, the "()" means tetration, but it does not equal to 39 () 14 = 39 × 14 = 546, similarly, 77 \uparrow^ 77 = 77 () 77 instead of 77 () 77.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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