In set theory, an ordinal number, or ordinal, is a generalization of ordinal numerals (first, second, nth, etc.) aimed to extend enumeration to infinite sets.^{[1]}
A finite set can be enumerated by successively labeling each element with the least natural number that has not been previously used. To extend this process to various infinite sets, ordinal numbers are defined more generally using linearly ordered greek letter variables that include the natural numbers and have the property that every set of ordinals has a least element (this is needed for giving a meaning to "the least unused element").^{[2]} This more general definition allows us to define an ordinal number (omega) to be the least element that is greater than every natural number, along with ordinal numbers , , etc., which are even greater than .
A linear order such that every nonempty subset has a least element is called a wellorder. The axiom of choice implies that every set can be wellordered, and given two wellordered sets, one is isomorphic to an initial segment of the other. So ordinal numbers exist and are essentially unique.
Ordinal numbers are distinct from cardinal numbers, which measure the size of sets. Although the distinction between ordinals and cardinals is not always apparent on finite sets (one can go from one to the other just by counting labels), they are very different in the infinite case, where different infinite ordinals can correspond to sets having the same cardinal. Like other kinds of numbers, ordinals can be added, multiplied, and exponentiated, although none of these operations are commutative.
Ordinals were introduced by Georg Cantor in 1883^{[3]} in order to accommodate infinite sequences and classify derived sets, which he had previously introduced in 1872 while studying the uniqueness of trigonometric series.^{[4]}
A natural number (which, in this context, includes the number 0) can be used for two purposes: to describe the size of a set, or to describe the position of an element in a sequence. When restricted to finite sets, these two concepts coincide, since all linear orders of a finite set are isomorphic.
When dealing with infinite sets, however, one has to distinguish between the notion of size, which leads to cardinal numbers, and the notion of position, which leads to the ordinal numbers described here. This is because while any set has only one size (its cardinality), there are many nonisomorphic wellorderings of any infinite set, as explained below.
Whereas the notion of cardinal number is associated with a set with no particular structure on it, the ordinals are intimately linked with the special kind of sets that are called wellordered. A wellordered set is a totally ordered set in which every nonempty subset has a least element (a totally ordered set is an ordered set such that, given two distinct elements, one is less than the other). Equivalently, assuming the axiom of dependent choice, it is a totally ordered set without any infinite decreasing sequence — though there may be infinite increasing sequences. Ordinals may be used to label the elements of any given wellordered set (the smallest element being labelled 0, the one after that 1, the next one 2, "and so on"), and to measure the "length" of the whole set by the least ordinal that is not a label for an element of the set. This "length" is called the order type of the set.
Any ordinal is defined by the set of ordinals that precede it. In fact, the most common definition of ordinals identifies each ordinal as the set of ordinals that precede it. For example, the ordinal 42 is generally identified as the set {0, 1, 2, ..., 41}. Conversely, any set S of ordinals that is downward closed — meaning that for any ordinal α in S and any ordinal β < α, β is also in S — is (or can be identified with) an ordinal.
This definition of ordinals in terms of sets allows for infinite ordinals. The smallest infinite ordinal is , which can be identified with the set of natural numbers (so that the ordinal associated with every natural number precedes ). Indeed, the set of natural numbers is wellordered—as is any set of ordinals—and since it is downward closed, it can be identified with the ordinal associated with it.
Perhaps a clearer intuition of ordinals can be formed by examining a first few of them: as mentioned above, they start with the natural numbers, 0, 1, 2, 3, 4, 5, ... After all natural numbers comes the first infinite ordinal, ω, and after that come ω+1, ω+2, ω+3, and so on. (Exactly what addition means will be defined later on: just consider them as names.) After all of these come ω·2 (which is ω+ω), ω·2+1, ω·2+2, and so on, then ω·3, and then later on ω·4. Now the set of ordinals formed in this way (the ω·m+n, where m and n are natural numbers) must itself have an ordinal associated with it: and that is ω^{2}. Further on, there will be ω^{3}, then ω^{4}, and so on, and ω^{ω}, then ω^{ωω}, then later ω^{ωωω}, and even later ε_{0} (epsilon nought) (to give a few examples of relatively small—countable—ordinals). This can be continued indefinitely (as every time one says "and so on" when enumerating ordinals, it defines a larger ordinal). The smallest uncountable ordinal is the set of all countable ordinals, expressed as ω_{1} or .^{[5]}
In a wellordered set, every nonempty subset contains a distinct smallest element. Given the axiom of dependent choice, this is equivalent to saying that the set is totally ordered and there is no infinite decreasing sequence (the latter being easier to visualize). In practice, the importance of wellordering is justified by the possibility of applying transfinite induction, which says, essentially, that any property that passes on from the predecessors of an element to that element itself must be true of all elements (of the given wellordered set). If the states of a computation (computer program or game) can be wellordered—in such a way that each step is followed by a "lower" step—then the computation will terminate.
It is inappropriate to distinguish between two wellordered sets if they only differ in the "labeling of their elements", or more formally: if the elements of the first set can be paired off with the elements of the second set such that if one element is smaller than another in the first set, then the partner of the first element is smaller than the partner of the second element in the second set, and vice versa. Such a onetoone correspondence is called an order isomorphism, and the two wellordered sets are said to be orderisomorphic or similar (with the understanding that this is an equivalence relation).
Formally, if a partial order ≤ is defined on the set S, and a partial order ≤' is defined on the set S' , then the posets (S,≤) and (S' ,≤') are order isomorphic if there is a bijection f that preserves the ordering. That is, f(a) ≤' f(b) if and only if a ≤ b. Provided there exists an order isomorphism between two wellordered sets, the order isomorphism is unique: this makes it quite justifiable to consider the two sets as essentially identical, and to seek a "canonical" representative of the isomorphism type (class). This is exactly what the ordinals provide, and it also provides a canonical labeling of the elements of any wellordered set. Every wellordered set (S,<) is orderisomorphic to the set of ordinals less than one specific ordinal number under their natural ordering. This canonical set is the order type of (S,<).
Essentially, an ordinal is intended to be defined as an isomorphism class of wellordered sets: that is, as an equivalence class for the equivalence relation of "being orderisomorphic". There is a technical difficulty involved, however, in the fact that the equivalence class is too large to be a set in the usual Zermelo–Fraenkel (ZF) formalization of set theory. But this is not a serious difficulty. The ordinal can be said to be the order type of any set in the class.
The original definition of ordinal numbers, found for example in the Principia Mathematica, defines the order type of a wellordering as the set of all wellorderings similar (orderisomorphic) to that wellordering: in other words, an ordinal number is genuinely an equivalence class of wellordered sets. This definition must be abandoned in ZF and related systems of axiomatic set theory because these equivalence classes are too large to form a set. However, this definition still can be used in type theory and in Quine's axiomatic set theory New Foundations and related systems (where it affords a rather surprising alternative solution to the BuraliForti paradox of the largest ordinal).
0  =  {}  =  ∅ 

1  =  {0}  =  {∅} 
2  =  {0,1}  =  {∅,{∅)) 
3  =  {0,1,2}  =  {∅,{∅},{∅,{∅))} 
4  =  {0,1,2,3}  =  {∅,{∅},{∅,{∅)),{∅,{∅},{∅,{∅)))) 
Rather than defining an ordinal as an equivalence class of wellordered sets, it will be defined as a particular wellordered set that (canonically) represents the class. Thus, an ordinal number will be a wellordered set; and every wellordered set will be orderisomorphic to exactly one ordinal number.
For each wellordered set T, defines an order isomorphism between T and the set of all subsets of T having the form ordered by inclusion. This motivates the standard definition, suggested by John von Neumann at the age of 19, now called definition of von Neumann ordinals: "each ordinal is the wellordered set of all smaller ordinals". In symbols, .^{[6]}^{[7]} Formally:
The natural numbers are thus ordinals by this definition. For instance, 2 is an element of 4 = {0, 1, 2, 3}, and 2 is equal to {0, 1} and so it is a subset of {0, 1, 2, 3}.
It can be shown by transfinite induction that every wellordered set is orderisomorphic to exactly one of these ordinals, that is, there is an order preserving bijective function between them.
Furthermore, the elements of every ordinal are ordinals themselves. Given two ordinals S and T, S is an element of T if and only if S is a proper subset of T. Moreover, either S is an element of T, or T is an element of S, or they are equal. So every set of ordinals is totally ordered. Further, every set of ordinals is wellordered. This generalizes the fact that every set of natural numbers is wellordered.
Consequently, every ordinal S is a set having as elements precisely the ordinals smaller than S. For example, every set of ordinals has a supremum, the ordinal obtained by taking the union of all the ordinals in the set. This union exists regardless of the set's size, by the axiom of union.
The class of all ordinals is not a set. If it were a set, one could show that it was an ordinal and thus a member of itself, which would contradict its strict ordering by membership. This is the BuraliForti paradox. The class of all ordinals is variously called "Ord", "ON", or "∞".
An ordinal is finite if and only if the opposite order is also wellordered, which is the case if and only if each of its nonempty subsets has a maximum.
There are other modern formulations of the definition of ordinal. For example, assuming the axiom of regularity, the following are equivalent for a set x:
These definitions cannot be used in nonwellfounded set theories. In set theories with urelements, one has to further make sure that the definition excludes urelements from appearing in ordinals.
If α is any ordinal and X is a set, an αindexed sequence of elements of X is a function from α to X. This concept, a transfinite sequence (if α is infinite) or ordinalindexed sequence, is a generalization of the concept of a sequence. An ordinary sequence corresponds to the case α = ω, while a finite α corresponds to a tuple, a.k.a. string.
Transfinite induction holds in any wellordered set, but it is so important in relation to ordinals that it is worth restating here.
That is, if P(α) is true whenever P(β) is true for all β < α, then P(α) is true for all α. Or, more practically: in order to prove a property P for all ordinals α, one can assume that it is already known for all smaller β < α.
Transfinite induction can be used not only to prove things, but also to define them. Such a definition is normally said to be by transfinite recursion – the proof that the result is welldefined uses transfinite induction. Let F denote a (class) function F to be defined on the ordinals. The idea now is that, in defining F(α) for an unspecified ordinal α, one may assume that F(β) is already defined for all β < α and thus give a formula for F(α) in terms of these F(β). It then follows by transfinite induction that there is one and only one function satisfying the recursion formula up to and including α.
Here is an example of definition by transfinite recursion on the ordinals (more will be given later): define function F by letting F(α) be the smallest ordinal not in the set {F(β)  β < α}, that is, the set consisting of all F(β) for β < α. This definition assumes the F(β) known in the very process of defining F; this apparent vicious circle is exactly what definition by transfinite recursion permits. In fact, F(0) makes sense since there is no ordinal β < 0, and the set {F(β)  β < 0} is empty. So F(0) is equal to 0 (the smallest ordinal of all). Now that F(0) is known, the definition applied to F(1) makes sense (it is the smallest ordinal not in the singleton set {F(0)} = {0}), and so on (the and so on is exactly transfinite induction). It turns out that this example is not very exciting, since provably F(α) = α for all ordinals α, which can be shown, precisely, by transfinite induction.
Any nonzero ordinal has the minimum element, zero. It may or may not have a maximum element. For example, 42 has maximum 41 and ω+6 has maximum ω+5. On the other hand, ω does not have a maximum since there is no largest natural number. If an ordinal has a maximum α, then it is the next ordinal after α, and it is called a successor ordinal, namely the successor of α, written α+1. In the von Neumann definition of ordinals, the successor of α is since its elements are those of α and α itself.^{[6]}
A nonzero ordinal that is not a successor is called a limit ordinal. One justification for this term is that a limit ordinal is the limit in a topological sense of all smaller ordinals (under the order topology).
When is an ordinalindexed sequence, indexed by a limit and the sequence is increasing, i.e. whenever its limit is defined as the least upper bound of the set that is, the smallest ordinal (it always exists) greater than any term of the sequence. In this sense, a limit ordinal is the limit of all smaller ordinals (indexed by itself). Put more directly, it is the supremum of the set of smaller ordinals.
Another way of defining a limit ordinal is to say that α is a limit ordinal if and only if:
So in the following sequence:
ω is a limit ordinal because for any smaller ordinal (in this example, a natural number) there is another ordinal (natural number) larger than it, but still less than ω.
Thus, every ordinal is either zero, or a successor (of a welldefined predecessor), or a limit. This distinction is important, because many definitions by transfinite recursion rely upon it. Very often, when defining a function F by transfinite recursion on all ordinals, one defines F(0), and F(α+1) assuming F(α) is defined, and then, for limit ordinals δ one defines F(δ) as the limit of the F(β) for all β<δ (either in the sense of ordinal limits, as previously explained, or for some other notion of limit if F does not take ordinal values). Thus, the interesting step in the definition is the successor step, not the limit ordinals. Such functions (especially for F nondecreasing and taking ordinal values) are called continuous. Ordinal addition, multiplication and exponentiation are continuous as functions of their second argument (but can be defined nonrecursively).
Any wellordered set is similar (orderisomorphic) to a unique ordinal number ; in other words, its elements can be indexed in increasing fashion by the ordinals less than . This applies, in particular, to any set of ordinals: any set of ordinals is naturally indexed by the ordinals less than some . The same holds, with a slight modification, for classes of ordinals (a collection of ordinals, possibly too large to form a set, defined by some property): any class of ordinals can be indexed by ordinals (and, when the class is unbounded in the class of all ordinals, this puts it in classbijection with the class of all ordinals). So the th element in the class (with the convention that the "0th" is the smallest, the "1st" is the next smallest, and so on) can be freely spoken of. Formally, the definition is by transfinite induction: the th element of the class is defined (provided it has already been defined for all ), as the smallest element greater than the th element for all .
This could be applied, for example, to the class of limit ordinals: the th ordinal, which is either a limit or zero is (see ordinal arithmetic for the definition of multiplication of ordinals). Similarly, one can consider additively indecomposable ordinals (meaning a nonzero ordinal that is not the sum of two strictly smaller ordinals): the th additively indecomposable ordinal is indexed as . The technique of indexing classes of ordinals is often useful in the context of fixed points: for example, the th ordinal such that is written . These are called the "epsilon numbers".
A class of ordinals is said to be unbounded, or cofinal, when given any ordinal , there is a in such that (then the class must be a proper class, i.e., it cannot be a set). It is said to be closed when the limit of a sequence of ordinals in the class is again in the class: or, equivalently, when the indexing (class)function is continuous in the sense that, for a limit ordinal, (the th ordinal in the class) is the limit of all for ; this is also the same as being closed, in the topological sense, for the order topology (to avoid talking of topology on proper classes, one can demand that the intersection of the class with any given ordinal is closed for the order topology on that ordinal, this is again equivalent).
Of particular importance are those classes of ordinals that are closed and unbounded, sometimes called clubs. For example, the class of all limit ordinals is closed and unbounded: this translates the fact that there is always a limit ordinal greater than a given ordinal, and that a limit of limit ordinals is a limit ordinal (a fortunate fact if the terminology is to make any sense at all!). The class of additively indecomposable ordinals, or the class of ordinals, or the class of cardinals, are all closed unbounded; the set of regular cardinals, however, is unbounded but not closed, and any finite set of ordinals is closed but not unbounded.
A class is stationary if it has a nonempty intersection with every closed unbounded class. All superclasses of closed unbounded classes are stationary, and stationary classes are unbounded, but there are stationary classes that are not closed and stationary classes that have no closed unbounded subclass (such as the class of all limit ordinals with countable cofinality). Since the intersection of two closed unbounded classes is closed and unbounded, the intersection of a stationary class and a closed unbounded class is stationary. But the intersection of two stationary classes may be empty, e.g. the class of ordinals with cofinality ω with the class of ordinals with uncountable cofinality.
Rather than formulating these definitions for (proper) classes of ordinals, one can formulate them for sets of ordinals below a given ordinal : A subset of a limit ordinal is said to be unbounded (or cofinal) under provided any ordinal less than is less than some ordinal in the set. More generally, one can call a subset of any ordinal cofinal in provided every ordinal less than is less than or equal to some ordinal in the set. The subset is said to be closed under provided it is closed for the order topology in , i.e. a limit of ordinals in the set is either in the set or equal to itself.
There are three usual operations on ordinals: addition, multiplication, and exponentiation. Each can be defined in essentially two different ways: either by constructing an explicit wellordered set that represents the operation or by using transfinite recursion. The Cantor normal form provides a standardized way of writing ordinals. It uniquely represents each ordinal as a finite sum of ordinal powers of ω. However, this cannot form the basis of a universal ordinal notation due to such selfreferential representations as ε_{0} = ω^{ε0}.
Ordinals are a subclass of the class of surreal numbers, and the socalled "natural" arithmetical operations for surreal numbers are an alternative way to combine ordinals arithmetically. They retain commutativity at the expense of continuity.
Interpreted as nimbers, a gametheoretic variant of numbers, ordinals can also be combined via nimber arithmetic operations. These operations are commutative but the restriction to natural numbers is generally not the same as ordinary addition of natural numbers.
Each ordinal associates with one cardinal, its cardinality. If there is a bijection between two ordinals (e.g. ω = 1 + ω and ω + 1 > ω), then they associate with the same cardinal. Any wellordered set having an ordinal as its ordertype has the same cardinality as that ordinal. The least ordinal associated with a given cardinal is called the initial ordinal of that cardinal. Every finite ordinal (natural number) is initial, and no other ordinal associates with its cardinal. But most infinite ordinals are not initial, as many infinite ordinals associate with the same cardinal. The axiom of choice is equivalent to the statement that every set can be wellordered, i.e. that every cardinal has an initial ordinal. In theories with the axiom of choice, the cardinal number of any set has an initial ordinal, and one may employ the Von Neumann cardinal assignment as the cardinal's representation. (However, we must then be careful to distinguish between cardinal arithmetic and ordinal arithmetic.) In set theories without the axiom of choice, a cardinal may be represented by the set of sets with that cardinality having minimal rank (see Scott's trick).
One issue with Scott's trick is that it identifies the cardinal number with , which in some formulations is the ordinal number . It may be clearer to apply Von Neumann cardinal assignment to finite cases and to use Scott's trick for sets which are infinite or do not admit well orderings. Note that cardinal and ordinal arithmetic agree for finite numbers.
The αth infinite initial ordinal is written , it is always a limit ordinal. Its cardinality is written . For example, the cardinality of ω_{0} = ω is , which is also the cardinality of ω^{2} or ε_{0} (all are countable ordinals). So ω can be identified with , except that the notation is used when writing cardinals, and ω when writing ordinals (this is important since, for example, = whereas ). Also, is the smallest uncountable ordinal (to see that it exists, consider the set of equivalence classes of wellorderings of the natural numbers: each such wellordering defines a countable ordinal, and is the order type of that set), is the smallest ordinal whose cardinality is greater than , and so on, and is the limit of the for natural numbers n (any limit of cardinals is a cardinal, so this limit is indeed the first cardinal after all the ).
The cofinality of an ordinal is the smallest ordinal that is the order type of a cofinal subset of . Notice that a number of authors define cofinality or use it only for limit ordinals. The cofinality of a set of ordinals or any other wellordered set is the cofinality of the order type of that set.
Thus for a limit ordinal, there exists a indexed strictly increasing sequence with limit . For example, the cofinality of ω^{2} is ω, because the sequence ω·m (where m ranges over the natural numbers) tends to ω^{2}; but, more generally, any countable limit ordinal has cofinality ω. An uncountable limit ordinal may have either cofinality ω as does or an uncountable cofinality.
The cofinality of 0 is 0. And the cofinality of any successor ordinal is 1. The cofinality of any limit ordinal is at least .
An ordinal that is equal to its cofinality is called regular and it is always an initial ordinal. Any limit of regular ordinals is a limit of initial ordinals and thus is also initial even if it is not regular, which it usually is not. If the Axiom of Choice, then is regular for each α. In this case, the ordinals 0, 1, , , and are regular, whereas 2, 3, , and ω_{ω·2} are initial ordinals that are not regular.
The cofinality of any ordinal α is a regular ordinal, i.e. the cofinality of the cofinality of α is the same as the cofinality of α. So the cofinality operation is idempotent.
As mentioned above (see Cantor normal form), the ordinal ε_{0} is the smallest satisfying the equation , so it is the limit of the sequence 0, 1, , , , etc. Many ordinals can be defined in such a manner as fixed points of certain ordinal functions (the th ordinal such that is called , then one could go on trying to find the th ordinal such that , "and so on", but all the subtlety lies in the "and so on"). One could try to do this systematically, but no matter what system is used to define and construct ordinals, there is always an ordinal that lies just above all the ordinals constructed by the system. Perhaps the most important ordinal that limits a system of construction in this manner is the Church–Kleene ordinal, (despite the in the name, this ordinal is countable), which is the smallest ordinal that cannot in any way be represented by a computable function (this can be made rigorous, of course). Considerably large ordinals can be defined below , however, which measure the "prooftheoretic strength" of certain formal systems (for example, measures the strength of Peano arithmetic). Large countable ordinals such as countable admissible ordinals can also be defined above the ChurchKleene ordinal, which are of interest in various parts of logic.^{[citation needed]}
Any ordinal number can be made into a topological space by endowing it with the order topology; this topology is discrete if and only if the ordinal is a countable cardinal, i.e. at most ω. A subset of ω + 1 is open in the order topology if and only if either it is cofinite or it does not contain ω as an element.
See the Topology and ordinals section of the "Order topology" article.
The transfinite ordinal numbers, which first appeared in 1883,^{[8]} originated in Cantor's work with derived sets. If P is a set of real numbers, the derived set P′ is the set of limit points of P. In 1872, Cantor generated the sets P^{(n)} by applying the derived set operation n times to P. In 1880, he pointed out that these sets form the sequence P' ⊇ ··· ⊇ P^{(n)} ⊇ P^{(n + 1)} ⊇ ···, and he continued the derivation process by defining P^{(∞)} as the intersection of these sets. Then he iterated the derived set operation and intersections to extend his sequence of sets into the infinite: P^{(∞)} ⊇ P^{(∞ + 1)} ⊇ P^{(∞ + 2)} ⊇ ··· ⊇ P^{(2∞)} ⊇ ··· ⊇ P^{(∞2)} ⊇ ···.^{[9]} The superscripts containing ∞ are just indices defined by the derivation process.^{[10]}
Cantor used these sets in the theorems:
These theorems are proved by partitioning P′ into pairwise disjoint sets: P′ = (P′\ P^{(2)}) ∪ (P^{(2)} \ P^{(3)}) ∪ ··· ∪ (P^{(∞)} \ P^{(∞ + 1)}) ∪ ··· ∪ P^{(α)}. For β < α: since P^{(β + 1)} contains the limit points of P^{(β)}, the sets P^{(β)} \ P^{(β + 1)} have no limit points. Hence, they are discrete sets, so they are countable. Proof of first theorem: If P^{(α)} = ∅ for some index α, then P′ is the countable union of countable sets. Therefore, P′ is countable.^{[11]}
The second theorem requires proving the existence of an α such that P^{(α)} = ∅. To prove this, Cantor considered the set of all α having countably many predecessors. To define this set, he defined the transfinite ordinal numbers and transformed the infinite indices into ordinals by replacing ∞ with ω, the first transfinite ordinal number. Cantor called the set of finite ordinals the first number class. The second number class is the set of ordinals whose predecessors form a countably infinite set. The set of all α having countably many predecessors—that is, the set of countable ordinals—is the union of these two number classes. Cantor proved that the cardinality of the second number class is the first uncountable cardinality.^{[12]}
Cantor's second theorem becomes: If P′ is countable, then there is a countable ordinal α such that P^{(α)} = ∅. Its proof uses proof by contradiction. Let P′ be countable, and assume there is no such α. This assumption produces two cases.
In both cases, P′ is uncountable, which contradicts P′ being countable. Therefore, there is a countable ordinal α such that P^{(α)} = ∅. Cantor's work with derived sets and ordinal numbers led to the CantorBendixson theorem.^{[14]}
Using successors, limits, and cardinality, Cantor generated an unbounded sequence of ordinal numbers and number classes.^{[15]} The (α + 1)th number class is the set of ordinals whose predecessors form a set of the same cardinality as the αth number class. The cardinality of the (α + 1)th number class is the cardinality immediately following that of the αth number class.^{[16]} For a limit ordinal α, the αth number class is the union of the βth number classes for β < α.^{[17]} Its cardinality is the limit of the cardinalities of these number classes.
If n is finite, the nth number class has cardinality . If α ≥ ω, the αth number class has cardinality .^{[18]} Therefore, the cardinalities of the number classes correspond onetoone with the aleph numbers. Also, the αth number class consists of ordinals different from those in the preceding number classes if and only if α is a nonlimit ordinal. Therefore, the nonlimit number classes partition the ordinals into pairwise disjoint sets.
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