Ordinal arithmetic

The union of two disjoint well-ordered sets S and T can be well-ordered. The order-type of that union is the ordinal that results from adding the order-types of S and T. If two well-ordered sets are not already disjoint, then they can be replaced by order-isomorphic disjoint sets, e.g. replace S by {0} × S and T by {1} × T. This way, the well-ordered set S is written "to the left" of the well-ordered set T, meaning one defines an order on S ${\displaystyle \cup }$ T in which every element of S is smaller than every element of T. The sets S and T themselves keep the ordering they already have.

The definition of addition α + β can also be given by transfinite recursion on β:

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

Ordinal addition on the natural numbers is the same as standard addition. The first transfinite ordinal is ω, 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 ω because in ω only 0 does not have a direct predecessor while in ω + ω the two elements 0 and 0' do not have direct predecessors.

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

The set { (n,0), (n,1) : n ∈ N }, under lexicographic order, has order type 2 • ω, which is equal to ω.

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

The definition of multiplication can also be given inductively (the following induction is on β):

• α·0 = 0.
• α · S(β) = (α · β) + α, for a successor ordinal S(β).
• ${\displaystyle \alpha \cdot \beta =\bigcup _{\delta <\beta }(\alpha \cdot \delta )}$, when β 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 ω. Thus, ω·2 = ω+ω ≠ ω = 2·ω, showing that multiplication of ordinals is not in general commutative, c.f. pictures.

Again ordinal multiplication on the natural numbers is the same as standard multiplication.

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 inductively (the following induction is on β, the exponent):

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

Both definitions simplify considerably if the exponent β is a finite number: α^β is then just the product of β copies of α; e.g. ω³ = ω·ω·ω, and the elements of ω³ can be viewed as triples of natural numbers, ordered lexicographically. 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 α, 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 ${\displaystyle \omega ^{n_{1}}c_{1}+\omega ^{n_{2}}c_{2}+\cdots +\omega ^{n_{k}}c_{k}}$ where k, n₁,...,n_k are natural numbers, c₁,...,c_k are positive integers, 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 ${\displaystyle \alpha ^{b_{1}}a_{1}+\alpha ^{b_{2}}a_{2}+\cdots +\alpha ^{b_{k}}a_{k}}$ where k is a natural number, b₁,...,b_k are ordinals smaller than β with b₁>...>b_k, and a₁,...,a_k are positive ordinals smaller than α. 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 β 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. ω) in the former and symbols for cardinals (e.g. ${\displaystyle \aleph _{0}}$) in the latter.

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.

Every ordinal number α can be uniquely written as ${\displaystyle \omega ^{\beta _{1}}c_{1}+\omega ^{\beta _{2}}c_{2}+\cdots +\omega ^{\beta _{k}}c_{k}}$, where k is a natural number, ${\displaystyle c_{1},c_{2},\ldots ,c_{k}}$ are positive integers, and ${\displaystyle \beta _{1}>\beta _{2}>\ldots >\beta _{k}\geq 0}$ are ordinal numbers. The degenerate case α=0 occurs when k=0 and there are no βs nor cs. This decomposition of α is called the Cantor normal form of α, and can be considered the base-ω positional numeral system. The highest exponent ${\displaystyle \beta _{1}}$ is called the degree of ${\displaystyle \alpha }$, and satisfies ${\displaystyle \beta _{1}\leq \alpha }$. The equality ${\displaystyle \beta _{1}=\alpha }$ applies if and only if ${\displaystyle \alpha =\omega ^{\alpha }}$. 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 ${\displaystyle \omega ^{\beta _{1}}+\omega ^{\beta _{2}}+\cdots +\omega ^{\beta _{k}}}$, where k is a natural number, and ${\displaystyle \beta _{1}\geq \beta _{2}\geq \ldots \geq \beta _{k}\geq 0}$ are ordinal numbers.

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

The Cantor normal form allows us to uniquely express—and order—the ordinals α that are built from the natural numbers by a finite number of arithmetical operations of addition, multiplication and exponentiation base-${\displaystyle \omega }$: in other words, assuming ${\displaystyle \beta _{1}<\alpha }$ in the Cantor normal form, we can also express the exponents ${\displaystyle \beta _{i}}$ in Cantor normal form, and making the same assumption for the ${\displaystyle \beta _{i}}$ as for α and so on recursively, we get a system of notation for these ordinals (for example,

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

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 ω, and the smallest ordinal such that ${\displaystyle \varepsilon _{0}=\omega ^{\varepsilon _{0}}}$, i.e. in Cantor normal form the exponent is not smaller than the ordinal itself. It is the limit of the sequence

${\displaystyle 0,\,1=\omega ^{0},\,\omega =\omega ^{1},\,\omega ^{\omega },\,\omega ^{\omega ^{\omega }},\,\ldots \,.}$

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 § Addition and § Multiplication) that

${\displaystyle \omega ^{\beta }c+\omega ^{\beta '}c'=\omega ^{\beta '}c'\,,}$

if ${\displaystyle \beta '>\beta }$ (if ${\displaystyle \beta '=\beta }$ one can apply the distributive law on the left and rewrite this as ${\displaystyle \omega ^{\beta }(c+c')}$, and if ${\displaystyle \beta '<\beta }$ the expression is already in Cantor normal form); and to compute products, the essential facts are that when ${\displaystyle 0<\alpha =\omega ^{\beta _{1}}c_{1}+\cdots +\omega ^{\beta _{k}}c_{k}}$ is in Cantor normal form and ${\displaystyle 0<\beta '}$, then

${\displaystyle \alpha \omega ^{\beta '}=\omega ^{\beta _{1}+\beta '}\,}$

and

${\displaystyle \alpha n=\omega ^{\beta _{1}}c_{1}n+\omega ^{\beta _{2}}c_{2}+\cdots +\omega ^{\beta _{k}}c_{k}\,,}$

if n is a non-zero natural number.

To compare two ordinals written in Cantor normal form, first compare ${\displaystyle \beta _{1}}$, then ${\displaystyle c_{1}}$, then ${\displaystyle \beta _{2}}$, then ${\displaystyle c_{2}}$, etc.. At the first difference, 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.

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 (Sierpiński 1958).

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, ... , ω, 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 α. 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 occurs before every successor prime
• If two consecutive primes of the prime factorization are both limits or both finite, then the second one is at most 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 α is the smallest power of ω in the Cantor normal form and β is a successor.
• If α=ω^γ then writing γ in Cantor normal form gives an expansion of α as a product of limit primes.
• Now look at the Cantor normal form of β. 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 an integer m. Repeating this and factorizing the integers into primes gives the prime factorization of β.

So the factorization of the Cantor normal form ordinal

${\displaystyle \omega ^{\alpha _{1}}n_{1}+\cdots +\omega ^{\alpha _{k}}n_{k}}$ (with ${\displaystyle \alpha _{1}>\cdots >\alpha _{k}}$)

into a minimal product of infinite primes and integers is

${\displaystyle \omega ^{\omega ^{\beta _{1}}}\cdots \omega ^{\omega ^{\beta _{m}}}n_{k}(\omega ^{\alpha _{k-1}-\alpha _{k}}+1)n_{k-1}\cdots n_{2}(\omega ^{\alpha _{1}-\alpha _{2}}+1)n_{1}}$

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

${\displaystyle \alpha _{k}=\omega ^{\beta _{1}}+\cdots +\omega ^{\beta _{m}}}$ with ${\displaystyle \beta _{1}\geq \cdots \geq \beta _{m}}$.

As discussed above, the Cantor normal form of ordinals below ${\displaystyle \varepsilon _{0}}$ 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 ${\displaystyle \omega }$. We can do away with the infinitely many numerals by using just the constant symbol 0 and the operation of successor, ${\displaystyle S}$ (for example, the integer 4 may be expressed as ${\displaystyle S(S(S(S(0))))}$). 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 ${\displaystyle \varepsilon _{0}}$, but cannot express ${\displaystyle \varepsilon _{0}}$. There are other ordinal notations capable of capturing ordinals well past ${\displaystyle \varepsilon _{0}}$, but because there are only countably many finite-length strings over any finite alphabet, for any given ordinal notation there will be ordinals below ${\displaystyle \omega _{1}}$ (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.

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) (Sierpiński 1958). These are the same as the addition and multiplication (restricted to ordinals) of John Conway's field of surreal numbers. They have the advantage that they are associative and commutative, and natural product distributes over natural sum. The cost of making these operations commutative is that they lose the continuity in the right argument, which is a property of the ordinary sum and product. The natural sum of α and β is often denoted by α ⊕ β or α # β, and the natural product by α ⊗ β or α ⨳ β.

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) ⊕ o(T), while the type of the direct product is o(S) ⊗ o(T).[2] One may take this relation as a definition of the natural operations by choosing S and T to be ordinals α and β; so α ⊕ β is the maximum order type of a total order extending the disjoint union (as a partial order) of α and β; while α ⊗ β is the maximum order type of a total order extending the direct product (as a partial order) of α and β.[3] A useful application of this is when α and β 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 of α and β inductively (by simultaneous induction on α and β) as the smallest ordinal greater than the natural sum of α and γ for all γ < β and of γ and β for all γ < α. There is also an inductive definition of the natural product (by mutual induction), but it is somewhat tedious to write down and we shall not do so (see the article on surreal numbers for the definition in that context, which, however, uses surreal subtraction, something that obviously cannot be 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 ω and 1 is ω+1 (the usual sum), but this is also the natural sum of 1 and ω. 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 ω and 2 is ω·2 (the usual product), but this is also the natural product of 2 and ω.

Yet another way to define the natural sum and product of two ordinals α and β is to use the Cantor normal form: one can find a sequence of ordinals γ₁ > ... > γ_n and two sequences (k₁, ..., k_n) and (j₁, ..., j_n) of natural numbers (including zero, but satisfying k_i + j_i > 0 for all i) such that

${\displaystyle \alpha =\omega ^{\gamma _{1}}\cdot k_{1}+\cdots +\omega ^{\gamma _{n}}\cdot k_{n}}$

${\displaystyle \beta =\omega ^{\gamma _{1}}\cdot j_{1}+\cdots +\omega ^{\gamma _{n}}\cdot j_{n}}$

and define

${\displaystyle \alpha \#\beta =\omega ^{\gamma _{1}}\cdot (k_{1}+j_{1})+\cdots +\omega ^{\gamma _{n}}\cdot (k_{n}+j_{n}).}$

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 x is any delta number, then

${\displaystyle x^{5}+x^{4}+x^{3}+x^{2}+x+1=(x+1)(x^{4}+x^{2}+1)=(x^{2}+x+1)(x^{3}+1)}$

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

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.

• Thomas Jech (21 March 2006). Set Theory: The Third Millennium Edition, revised and expanded. Springer Science & Business Media. ISBN 978-3-540-44085-7.
• Kunen, Kenneth, 1980. Set Theory: An Introduction to Independence Proofs. Elsevier. ISBN 0-444-86839-9.
• Sierpiński, Wacław (1958), Cardinal and ordinal numbers, Polska Akademia Nauk Monografie Matematyczne, vol. 34, Warsaw: Państwowe Wydawnictwo Naukowe, MR 0095787

• ordCalc ordinal calculator