Ordinal arithmetic

Template:Short description In the mathematical field of set theory, ordinal arithmetic describes the three usual operations on ordinal numbers: addition, multiplication, and exponentiation. Each can be defined in essentially two different ways: either by constructing an explicit well-ordered set that represents the result of the operation or by using transfinite recursion. Cantor normal form provides a standardized way of writing ordinals. In addition to these usual ordinal operations, there are also the "natural" arithmetic of ordinals and the nimber operations.

Addition

The sum of two well-ordered sets Template:Mvar and Template:Mvar is the ordinal representing the variant of lexicographical order with least significant position first, on the union of the Cartesian products S × Template:Mset and T × Template:Mset. This way, every element of Template:Mvar is smaller than every element of Template:Mvar, comparisons within Template:Mvar keep the order they already have, and likewise for comparisons within Template:Mvar.

The definition of addition α + β can also be given by transfinite recursion on Template:Mvar. When the right addend β = 0, ordinary addition gives α + 0 = α for any Template:Mvar. For β > 0, the value of α + β is the smallest ordinal strictly greater than the sum of Template:Mvar and Template:Mvar for all δ < β. Writing the successor and limit ordinals cases separately:

• α + 0 = α
• α + S(β) = S(α + β), where Template:Mvar denotes the successor function.
• $\alpha +\beta =\bigcup _{\delta <\beta }(\alpha +\delta )$ when Template:Mvar is a limit ordinal.

Ordinal addition on the natural numbers is the same as standard addition. The first transfinite ordinal is Template:Mvar, the set of all natural numbers, followed by ω + 1, ω + 2, etc. The ordinal ω + ω is obtained by two copies of the natural numbers ordered in the usual fashion and the second copy completely to the right of the first. Writing 0' < 1' < 2' < ... for the second copy, ω + ω looks like

0 < 1 < 2 < 3 < ... < 0' < 1' < 2' < ...

This is different from Template:Mvar because in Template:Mvar only 0 does not have a direct predecessor while in ω + ω the two elements 0 and 0' do not have direct predecessors.

Properties

Ordinal addition is, in general, not commutative. For example, 3 + ω = ω since the order relation for 3 + ω is 0 < 1 < 2 < 0' < 1' < 2' < ..., which can be relabeled to Template:Mvar. In contrast ω + 3 is not equal to Template:Mvar since the order relation 0 < 1 < 2 < ... < 0' < 1' < 2' has a largest element (namely, 2') and Template:Mvar does not (Template:Mvar and ω + 3 are equipotent, but not order isomorphic).

Ordinal addition is still associative; one can see for example that (ω + 4) + ω = ω + (4 + ω) = ω + ω.

Addition is strictly increasing and continuous in the right argument:

$\alpha <\beta \Rightarrow \gamma +\alpha <\gamma +\beta$

but the analogous relation does not hold for the left argument; instead we only have:

$\alpha <\beta \Rightarrow \alpha +\gamma \le \beta +\gamma$

Ordinal addition is left-cancellative: if α + β = α + γ, then β = γ. Furthermore, one can define left subtraction for ordinals β ≤ α: there is a unique Template:Mvar such that α = β + γ. On the other hand, right cancellation does not work:

$3+\omega =0+\omega =\omega$ but $3\ne 0$

Nor does right subtraction, even when β ≤ α: for example, there does not exist any Template:Mvar such that γ + 42 = ω.

If the ordinals less than Template:Mvar are closed under addition and contain 0, then Template:Mvar is occasionally called a Template:Mvar-number (see additively indecomposable ordinal). These are exactly the ordinals of the form ω^β.

Multiplication

File:OmegaPlusOmega svg.svg

The disjoint union { (n,0) : n ∈ N } ∪ { (n,1) : n ∈ N } , using lexicographic order with least significant position first, has order type ω • 2. This is different from Template:Mvar.

File:TwoTimesOmega svg.svg

The set { (0,n), (1,n) : n ∈ N } , under lexicographic order with least significant position first, has order type 2 • ω, which is equal to Template:Mvar.

The Cartesian product, S × T, of two well-ordered sets Template:Mvar and Template:Mvar can be well-ordered by a variant of lexicographical order that puts the least significant position first. Effectively, each element of Template:Mvar is replaced by a disjoint copy of Template:Mvar. The order-type of the Cartesian product is the ordinal that results from multiplying the order-types of Template:Mvar and Template:Mvar.

The definition of multiplication can also be given by transfinite recursion on Template:Mvar. When the right factor β = 0, ordinary multiplication gives α · 0 = 0 for any Template:Mvar. For β > 0, the value of α · β is the smallest ordinal greater than or equal to (α · δ) + α for all δ < β. Writing the successor and limit ordinals cases separately:

• Template:Mvar · 0 = 0.
• α · S(β) = (α · β) + α, for a successor ordinal S(β).
• $\alpha \cdot \beta =\bigcup _{\delta <\beta }(\alpha \cdot \delta )$, when Template:Mvar is a limit ordinal.

As an example, here is the order relation for ω · 2:

0₀ < 1₀ < 2₀ < 3₀ < ... < 0₁ < 1₁ < 2₁ < 3₁ < ...,

which has the same order type as ω + ω. In contrast, 2 · ω looks like this:

0₀ < 1₀ < 0₁ < 1₁ < 0₂ < 1₂ < 0₃ < 1₃ < ...

and after relabeling, this looks just like Template:Mvar. Thus, ω · 2 = ω+ω ≠ ω = 2 · ω, showing that multiplication of ordinals is not in general commutative, c.f. pictures.

As is the case with addition, ordinal multiplication on the natural numbers is the same as standard multiplication.

Properties

α · 0 = 0 · α = 0, and the zero-product property holds: α · β = 0 → α = 0 or β = 0. The ordinal 1 is a multiplicative identity, α · 1 = 1 · α = α. Multiplication is associative, (α · β) · γ = α · (β · γ). Multiplication is strictly increasing and continuous in the right argument: (α < β and γ > 0) → γ·α < γ·β. Multiplication is not strictly increasing in the left argument, for example, 1 < 2 but 1 · ω = 2 · ω = ω. However, it is (non-strictly) increasing, i.e. α ≤ β → α·γ ≤ β·γ.

Multiplication of ordinals is not in general commutative. Specifically, a natural number greater than 1 never commutes with any infinite ordinal, and two infinite ordinals Template:Mvar and Template:Mvar commute if and only if α^m = βⁿ for some nonzero natural numbers Template:Mvar and Template:Mvar. The relation "Template:Mvar commutes with Template:Mvar" is an equivalence relation on the ordinals greater than 1, and all equivalence classes are countably infinite.

Distributivity holds, on the left: α(β + γ) = αβ + αγ. However, the distributive law on the right (β + γ)α = βα+γα is not generally true: (1 + 1) · ω = 2 · ω = ω while 1 · ω + 1 · ω = ω + ω, which is different. There is a left cancellation law: If α > 0 and α · β = α · γ, then β = γ. Right cancellation does not work, e.g. 1 · ω = 2 · ω = ω, but 1 and 2 are different. A left division with remainder property holds: for all Template:Mvar and Template:Mvar, if β > 0, then there are unique Template:Mvar and Template:Mvar such that α = β · γ + δ and δ < β. Right division does not work: there is no Template:Mvar such that α · ω ≤ ω^ω ≤ (α + 1) · ω.

The ordinal numbers form a left near-semiring, but do not form a ring. Hence the ordinals are not a Euclidean domain, since they are not even a ring; furthermore the Euclidean "norm" would be ordinal-valued using the left division here.

A Template:Mvar-number (see Multiplicatively indecomposable ordinal) is an ordinal Template:Mvar greater than 1 such that αβ = β whenever 0 < α < β. These consist of the ordinal 2 and the ordinals of the form β = ω^ωγ.

Exponentiation

The definition of exponentiation via order types is most easily explained using Von Neumann's definition of an ordinal as the set of all smaller ordinals. Then, to construct a set of order type α^β consider the set of all functions f : β → α such that f(x) = 0 for all but finitely many elements x ∈ β (essentially, we consider the functions with finite support). This set is ordered lexicographically with the least significant position first: we write f < g if and only if there exists x ∈ β with f(x) < g(x) and f(y) = g(y) for all y ∈ β with x < y. This is a well-ordering and hence gives an ordinal number.

The definition of exponentiation can also be given by transfinite recursion on the exponent Template:Mvar. When the exponent β = 0, ordinary exponentiation gives α⁰ = 1 for any Template:Mvar. For β > 0, the value of α^β is the smallest ordinal greater than or equal to α^δ · α for all δ < β. Writing the successor and limit ordinals cases separately:

• α⁰ = 1.
• α^S(β) = (α^β) · α, for a successor ordinal S(β).
• ${\alpha }^{\beta }=\bigcup _{0<\delta <\beta }({\alpha }^{\delta })$, when Template:Mvar is a limit ordinal.

Both definitions simplify considerably if the exponent Template:Mvar is a finite number: α^β is then just the product of Template:Mvar copies of Template:Mvar; e.g. ω³ = ω·ω·ω, and the elements of ω³ can be viewed as triples of natural numbers, ordered lexicographically with least significant position first. This agrees with the ordinary exponentiation of natural numbers.

But for infinite exponents, the definition may not be obvious. For example, α^ω can be identified with a set of finite sequences of elements of Template:Mvar, properly ordered. The equation 2^ω = ω expresses the fact that finite sequences of zeros and ones can be identified with natural numbers, using the binary number system. The ordinal ω^ω can be viewed as the order type of finite sequences of natural numbers; every element of ω^ω (i.e. every ordinal smaller than ω^ω) can be uniquely written in the form ${\omega }^{n_1}{c}_1+{\omega }^{n_2}{c}_2+\cdots +{\omega }^{n_k}{c}_k$ where Template:Mvar, n₁, ..., n_k are natural numbers, c₁, ..., c_k are nonzero natural numbers, and n₁ > ... > n_k.

The same is true in general: every element of α^β (i.e. every ordinal smaller than α^β) can be uniquely written in the form ${\alpha }^{b_1}{a}_1+{\alpha }^{b_2}{a}_2+\cdots +{\alpha }^{b_k}{a}_k$ where Template:Mvar is a natural number, b₁, ..., b_k are ordinals smaller than Template:Mvar with b₁ > ... > b_k, and a₁, ..., a_k are nonzero ordinals smaller than Template:Mvar. This expression corresponds to the function f : β → α which sends b_i to a_i for i = 1, ..., k and sends all other elements of Template:Mvar to 0.

While the same exponent-notation is used for ordinal exponentiation and cardinal exponentiation, the two operations are quite different and should not be confused. The cardinal exponentiation A^B is defined to be the cardinal number of the set of all functions B → A, while the ordinal exponentiation α^β only contains the functions β → α with finite support, typically a set of much smaller cardinality. To avoid confusing ordinal exponentiation with cardinal exponentiation, one can use symbols for ordinals (e.g. Template:Mvar) in the former and symbols for cardinals (e.g. ${\mathrm{\aleph }}_0$) in the latter.

Properties

• α⁰ = 1.
• If 0 < α, then 0^α = 0.
• 1^α = 1.
• α¹ = α.
• α^β·α^γ = α^{β + γ}.
• (α^β)^γ = α^β·γ.
• There are Template:Mvar, Template:Mvar, and Template:Mvar for which (α · β)^γ ≠ α^γ · β^γ. For instance, (ω · 2)² = ω·2·ω·2 = ω² · 2 ≠ ω² · 4.
• Ordinal exponentiation is strictly increasing and continuous in the right argument: If γ > 1 and α < β, then γ^α < γ^β.
• If α < β, then α^γ ≤ β^γ. Note, for instance, that 2 < 3 and yet 2^ω = 3^ω = ω.
• If α > 1 and α^β = α^γ, then β = γ. If α = 1 or α = 0 this is not the case.
• For all Template:Mvar and Template:Mvar, if β > 1 and α > 0 then there exist unique Template:Mvar, Template:Mvar, and Template:Mvar such that α = β^γ · δ + ρ such that 0 < δ < β and ρ < β^γ.

Jacobsthal showed that the only solutions of α^β = β^α with α ≤ β are given by α = β, or α = 2 and β = 4, or Template:Mvar is any limit ordinal and β = εα where Template:Mvar is an [[Epsilon numbers (mathematics)|Template:Mvar-number]] larger than Template:Mvar.

Beyond exponentiation

There are ordinal operations that continue the sequence begun by addition, multiplication, and exponentiation, including ordinal versions of tetration, pentation, and hexation. See also Veblen function.

Cantor normal form

Every ordinal number Template:Mvar can be uniquely written as ${\omega }^{{\beta }_1}{c}_1+{\omega }^{{\beta }_2}{c}_2+\cdots +{\omega }^{{\beta }_k}{c}_k$, where Template:Mvar is a natural number, $c_1,c_2,\dots ,c_k$ are nonzero natural numbers, and ${\beta }_1>{\beta }_2>\dots >{\beta }_k\ge 0$ are ordinal numbers. The degenerate case α = 0 occurs when k = 0 and there are no Template:Mvars nor Template:Mvars. This decomposition of Template:Mvar is called the Cantor normal form of Template:Mvar, and can be considered the base-Template:Mvar positional numeral system. The highest exponent ${\beta }_1$ is called the degree of $\alpha$, and satisfies ${\beta }_1\le \alpha$. The equality ${\beta }_1=\alpha$ applies if and only if $\alpha ={\omega }^{\alpha }$. In that case Cantor normal form does not express the ordinal in terms of smaller ones; this can happen as explained below.

A minor variation of Cantor normal form, which is usually slightly easier to work with, is to set all the numbers c_i equal to 1 and allow the exponents to be equal. In other words, every ordinal number α can be uniquely written as ${\omega }^{{\beta }_1}+{\omega }^{{\beta }_2}+\cdots +{\omega }^{{\beta }_k}$, where Template:Mvar is a natural number, and ${\beta }_1\ge {\beta }_2\ge \dots \ge {\beta }_k\ge 0$ are ordinal numbers.

Another variation of the Cantor normal form is the "base Template:Mvar expansion", where Template:Mvar is replaced by any ordinal δ > 1, and the numbers c_i are nonzero ordinals less than Template:Mvar.

The Cantor normal form allows us to uniquely express—and order—the ordinals Template:Mvar that are built from the natural numbers by a finite number of arithmetical operations of addition, multiplication and exponentiation base-$\omega$: in other words, assuming ${\beta }_1<\alpha$ in the Cantor normal form, we can also express the exponents ${\beta }_i$ in Cantor normal form, and making the same assumption for the ${\beta }_i$ as for Template:Mvar and so on recursively, we get a system of notation for these ordinals (for example,

${\omega }^{{\omega }^{{\omega }^7\cdot 6+\omega +42}\cdot 1729+{\omega }^9+88}\cdot 3+{\omega }^{{\omega }^{\omega }}\cdot 5+65537$

denotes an ordinal).

The ordinal ε₀ (epsilon nought) is the set of ordinal values α of the finite-length arithmetical expressions of Cantor normal form that are hereditarily non-trivial where non-trivial means β₁<α when 0<α. It is the smallest ordinal that does not have a finite arithmetical expression in terms of Template:Mvar, and the smallest ordinal such that ${\epsilon }_0={\omega }^{{\epsilon }_0}$, i.e. in Cantor normal form the exponent is not smaller than the ordinal itself. It is the limit of the sequence

$0,1={\omega }^0,\omega ={\omega }^1,{\omega }^{\omega },{\omega }^{{\omega }^{\omega }},\dots .$

The ordinal ε₀ is important for various reasons in arithmetic (essentially because it measures the proof-theoretic strength of the first-order Peano arithmetic: that is, Peano's axioms can show transfinite induction up to any ordinal less than ε₀ but not up to ε₀ itself).

The Cantor normal form also allows us to compute sums and products of ordinals: to compute the sum, for example, one need merely know (see the properties listed in Template:Sectionlink and Template:Sectionlink) that

${\omega }^{\beta }c+{\omega }^{{\beta }^{\prime }}{c}^{\prime }={\omega }^{{\beta }^{\prime }}{c}^{\prime },$

if ${\beta }^{\prime }>\beta$ (if ${\beta }^{\prime }=\beta$ one can apply the distributive law on the left and rewrite this as ${\omega }^{\beta }(c+c^{\prime })$, and if ${\beta }^{\prime }<\beta$ the expression is already in Cantor normal form); and to compute products, the essential facts are that when $0<\alpha ={\omega }^{{\beta }_1}{c}_1+\cdots +{\omega }^{{\beta }_k}{c}_k$ is in Cantor normal form and $0<{\beta }^{\prime }$, then

$\alpha {\omega }^{{\beta }^{\prime }}={\omega }^{{\beta }_1+{\beta }^{\prime }}$

and

$\alpha n={\omega }^{{\beta }_1}{c}_{1}n+{\omega }^{{\beta }_2}{c}_2+\cdots +{\omega }^{{\beta }_k}{c}_k,$

if Template:Mvar is a non-zero natural number.

To compare two ordinals written in Cantor normal form, first compare ${\beta }_1$, then $c_1$, then ${\beta }_2$, then $c_2$, and so on. At the first occurrence of inequality, the ordinal that has the larger component is the larger ordinal. If they are the same until one terminates before the other, then the one that terminates first is smaller.

Factorization into primes

Ernst Jacobsthal showed that the ordinals satisfy a form of the unique factorization theorem: every nonzero ordinal can be written as a product of a finite number of prime ordinals. This factorization into prime ordinals is in general not unique, but there is a "minimal" factorization into primes that is unique up to changing the order of finite prime factors .

A prime ordinal is an ordinal greater than 1 that cannot be written as a product of two smaller ordinals. Some of the first primes are 2, 3, 5, ... , Template:Mvar, ω + 1, ω² + 1, ω³ + 1, ..., ω^ω, ω^ω + 1, ω^{ω + 1} + 1, ... There are three sorts of prime ordinals:

• The finite primes 2, 3, 5, ...
• The ordinals of the form ω^ωα for any ordinal Template:Mvar. These are the prime ordinals that are limits, and are the delta numbers, the transfinite ordinals that are closed under multiplication.
• The ordinals of the form ω^α + 1 for any ordinal α > 0. These are the infinite successor primes, and are the successors of gamma numbers, the additively indecomposable ordinals.

Factorization into primes is not unique: for example, 2×3 = 3×2, 2×ω = ω, (ω+1)×ω = ω×ω and ω×ω^ω = ω^ω. However, there is a unique factorization into primes satisfying the following additional conditions:

• Every limit prime must occur before any successor prime
• If two consecutive primes of the prime factorization are both limits or both finite, the second one must be less than or equal to the first one.

This prime factorization can easily be read off using the Cantor normal form as follows:

• First write the ordinal as a product αβ where Template:Mvar is the smallest power of Template:Mvar in the Cantor normal form and Template:Mvar is a successor.
• If α = ω^γ then writing Template:Mvar in Cantor normal form gives an expansion of Template:Mvar as a product of limit primes.
• Now look at the Cantor normal form of Template:Mvar. If β = ω^λm + ω^μn + smaller terms, then β = (ω^μn + smaller terms)(ω^λ−μ + 1)m is a product of a smaller ordinal and a prime and a natural number Template:Mvar. Repeating this and factorizing the natural numbers into primes gives the prime factorization of Template:Mvar.

So the factorization of the Cantor normal form ordinal

ω^α₁n₁ + ⋯ + ω^α_kn_k (with α₁ > ⋯ > α_k)

into a minimal product of infinite primes and natural numbers is

(ω^ωβ₁ ⋯ ω^ωβ_m)n_k (ω^{α_k−1−α_k} + 1)n_k−1 ⋯ (ω^{α₁−α₂} + 1)n₁

where each n_i should be replaced by its factorization into a non-increasing sequence of finite primes and

α_k = ω^β₁ + ⋯ + ω^β_m with β₁ ≥ ⋯ ≥ β_m.

Large countable ordinals

As discussed above, the Cantor normal form of ordinals below ε₀ can be expressed in an alphabet containing only the function symbols for addition, multiplication and exponentiation, as well as constant symbols for each natural number and for Template:Mvar. We can do away with the infinitely many numerals by using just the constant symbol 0 and the operation of successor, S (for example, the natural number 4 may be expressed as S(S(S(S(0))))). This describes an ordinal notation: a system for naming ordinals over a finite alphabet. This particular system of ordinal notation is called the collection of arithmetical ordinal expressions, and can express all ordinals below ε₀, but cannot express ε₀. There are other ordinal notations capable of capturing ordinals well past ε₀, but because there are only countably many finite-length strings over any finite alphabet, for any given ordinal notation there will be ordinals below ω₁ (the first uncountable ordinal) that are not expressible. Such ordinals are known as large countable ordinals.

The operations of addition, multiplication and exponentiation are all examples of primitive recursive ordinal functions, and more general primitive recursive ordinal functions can be used to describe larger ordinals.

Natural operations

The natural sum and natural product operations on ordinals were defined in 1906 by Gerhard Hessenberg, and are sometimes called the Hessenberg sum (or product) . The natural sum of Template:Mvar and Template:Mvar is often denoted by α ⊕ β or α # β, and the natural product by α ⊗ β or α ⨳ β.

The natural sum and product are defined as follows. Let $\alpha ={\omega }^{{\alpha }_1}+\cdots +{\omega }^{{\alpha }_k}$ and $\beta ={\omega }^{{\beta }_1}+\cdots +{\omega }^{{\beta }_{\ell }}$ be in Cantor normal form (i.e. ${\alpha }_1\ge \cdots \ge {\alpha }_k$ and ${\beta }_1\ge \cdots \ge {\beta }_{\ell }$). Let ${\gamma }_1,\dots ,{\gamma }_{k+\ell }$ be the exponents ${\alpha }_1,\dots ,{\alpha }_k,{\beta }_1,\dots ,{\beta }_{\ell }$ sorted in nonincreasing order. Then $\alpha \oplus \beta$ is defined as $$\alpha \oplus \beta ={\omega }^{{\gamma }_1}+\cdots +{\omega }^{{\gamma }_{k+\ell }}.$$ The natural product of $\alpha$ and $\beta$ is defined as $$\alpha \otimes \beta =\underset{\begin{array}{rl}& 1\le i\le k\\ & 1\le j\le \ell \end{array}}{⨁}{\omega }^{{\alpha }_i\oplus {\beta }_j}.$$ For example, suppose $\alpha ={\omega }^{{\omega }^{\omega }}+\omega$ and $\beta ={\omega }^{\omega }+{\omega }^5$. Then $\alpha \oplus \beta ={\omega }^{{\omega }^{\omega }}+{\omega }^{\omega }+{\omega }^5+\omega$, whereas $\alpha +\beta ={\omega }^{{\omega }^{\omega }}+{\omega }^{\omega }+{\omega }^5$. And $\alpha \otimes \beta ={\omega }^{{\omega }^{\omega }+\omega }+{\omega }^{{\omega }^{\omega }+5}+{\omega }^{\omega +1}+{\omega }^6$, whereas $\alpha \beta ={\omega }^{{\omega }^{\omega }+\omega }+{\omega }^{{\omega }^{\omega }+5}$.

The natural sum and product are commutative and associative, and natural product distributes over natural sum. The operations are also monotonic, in the sense that if $\alpha <\beta$ then $\alpha \oplus \gamma <\beta \oplus \gamma$; if $\alpha \le \beta$ then $\alpha \otimes \gamma \le \beta \otimes \gamma$; and if $\alpha <\beta$ and $\gamma >0$ then $\alpha \otimes \gamma <\beta \otimes \gamma$.

We have $\underset{n}{\underbrace{\alpha \oplus \cdots \oplus \alpha }}=\alpha \otimes n$.

We always have $\alpha +\beta \le \alpha \oplus \beta$ and $\alpha \beta \le \alpha \otimes \beta$. If both $\alpha <{\omega }^{\gamma }$ and $\beta <{\omega }^{\gamma }$ then $\alpha \oplus \beta <{\omega }^{\gamma }$. If both $\alpha <{\omega }^{{\omega }^{\gamma }}$ and $\beta <{\omega }^{{\omega }^{\gamma }}$ then $\alpha \otimes \beta <{\omega }^{{\omega }^{\gamma }}$.

Natural sum and product are not continuous in the right argument, since, for example $\underset{n<\omega }{\lim }\alpha \oplus n=\alpha +\omega$, and not $\alpha \oplus \omega$; and $\underset{n<\omega }{\lim }\alpha \otimes n=\alpha \omega$, and not $\alpha \otimes \omega$.

The natural sum and product are the same as the addition and multiplication (restricted to ordinals) of John Conway's field of surreal numbers.

The natural operations come up in the theory of well partial orders; given two well partial orders $S$ and $T$, of types (maximum linearizations) $o(S)$ and $o(T)$, the type of the disjoint union is $o(S)\oplus o(T)$, while the type of the direct product is $o(S)\otimes o(T)$. One may take this relation as a definition of the natural operations by choosing Template:Mvar and Template:Mvar to be ordinals Template:Mvar and Template:Mvar; so α ⊕ β is the maximum order type of a total order extending the disjoint union (as a partial order) of Template:Mvar and Template:Mvar; while α ⊗ β is the maximum order type of a total order extending the direct product (as a partial order) of Template:Mvar and Template:Mvar. A useful application of this is when Template:Mvar and Template:Mvar are both subsets of some larger total order; then their union has order type at most α ⊕ β. If they are both subsets of some ordered abelian group, then their sum has order type at most α ⊗ β.

We can also define the natural sum α ⊕ β by simultaneous transfinite recursion on Template:Mvar and Template:Mvar, as the smallest ordinal strictly greater than the natural sum of Template:Mvar and Template:Mvar for all γ < β and of Template:Mvar and Template:Mvar for all γ < α. Similarly, we can define the natural product α ⊗ β by simultaneous transfinite recursion on Template:Mvar and Template:Mvar, as the smallest ordinal Template:Mvar such that (α ⊗ δ) ⊕ (ε ⊗ β) < γ ⊕ (ε ⊗ δ) for all ε < α and δ < β. Also, see the article on surreal numbers for the definition of natural multiplication in that context; however, it uses surreal subtraction, which is not defined on ordinals.

The natural sum is associative and commutative. It is always greater or equal to the usual sum, but it may be strictly greater. For example, the natural sum of Template:Mvar and 1 is ω + 1 (the usual sum), but this is also the natural sum of 1 and Template:Mvar. The natural product is associative and commutative and distributes over the natural sum. The natural product is always greater or equal to the usual product, but it may be strictly greater. For example, the natural product of Template:Mvar and 2 is ω · 2 (the usual product), but this is also the natural product of 2 and Template:Mvar.

Under natural addition, the ordinals can be identified with the elements of the free commutative monoid generated by the gamma numbers ω^α. Under natural addition and multiplication, the ordinals can be identified with the elements of the free commutative semiring generated by the delta numbers ω^ωα. The ordinals do not have unique factorization into primes under the natural product. While the full polynomial ring does have unique factorization, the subset of polynomials with non-negative coefficients does not: for example, if Template:Mvar is any delta number, then

x⁵ + x⁴ + x³ + x² + x + 1 = (x + 1) (x⁴ + x² + 1) = (x² + x + 1) (x³ + 1)

has two incompatible expressions as a natural product of polynomials with non-negative coefficients that cannot be decomposed further.

Nimber arithmetic

There are arithmetic operations on ordinals by virtue of the one-to-one correspondence between ordinals and nimbers. Three common operations on nimbers are nimber addition, nimber multiplication, and minimum excludance (mex). Nimber addition is a generalization of the bitwise exclusive or operation on natural numbers. The mex of a set of ordinals is the smallest ordinal not present in the set.

References

• Kunen, Kenneth, 1980. Set Theory: An Introduction to Independence Proofs. Elsevier. Template:Isbn.

External links

• ordCalc ordinal calculator