This page is being constructed. It will cover issues to do with cofinality, and some applications of König's inequality on cardinals.
We assume the Axiom of Choice throughout and identify cardinals with initial ordinals. Arithmetic, such as addition, multiplication and exponentiation is always cardinal arithmetic unless stated otherwise.
A cardinal is a successor cardinal if it is for a successor ordinal . It is a limit cardinal if it is for a limit ordinal .
A cardinal is said to be regular if it is not the sum of a collection of strictly smaller cardinals, where runs over an index set with . A cardinal is singular if it is not regular.
It is easy to see from that if and then . Thus we derive the important result that
Every succesor cardinal is regular.
The cofinality of a cardinal or initial ordinal is the least ordinal such that there are sets for all with each having cardinality strictly less than and with .
Thus a regular cardinal is one, , such that . A singular cardinal has .
The following is easy.
The cofinality of a cardinal is an initial ordinal (i.e. a cardinal).
The next theorem is more interesting.
If is a cardinal then .
Suppose cardinals are given for such that . Then by the König inequality
If and are cardinals then .
Let . If then by the previous theorem
In particular so for example .
We saw that every succesor cardinal is regular. Whether every limit cardinal is singular is more difficult. A regular limit cardinal is said to be (weakly) inaccessible. It is strongly inaccessible if in addition implies for all cardinals .
The non provability of the existence of strongly inaccessible cardinals is rather easier to show.
If is a strongly inaccessible cardinal then is a model of .
ZFC cannot prove the existence of a strongly inaccessible cardinal.
Otherwise ZFC would prove the existence of a model of ZFC which by the Soundness Theorem (formalised and proved inside ZFC) would imply that contradicting Gödel's second incompleteness theorem.
If ZFC is consistent so is ZFC plus the assertion that there is no strongly inaccessible cardinal.
If is a model of ZFC then take the smallest strongly inaccessible cardinal in and observe that satisfies the required theory, so this theory is consistent by soundness.
It turns out that by a construction using Gödel's constructible universe , similar results apply to weakly inaccessible cardinals too.