Regular cardinal

Regular cardinal

In set theory, a regular cardinal is a cardinal number that is equal to its own cofinality. So, crudely speaking, a regular cardinal is one which cannot be broken into a smaller collection of smaller parts.

(The situation is slightly more complicated in contexts where the axiom of choice might fail; in that case not all cardinals are necessarily the cardinalities of well-ordered sets. In that case, the above definition is restricted to well-orderable cardinals only.) If the axiom of choice holds (so that any cardinal number can be well-ordered), an infinite cardinal κ is regular if and only if it cannot be expressed as the cardinal sum of a set of cardinality less than κ, the elements of which are cardinals less than κ.

An infinite ordinal α is regular if and only if it is a limit ordinal which is not the limit of a set of smaller ordinals which set has order type less than α. A regular ordinal is always an initial ordinal, though some initial ordinals are not regular.

Infinite well-ordered cardinals which are not regular are called singular cardinals. Finite cardinal numbers are typically not called regular or singular.

Examples

The ordinals less than omega are finite. A finite sequence of finite ordinals always has a finite maximum, so omega cannot be the limit of any sequence of type less than omega whose elements are ordinals less than omega, and is therefore a regular ordinal. aleph_0 (aleph-null) is a regular cardinal because its initial ordinal, omega, is regular. It can also be seen directly to be regular, as the cardinal sum of a finite number of finite cardinal numbers is itself finite.

omega+1 is the next ordinal number greater than omega. It is singular, since it is not a limit ordinal. omega+omega is the next limit ordinal after omega. It can be written as the limit of the sequence omega, omega+1, omega+2, omega+3, and so on. This sequence has order type omega, so omega+omega is the limit of a sequence of type less than omega+omega whose elements are ordinals less than omega+omega, therefore it is singular.

aleph_1 is the next cardinal number greater than aleph_0, so the cardinals less than aleph_1 are countable (finite or denumerable). Assuming the axiom of choice, the union of a countable set of countable sets is itself countable. So aleph_1 cannot be written as the sum of a countable set of countable cardinal numbers, and is regular.

aleph_omega is the next cardinal number after the sequence aleph_0, aleph_1, aleph_2, aleph_3, and so on. Its initial ordinal omega_omega is the limit of the sequence omega, omega_1, omega_2, omega_3, and so on, which has order type omega, so omega_omega is singular, and so is aleph_omega. Assuming the axiom of choice, aleph_omega is the first infinite cardinal which is singular (the first infinite "ordinal" which is singular is omega+1). Proving the existence of singular cardinals requires the axiom of replacement, and in fact the inability to prove the existence of aleph_omega in Zermelo set theory is what led Fraenkel to postulate this axiom.

Properties

Uncountable limit cardinals that are also regular are known as weakly inaccessible cardinals. They cannot be proved to exist within ZFC, though their existence is not known to be inconsistent with ZFC. Their existence is sometimes taken as an additional axiom. Inaccessible cardinals are necessarily fixed points of the aleph function, though not all fixed points are regular. For instance, the first fixed point is the limit of the ω-sequence aleph_0, aleph_{aleph_0}, aleph_{aleph_{aleph_0, ... and is therefore singular.

If the axiom of choice holds, then every successor cardinal is regular. Thus the regularity or singularity of most aleph numbers can be checked depending on whether the cardinal is a successor cardinal or a limit cardinal. Some cardinal numbers cannot be proven to be equal to any particular aleph, for instance the cardinality of the continuum, whose value in ZFC may be any uncountable cardinal of uncountable cofinality. The continuum hypothesis postulates that the cardinality of the continuum is equal to aleph_1 which is regular.

Without the axiom of choice, there would be cardinal numbers which were not well-orderable. Moreover, the cardinal sum of an arbitrary collection could not be defined. Therefore only the aleph numbers can meaningfully be called regular or singular cardinals. Furthermore, a successor aleph need not be regular. For instance, the union of a countable set of countable sets need not be countable. It is consistent with ZF that omega_1 be the limit of a countable sequence of countable ordinals as well as the set of real numbers is countable union of countable sets. Indeed, the existence of any regular aleph other than aleph_0 is not provable without the axiom of choice.

See also

* Inaccessible cardinal

References

* "Elements of Set Theory", by Herbert B. Enderton, ISBN 0-12-238440-7
* "Set Theory, An Introduction to Independence Proofs", by Kenneth Kunen, ISBN 0-444-85401-0


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