Pentation

In mathematics, pentation (or hyper-5) is the fifth hyperoperation. Pentation is defined to be repeated tetration, similarly to how tetration is repeated exponentiation, exponentiation is repeated multiplication, and multiplication is repeated addition. The concept of "pentation" was named by English mathematician Reuben Goodstein in 1947, when he came up with the naming scheme for hyperoperations.

The number a pentated to the number b is defined as a tetrated to itself b - 1 times. This may variously be denoted as a[5]b, a↑↑↑b, a↑3b, a->b->3, or ba, depending on one's choice of notation.

For example, 2 pentated to 2 is 2 tetrated to 2, or 2 raised to the power of 2, which is 22=4. As another example, 2 pentated to 3 is 2 tetrated to the result of 2 tetrated to 2. Since 2 tetrated to 2 is 4, 2 pentated to 3 is 2 tetrated to 4, which is 2222=65536.

Based on this definition, pentation is only defined when a and b are both positive integers, though progress has been made to allow for any value of a.

## Contents

* 1 Definition
* 2 Etymology
* 3 Notation
* 4 Examples
* 5 See also
* 6 References

## Definition

Pentation is the next hyperoperation (infinite sequence of arithmetic operations, based on the previous one each time) after tetration and before hexation. It is defined as iterated (repeated) tetration (assuming right-associativity). This is similar to tetration, as tetration is iterated right-associative exponentiation.[1] It is a binary operation defined with two numbers a and b, where a is tetrated to itself b − 1 times.

The type of hyperoperation is typically denoted by a number in brackets, []. For instance, using hyperoperation notation for pentation and tetration, 2[5]3 means 2 to itself 2 times, or 2[4](2[4]2). This can then be reduced to 2[4](22)=2[4]4=2222=224=216=65,536.

## Etymology

The word "pentation" was coined by Reuben Goodstein in 1947 from the roots penta- (five) and iteration. It is part of his general naming scheme for hyperoperations.[2]

## Notation

There is little consensus on the notation for pentation; as such, there are many different ways to write the operation. However, some are more used than others, and some have clear advantages or disadvantages compared to others.

* Pentation can be written as a hyperoperation as a[5]b. In this format, a[3]b may be interpreted as the result of repeatedly applying the function x↦a[2]x, for b repetitions, starting from the number 1\. Analogously, a[4]b, tetration, represents the value obtained by repeatedly applying the function x↦a[3]x, for b repetitions, starting from the number 1, and the pentation a[5]b represents the value obtained by repeatedly applying the function x↦a[4]x, for b repetitions, starting from the number 1.[3][4] This will be the notation used in the rest of the article.

* In Knuth's up-arrow notation, a[5]b is represented as a↑↑↑b or a↑3b. In this notation, a↑b represents the exponentiation function ab and a↑↑b represents tetration. The operation can be easily adapted for hexation by adding another arrow.
* In Conway chained arrow notation, a[5]b=a->b->3.[5]
* Another proposed notation is ba, though this is not extensible to higher hyperoperations.[6]

## Examples

The values of the pentation function may also be obtained from the values in the fourth row of the table of values of a variant of the Ackermann function: if A(n,m) is defined by the Ackermann recurrence A(m−1,A(m,n−1)) with the initial conditions A(1,n)=an and A(m,1)=a, then a[5]b=A(4,b).[7]

Although tetration, its base operation, was extended to non-integer heights (more generally any height, real or complex) in 2017,[8] building on Hellmuth Kneser's work,[9] pentation a[5]b is currently only defined for integer values of b where a > 0 and b ≥ −2, such as e[5]2=e[4]e≈2075.96834.... As with all hyperoperations of order 3 (exponentiation) and higher, pentation has the following trivial cases (identities) which holds for all values of a and b within its domain:

* 1[5]b=1
* a[5]1=a

Additionally, we can also introduce the following defining relations:

* a[5]2=a[4]a
* a[5]0=1
* a[5](−1)=0
* a[5](−2)=−1
* a[5](b+1)=a[4](a[5]b)

Other than the trivial cases shown above, pentation generates extremely large numbers very quickly. As a result, there are only a few non-trivial cases that produce numbers that can be written in conventional notation, which are all listed below.

Some of these numbers are written in power tower notation due to their extreme size. Note that exp10(n)=10n.

* 2[5]2=2[4]2=22=4
* 2[5]3=2[4](2[5]2)=2[4](2[4]2)=2[4]4=2222=224=216=65,536
* 2[5]4=2[4](2[5]3)=2[4](2[4](2[4]2))=2[4](2[4]4)=2[4]65,536=222⋅⋅⋅2 (a power tower of height 65,536) ≈exp1065,533(4.29508)
* 2[5]5=2[4](2[5]4)=2[4](2[4](2[4](2[4]2)))=2[4](2[4](2[4]4))=2[4](2[4]65,536)=222⋅⋅⋅2 (a power tower of height 2[4]65,536) ≈exp102[4]65,536−3(4.29508)
* 3[5]2=3[4]3=333=327=7,625,597,484,987
* 3[5]3=3[4](3[5]2)=3[4](3[4]3)=3[4]7,625,597,484,987=333⋅⋅⋅3 (a power tower of height 7,625,597,484,987) ≈exp107,625,597,484,986(1.09902)
* 3[5]4=3[4](3[5]3)=3[4](3[4](3[4]3))=3[4](3[4]7,625,597,484,987)=333⋅⋅⋅3 (a power tower of height 3[4]7,625,597,484,987) ≈exp103[4]7,625,597,484,987−1(1.09902)
* 4[5]2=4[4]4=4444=44256≈exp103(2.19) (a number with over 10153 digits)
* 5[5]2=5[4]5=55555=5553125≈exp104(3.33928) (a number with more than 10102184 digits)

## See also

* Ackermann function
* Large numbers
* Graham's number
* History of large numbers

## References

1. ↑ Perstein, Millard H. (June 1961), "Algorithm 93: General Order Arithmetic", Communications of the ACM 5 (6): 344, doi:10.1145/367766.368160 .
2. ↑ "Transfinite ordinals in recursive number theory", The Journal of Symbolic Logic 12 (4): 123–129, 1947, doi:10.2307/2266486 .
3. ↑ "Mathematics and computer science: Coping with finiteness", Science 194 (4271): 1235–1242, 1976, doi:10.1126/science.194.4271.1235, PMID 17797067, Bibcode: 1976Sci...194.1235K .
4. ↑ Blakley, G. R.; Borosh, I. (1979), "Knuth's iterated powers", Advances in Mathematics 34 (2): 109–136, doi:10.1016/0001-8708(79)90052-5 .
5. ↑ Conway, John Horton; Guy, Richard (1996), The Book of Numbers, Springer, p. 61, ISBN 9780387979939, https://books.google.com/books?id=0--3rcO7dMYC&pg=PA61 .
6. ↑ "Tetration.org - Tetration". http://www.tetration.org/Tetration/index.html.
7. ↑ Nambiar, K. K. (1995), "Ackermann functions and transfinite ordinals", Applied Mathematics Letters 8 (6): 51–53, doi:10.1016/0893-9659(95)00084-4 .
8. ↑ "Solving F(z+1)=bF(z) in the complex plane". Advances in Computational Mathematics 43: 1–22. March 2017. doi:10.1007/s10444-017-9524-1. http://myweb.astate.edu/wpaulsen/tetration2.pdf.
9. ↑ "Reelle analytische Lösungen der Gleichung φ(φ(x))=ex und verwandter Funktionalgleichungen" (in de). Journal für die reine und angewandte Mathematik 187: 56–67. 1950.

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Hyperoperations

Primary|

* Successor (0)
* Addition (1)
* Multiplication (2)
* Exponentiation (3)
* Tetration (4)
* Pentation (5)

Inverse for left argument|

* Subtraction (1)
* Division (2)
* nth root (3)
* Super-root (4)

Inverse for right argument|

* Subtraction (1)
* Division (2)
* Logarithm (3)
* Super-logarithm (4)

Related articles|

* Ackermann function
* Bowers's operators
* Conway chained arrow notation
* Grzegorczyk hierarchy
* Knuth's up-arrow notation
* Steinhaus–Moser notation

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Large numbers

Examples in
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* Thousand
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* Googol
* Googolplex
* Skewes's number
* Moser's number
* Graham's number
* TREE(3)
* SSCG(3)
* Rayo's number
* Transfinite numbers

Expression
methods|

| Notations|

* Scientific notation
* Knuth's up-arrow notation
* Conway chained arrow notation
* Steinhaus–Moser notation

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Operators|

* Hyperoperation
* Tetration
* Pentation
* Ackermann function
* Bowers's operators
* Grzegorczyk hierarchy

Related
articles
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* Extended real number line
* Gigantic prime
* Indefinite and fictitious numbers
* Infinitesimal
* Largest known prime number
* List of numbers
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