Ordinal Number

In common usage, an ordinal number is an adjective which describes the numerical position of an object, e.g., first, second, third, etc.

In formal set theory, an ordinal number (sometimes simply called an "ordinal" for short) is one of the numbers in Georg Cantor's extension of the whole numbers. An ordinal number is defined as the order type of a well ordered set (Dauben 1990, p. 199; Moore 1982, p. 52; Suppes 1972, p. 129). Finite ordinal numbers are commonly denoted using arabic numerals, while transfinite ordinals are denoted using lower case Greek letters.

It is easy to see that every finite totally ordered set is well ordered. Any two totally ordered sets with k elements (for k a nonnegative integer) are order isomorphic, and therefore have the same order type (which is also an ordinal number). The ordinals for finite sets are denoted 0, 1, 2, 3, ..., i.e., the integers one less than the corresponding nonnegative integers.

The first transfinite ordinal, denoted omega, is the order type of the set of nonnegative integers (Dauben 1990, p. 152; Moore 1982, p. viii; Rubin 1967, pp. 86 and 177; Suppes 1972, p. 128). This is the "smallest" of Cantor's transfinite numbers, defined to be the smallest ordinal number greater than the ordinal number of the whole numbers. Conway and Guy (1996) denote it with the notation omega={0,1,...|}.

From the definition of ordinal comparison, it follows that the ordinal numbers are a well ordered set. In order of increasing size, the ordinal numbers are 0, 1, 2, ..., omega, omega+1, omega+2, ..., omega+omega, omega+omega+1, .... The notation of ordinal numbers can be a bit counterintuitive, e.g., even though 1+omega=omega, omega+1>omega. The cardinal number of the set of countable ordinal numbers is denoted aleph_1 (aleph-1).

If (A,<=) is a well ordered set with ordinal number alpha, then the set of all ordinals <alpha is order isomorphic to A. This provides the motivation to define an ordinal as the set of all ordinals less than itself. John von Neumann defined a set alpha to be an ordinal number iff

1. If beta is a member of alpha, then beta is a proper subset of alpha.

2. If beta and gamma are members of alpha then one of the following is true: beta=gamma, beta is a member of gamma, or gamma is a member of beta.

3. If beta is a nonempty proper subset of alpha, then there exists a gamma member of alpha such that the intersection gamma intersection beta is empty.

(Rubin 1967, p. 176; Ciesielski 1997, p. 44). This is the standard representation of ordinals. In this representation,

0{}empty set
1{0}set of one element
2{0,1}set of two elements
3{0,1,2}set of three elements
omega{0,1,2,...}set of all finite ordinals
omega_1set of all countable ordinals
omega_2set of all countable and aleph_1 ordinals
omega_omegaset all finite ordinals and aleph_k ordinals for all nonnegative integers k

Rubin (1967, p. 272) provides a nice definition of the omega_alpha ordinals.

Since for any ordinal alpha, the union alpha union {alpha} is a bigger ordinal alpha+1, there is no largest ordinal, and the class of all ordinals is therefore a proper class (as shown by the Burali-Forti paradox).

Ordinal numbers have some other rather peculiar properties. The sum of two ordinal numbers can take on two different values, the sum of three can take on five values. The first few terms of this sequence are 2, 5, 13, 33, 81, 193, 449, 33^2, 33·81, 81^2, 81·193, 193^2, ..., namely 2, 5, 13, 33, 81, 193, 449, 1089, 2673, 6561, 15633, 37249, ... (Conway and Guy 1996, OEIS A005348). The sum of n ordinals has either 193^a81^b or 33·81^a possible answers for n>=15 (Conway and Guy 1996).

r×omega is the same as omega, but omega×r is equal to omega+...+omega_()_(r). omega^2 is larger than any number of the form omega×r, omega^3 is larger than omega^2, and so on.

There exist ordinal numbers which cannot be constructed from smaller ones by finite additions, multiplications, and exponentiations. These ordinals obey Cantor's equation. The first such ordinal is


The next is


then follow epsilon_2, epsilon_3, ..., epsilon_omega, epsilon_(omega+1), ..., epsilon_(omega×2), ..., epsilon_(omega^2), epsilon_(omega^omega), ..., epsilon_(epsilon_0), epsilon_(epsilon_0+1), ..., epsilon_(epsilon_0+omega), ..., epsilon_(epsilon_0+omega^omega), ..., epsilon_(epsilon_0×2), ..., epsilon_(epsilon_1), ..., epsilon_(epsilon_2), ..., epsilon_(epsilon_omega), ..., epsilon_(epsilon_(epsilon_0)), ..., epsilon_(epsilon_(epsilon_1)), ..., epsilon_(epsilon_(epsilon_omega)), ..., epsilon_(epsilon_(epsilon_(epsilon_0))), ... (Conway and Guy 1996).

Ordinal addition, ordinal multiplication, and ordinal exponentiation can all be defined. Although these definitions also work perfectly well for order types, this does not seem to be commonly done. There are two methods commonly used to define operations on the ordinals: one is using sets, and the other is inductively.

See also

Aleph-1, Axiom of Choice, Burali-Forti Paradox, Cantor's Equation, Cardinal Number, Initial Ordinal, Order Statistic, Order Type, Power Set, Surreal Number, Well Ordered Set

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Cantor, G. Über unendliche, lineare Punktmannigfältigkeiten, Arbeiten zur Mengenlehre aus dem Jahren 1872-1884. Leipzig, Germany: Teubner-Archiv zur Mathematik, 1884.Ciesielski, K. Set Theory for the Working Mathematician. Cambridge, England: Cambridge University Press, 1997.Conway, J. H. and Guy, R. K. "Cantor's Ordinal Numbers." In The Book of Numbers. New York: Springer-Verlag, pp. 266-267 and 274, 1996.Dauben, J. W. Georg Cantor: His Mathematics and Philosophy of the Infinite. Princeton, NJ: Princeton University Press, 1990.Moore, G. H. Zermelo's Axiom of Choice: Its Origin, Development, and Influence. New York: Springer-Verlag, 1982.Rubin, J. E. Set Theory for the Mathematician. New York: Holden-Day, 1967.Suppes, P. Axiomatic Set Theory. New York: Dover, 1972.Sloane, N. J. A. Sequence A005348/M1435 in "The On-Line Encyclopedia of Integer Sequences."

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Ordinal Number

Cite this as:

Weisstein, Eric W. "Ordinal Number." From MathWorld--A Wolfram Web Resource.

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