In set theory, a limit ordinal is an ordinal number that is neither zero nor a successor ordinal. Alternatively, an ordinal
![](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Omega-exp-omega-labeled.svg/250px-Omega-exp-omega-labeled.svg.png)
For example, the smallest limit ordinal is
Using the von Neumann definition of ordinals, every ordinal is the well-ordered set of all smaller ordinals. The union of a nonempty set of ordinals that has no greatest element is then always a limit ordinal. Using von Neumann cardinal assignment, every infinite cardinal number is also a limit ordinal.
Alternative definitions
editVarious other ways to define limit ordinals are:
- It is equal to the supremum of all the ordinals below it, but is not zero. (Compare with a successor ordinal: the set of ordinals below it has a maximum, so the supremum is this maximum, the previous ordinal.)
- It is not zero and has no maximum element.
- It can be written in the form
ω α forα > 0. That is, in the Cantor normal form there is no finite number as last term, and the ordinal is nonzero. - It is a limit point of the class of ordinal numbers, with respect to the order topology. (The other ordinals are isolated points.)
Some contention exists on whether or not 0 should be classified as a limit ordinal, as it does not have an immediate predecessor; some textbooks include 0 in the class of limit ordinals[1] while others exclude it.[2]
Examples
editBecause the class of ordinal numbers is well-ordered, there is a smallest infinite limit ordinal; denoted by
In general, all of these recursive definitions via multiplication, exponentiation, repeated exponentiation, etc. yield limit ordinals. All of the ordinals discussed so far are still countable ordinals. However, there is no recursively enumerable scheme for systematically naming all ordinals less than the Church–Kleene ordinal, which is a countable ordinal.
Beyond the countable, the first uncountable ordinal is usually denoted
Continuing, one can obtain the following (all of which are now increasing in cardinality):
In general, we always get a limit ordinal when taking the union of a nonempty set of ordinals that has no maximum element.
The ordinals of the form
Properties
editThe classes of successor ordinals and limit ordinals (of various cofinalities) as well as zero exhaust the entire class of ordinals, so these cases are often used in proofs by transfinite induction or definitions by transfinite recursion. Limit ordinals represent a sort of "turning point" in such procedures, in which one must use limiting operations such as taking the union over all preceding ordinals. In principle, one could do anything at limit ordinals, but taking the union is continuous in the order topology and this is usually desirable.
If we use the von Neumann cardinal assignment, every infinite cardinal number is also a limit ordinal (and this is a fitting observation, as cardinal derives from the Latin cardo meaning hinge or turning point): the proof of this fact is done by simply showing that every infinite successor ordinal is equinumerous to a limit ordinal via the Hotel Infinity argument.
Cardinal numbers have their own notion of successorship and limit (everything getting upgraded to a higher level).
Indecomposable ordinals
editAdditively indecomposable
A limit ordinal
Multiplicatively indecomposable
A limit ordinal
Exponentially indecomposable and beyond
The term "exponentially indecomposable" does not refer to ordinals not expressible as the exponential product (?) of
See also
editReferences
editFurther reading
edit- Cantor, G., (1897), Beitrage zur Begrundung der transfiniten Mengenlehre. II (tr.: Contributions to the Founding of the Theory of Transfinite Numbers II), Mathematische Annalen 49, 207-246 English translation.
- 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.
- Sierpiński, W. (1965). Cardinal and Ordinal Numbers (2nd ed.). Warszawa: Państwowe Wydawnictwo Naukowe. Also defines ordinal operations in terms of the Cantor Normal Form.