Ordinal number
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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
A linear order such that every non-empty subset has a least element is called a
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]
Ordinals extend the natural numbers
A
When dealing with infinite sets, however, one has to distinguish between the notion of size, which leads to
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
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
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 well-ordered—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 ω2. Further on, there will be ω3, then ω4, and so on, and ωω, then ωωω, then later ωωωω, and even later ε0 (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 ω1 or .[5]
Definitions
Well-ordered sets
In a
It is inappropriate to distinguish between two well-ordered 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 one-to-one correspondence is called an order isomorphism, and the two well-ordered sets are said to be order-isomorphic or similar (with the understanding that this is an equivalence relation).
Formally, if a
Essentially, an ordinal is intended to be defined as an
Definition of an ordinal as an equivalence class
The original definition of ordinal numbers, found for example in the
Von Neumann definition of ordinals
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 well-ordered sets, it will be defined as a particular well-ordered set that (canonically) represents the class. Thus, an ordinal number will be a well-ordered set; and every well-ordered set will be order-isomorphic to exactly one ordinal number.
For each well-ordered 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 well-ordered set of all smaller ordinals". In symbols, .[6][7] Formally:
- A set S is an ordinal strictlywell-ordered with respect to set membership and every element of S is also a subset of S.
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
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
Consequently, every ordinal S is a set having as elements precisely the ordinals smaller than S. For example, every set of ordinals has a
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 Burali-Forti 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 well-ordered, which is the case if and only if each of its non-empty subsets has a greatest element.
Other definitions
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:
- x is a (von Neumann) ordinal,
- x is a trichotomouson x,
- x is a transitive set totally ordered by set inclusion,
- x is a transitive set of transitive sets.
These definitions cannot be used in non-well-founded set theories. In set theories with urelements, one has to further make sure that the definition excludes urelements from appearing in ordinals.
Transfinite sequence
If α is any ordinal and X is a set, an α-indexed sequence of elements of X is a function from α to X. This concept, a transfinite sequence (if α is infinite) or ordinal-indexed sequence, is a generalization of the concept of a
Transfinite induction
Transfinite induction holds in any well-ordered set, but it is so important in relation to ordinals that it is worth restating here.
- Any property that passes from the set of ordinals smaller than a given ordinal α to α itself, is true of all ordinals.
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 recursion
Transfinite induction can be used not only to prove things, but also to define them. Such a definition is normally said to be by
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.
Successor and limit ordinals
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
When is an ordinal-indexed 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:
- There is an ordinal less than α and whenever ζ is an ordinal less than α, then there exists an ordinal ξ such that ζ < ξ < α.
So in the following sequence:
- 0, 1, 2, ..., ω, ω+1
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 well-defined 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 non-recursively).
Indexing classes of ordinals
Any well-ordered set is similar (order-isomorphic) 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 class-bijection with the class of all ordinals). So the -th element in the class (with the convention that the "0-th" is the smallest, the "1-st" 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 "
Closed unbounded sets and classes
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.
Arithmetic of ordinals
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 well-ordered 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 self-referential representations as ε0 = ωε0.
Ordinals are a subclass of the class of surreal numbers, and the so-called "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 game-theoretic 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.
Ordinals and cardinals
Initial ordinal of a cardinal
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 well-ordered set having an ordinal as its order-type 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 well-ordered, 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 α-th infinite initial ordinal is written , it is always a limit ordinal. Its cardinality is written . For example, the cardinality of ω0 = ω is , which is also the cardinality of ω2 or ε0 (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 well-orderings of the natural numbers: each such well-ordering 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 ).
Cofinality
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 well-ordered 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 ω2 is ω, because the sequence ω·m (where m ranges over the natural numbers) tends to ω2; 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 ωω·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
Some "large" countable ordinals
As mentioned above (see Cantor normal form), the ordinal ε0 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
Topology and ordinals
Any ordinal number can be made into a
See the Topology and ordinals section of the "Order topology" article.
History
The transfinite ordinal numbers, which first appeared in 1883,
Cantor used these sets in the theorems:
- If P(α) = ∅ for some index α, then P′ is countable;
- Conversely, if P′ is countable, then there is an index α such that P(α) = ∅.
These theorems are proved by partitioning P′ into
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.
- Case 1: P(β) \ P(β + 1) is non-empty for all countable β. Since there are uncountably many of these pairwise disjoint sets, their union is uncountable. This union is a subset of P′, so P' is uncountable.
- Case 2: P(β) \ P(β + 1) is empty for some countable β. Since P(β + 1) ⊆ P(β), this implies P(β + 1) = P(β). Thus, P(β) is a perfect set, so it is uncountable.[13] Since P(β) ⊆ P′, the set P′ is uncountable.
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
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 α-th number class. The cardinality of the (α + 1)-th number class is the cardinality immediately following that of the α-th number class.[16] For a limit ordinal α, the α-th number class is the union of the β-th number classes for β < α.[17] Its cardinality is the limit of the cardinalities of these number classes.
If n is finite, the n-th number class has cardinality . If α ≥ ω, the α-th number class has cardinality .[18] Therefore, the cardinalities of the number classes correspond one-to-one with the aleph numbers. Also, the α-th number class consists of ordinals different from those in the preceding number classes if and only if α is a non-limit ordinal. Therefore, the non-limit number classes partition the ordinals into pairwise disjoint sets.
See also
- Counting
- Even and odd ordinals
- First uncountable ordinal
- Ordinal space
- Surreal number, a generalization of ordinals which includes negative, real, and infinitesimal values.
Notes
- ^ "Ordinal Number - Examples and Definition of Ordinal Number". Literary Devices. 2017-05-21. Retrieved 2021-08-31.
- ISBN 978-0-8225-8846-7.
- ^ Thorough introductions are given by Levy 1979 and Jech 2003.
- MR 0532548. See the footnote on p. 12.
- ^ Weisstein, Eric W. "Ordinal Number". mathworld.wolfram.com. Retrieved 2020-08-12.
- ^ a b von Neumann 1923
- ^ Levy 1979, p. 52 attributes the idea to unpublished work of Zermelo in 1916 and several papers by von Neumann the 1920s.
- ^ Cantor 1883. English translation: Ewald 1996, pp. 881–920
- ^ Ferreirós 1995, pp. 34–35; Ferreirós 2007, pp. 159, 204–5
- ^ Ferreirós 2007, p. 269
- ^ Ferreirós 1995, pp. 35–36; Ferreirós 2007, p. 207
- ^ Ferreirós 1995, pp. 36–37; Ferreirós 2007, p. 271
- ^ Dauben 1979, p. 111
- ^ Ferreirós 2007, pp. 207–8
- ^ Dauben 1979, pp. 97–98
- ^ Hallett 1986, pp. 61–62
- ^ Tait 1997, p. 5 footnote
- ^ The first number class has cardinality . Mathematical induction proves that the n-th number class has cardinality . Since the ω-th number class is the union of the n-th number classes, its cardinality is , the limit of the . Transfinite induction proves that if α ≥ ω, the α-th number class has cardinality .
References
- S2CID 121930608. Published separately as: Grundlagen einer allgemeinen Mannigfaltigkeitslehre.
- Cantor, Georg (1897), "Beitrage zur Begrundung der transfiniten Mengenlehre. II", Mathematische Annalen, vol. 49, no. 2, pp. 207–246, .
- ISBN 978-1-4612-4072-3
- ISBN 0-674-34871-0.
- Ewald, William B., ed. (1996), From Immanuel Kant to David Hilbert: A Source Book in the Foundations of Mathematics, Volume 2, ISBN 0-19-850536-1.
- Ferreirós, José (1995), "'What fermented in me for years': Cantor's discovery of transfinite numbers" (PDF), .
- Ferreirós, José (2007), Labyrinth of Thought: A History of Set Theory and Its Role in Mathematical Thought (2nd revised ed.), ISBN 978-3-7643-8349-7.
- Hallett, Michael (1986), Cantorian Set Theory and Limitation of Size, Oxford University Press, ISBN 0-19-853283-0.
- Hamilton, A. G. (1982), "6. Ordinal and cardinal numbers", Numbers, Sets, and Axioms : the Apparatus of Mathematics, New York: Cambridge University Press, ISBN 0-521-24509-5.
- ISBN 978-0-444-51621-3.
- Levy, A. (2002) [1979], Basic Set Theory, ISBN 0-486-42079-5.
- Jech, Thomas (2013), Set Theory (2nd ed.), Springer, ISBN 978-3-662-22400-7.
- Sierpiński, W. (1965), Cardinal and Ordinal Numbers (2nd ed.), Warszawa: Państwowe Wydawnictwo Naukowe Also defines ordinal operations in terms of the Cantor Normal Form.
- ISBN 0-486-61630-4.
- ISBN 0-8126-9344-2.
- von Neumann, John (1923), "Zur Einführung der transfiniten Zahlen", Acta litterarum ac scientiarum Ragiae Universitatis Hungaricae Francisco-Josephinae, Sectio scientiarum mathematicarum, vol. 1, pp. 199–208, archived from the original on 2014-12-18, retrieved 2013-09-15
- ISBN 0-674-32449-8 - English translation of von Neumann 1923.
External links
- "Ordinal number", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
- Ordinals at ProvenMath
- Ordinal calculator GPL'd free software for computing with ordinals and ordinal notations
- Chapter 4 of Don Monk's lecture notes on set theory is an introduction to ordinals.