Supernatural number

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Hasse diagram of the lattice of supernatural numbers; primes other than 2 and 3 are omitted for simplicity.

In mathematics, the supernatural numbers, sometimes called generalized natural numbers or Steinitz numbers, are a generalization of the natural numbers. They were used by Ernst Steinitz[1]: 249–251 in 1910 as a part of his work on field theory.

A supernatural number  ω {\displaystyle \omega } is a formal product:

ω = ∏ p p n p , {\displaystyle \omega =\prod _{p}p^{n_{p}},}

where  p {\displaystyle p} runs over all prime numbers, and each  n p {\displaystyle n_{p}} is zero, a natural number or infinity. Sometimes  v p ( ω ) {\displaystyle v_{p}(\omega )} is used instead of  n p {\displaystyle n_{p}} . If no  n p = ∞ {\displaystyle n_{p}=\infty } and there are only a finite number of non-zero  n p {\displaystyle n_{p}} then we recover the positive integers. Slightly less intuitively, if all  n p {\displaystyle n_{p}} are  ∞ {\displaystyle \infty } , we get zero.[citation needed] Supernatural numbers extend beyond natural numbers by allowing the possibility of infinitely many prime factors, and by allowing any given prime to divide  ω {\displaystyle \omega } "infinitely often," by taking that prime's corresponding exponent to be the symbol  ∞ {\displaystyle \infty } .

There is no natural way to add supernatural numbers, but they can be multiplied, with  ∏ p p n p ⋅ ∏ p p m p = ∏ p p n p + m p {\displaystyle \prod _{p}p^{n_{p}}\cdot \prod _{p}p^{m_{p}}=\prod _{p}p^{n_{p}+m_{p}}} . Similarly, the notion of divisibility extends to the supernaturals with  ω 1 ∣ ω 2 {\displaystyle \omega _{1}\mid \omega _{2}} if  v p ( ω 1 ) ≤ v p ( ω 2 ) {\displaystyle v_{p}(\omega _{1})\leq v_{p}(\omega _{2})} for all  p {\displaystyle p} . The notion of the least common multiple and greatest common divisor can also be generalized for supernatural numbers, by defining

lcm ⁡ ( { ω i } ) = ∏ p p sup ( v p ( ω i ) ) {\displaystyle \displaystyle \operatorname {lcm} (\\{\omega _{i}\\})\displaystyle =\prod _{p}p^{\sup(v_{p}(\omega _{i}))}}

and

gcd ⁡ ( { ω i } ) = ∏ p p inf ( v p ( ω i ) ) {\displaystyle \displaystyle \operatorname {gcd} (\\{\omega _{i}\\})\displaystyle =\prod _{p}p^{\inf(v_{p}(\omega _{i}))}} .

With these definitions, the gcd or lcm of infinitely many natural numbers (or supernatural numbers) is a supernatural number. We can also extend the usual  p {\displaystyle p} -adic order functions to supernatural numbers by defining  v p ( ω ) = n p {\displaystyle v_{p}(\omega )=n_{p}} for each  p {\displaystyle p} .

Supernatural numbers are used to define orders and indices of profinite groups and subgroups, in which case many of the theorems from finite group theory carry over exactly. They are used to encode the algebraic extensions of a finite field.[2]

Supernatural numbers also arise in the classification of uniformly hyperfinite algebras.

## See also[edit]

* Profinite integer

## References[edit]

1. ^ Steinitz, Ernst (1910). "Algebraische Theorie der Körper". Journal für die reine und angewandte Mathematik (in German). 137: 167–309. ISSN 0075-4102. JFM 41.0445.03.
2. ^ Brawley & Schnibben (1989) pp.25-26

* Brawley, Joel V.; Schnibben, George E. (1989). Infinite algebraic extensions of finite fields. Contemporary Mathematics. Vol. 95\. Providence, RI: American Mathematical Society. pp. 23–26. ISBN 0-8218-5101-2. Zbl 0674.12009.
* Efrat, Ido (2006). Valuations, orderings, and Milnor K-theory. Mathematical Surveys and Monographs. Vol. 124\. Providence, RI: American Mathematical Society. p. 125\. ISBN 0-8218-4041-X. Zbl 1103.12002.
* Fried, Michael D.; Jarden, Moshe (2008). Field arithmetic. Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge. Vol. 11 (3rd ed.). Springer-Verlag. p. 520\. ISBN 978-3-540-77269-9. Zbl 1145.12001.

## External links[edit]

* Planet Math: Supernatural number

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Number systems

Sets of definable numbers|

* Natural numbers ( N {\displaystyle \mathbb {N} } )
* Integers ( Z {\displaystyle \mathbb {Z} } )
* Rational numbers ( Q {\displaystyle \mathbb {Q} } )
* Constructible numbers
* Algebraic numbers ( A {\displaystyle \mathbb {A} } )
* Closed-form numbers
* Periods
* Computable numbers
* Arithmetical numbers
* Set-theoretically definable numbers
* Gaussian integers

Composition algebras|

* Division algebras: Real numbers ( R {\displaystyle \mathbb {R} } )
* Complex numbers ( C {\displaystyle \mathbb {C} } )
* Quaternions ( H {\displaystyle \mathbb {H} } )
* Octonions ( O {\displaystyle \mathbb {O} } )

Split
types|

* Over  R {\displaystyle \mathbb {R} } :
* Split-complex numbers
* Split-quaternions
* Split-octonions
Over  C {\displaystyle \mathbb {C} } :
* Bicomplex numbers
* Biquaternions
* Bioctonions

Other hypercomplex|

* Dual numbers
* Dual quaternions
* Dual-complex numbers
* Hyperbolic quaternions
* Sedenions ( S {\displaystyle \mathbb {S} } )
* Split-biquaternions
* Multicomplex numbers
* Geometric algebra/Clifford algebra
* Algebra of physical space
* Spacetime algebra
* Plane-based geometric algebra

Other types|

* Cardinal numbers
* Extended natural numbers
* Irrational numbers
* Fuzzy numbers
* Hyperreal numbers
* Levi-Civita field
* Surreal numbers
* Transcendental numbers
* Ordinal numbers
* p-adic numbers (p-adic solenoids)
* Supernatural numbers
* Profinite integers
* Superreal numbers
* Normal numbers

* Classification
* List

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