Ultrafilters and Large Cardinals

Oct 13, 2025 by wbrum.org 34 views

Introduction

In the realm of set theory, ultrafilters play a crucial role in the study of large cardinals and their properties. A coherent system of ultrafilters is a collection of ultrafilters on a regular cardinal $\kappa$ that satisfy certain properties. In this article, we will delve into the consistency strength of a coherent system of ultrafilters, exploring the implications of such a system on the cardinality of $\kappa$ .

Ultrafilters and Large Cardinals

Before we dive into the specifics of a coherent system of ultrafilters, let's briefly discuss the concept of ultrafilters and their relationship with large cardinals.

Ultrafilters

An ultrafilter on a set $X$ is a collection $\mathcal{U}$ of subsets of $X$ that satisfies the following properties:

$\emptyset \notin \mathcal{U}$
If $A \in \mathcal{U}$ and $A \subseteq B \subseteq X$ , then $B \in \mathcal{U}$
If $A, B \in \mathcal{U}$ , then $A \cap B \in \mathcal{U}$
For every $A \subseteq X$ , either $A \in \mathcal{U}$ or $X \setminus A \in \mathcal{U}$

Ultrafilters are used to define various large cardinal properties, such as the existence of measurable cardinals and strong cardinals.

Large Cardinals

Large cardinals are cardinals that possess certain properties that are not provable in ZFC (Zermelo-Fraenkel set theory with the axiom of choice). Some examples of large cardinals include:

Measurable cardinals: A cardinal $\kappa$ is measurable if there exists a $\kappa$ -complete non-principal ultrafilter on $\kappa$ .
Strong cardinals: A cardinal $\kappa$ is strong if there exists a cardinal $\lambda$ such that for any set $A \subseteq \lambda$ , there exists a $\kappa$ -complete non-principal ultrafilter on $\lambda$ that is generic over $A$ .

Coherent Systems of Ultrafilters

A coherent system of ultrafilters on a regular cardinal $\kappa$ is a collection $\mathcal{U}$ of ultrafilters on $\kappa$ that satisfy the following properties:

$\mathcal{U}$ is a filter on $\kappa$
For every $A \subseteq \kappa$ , either $A \in \mathcal{U}$ or $\kappa \setminus A \in \mathcal{U}$
For every $A \subseteq \kappa$ , if $A \in \mathcal{U}$ , then the set of accumulation points of $A$ is also in $\mathcal{U}$

Properties of Coherent Systems

Coherent systems of ultrafilters have several interesting properties, including:

Club property: If $C \subseteq \kappa$ is a club, then the set of accumulation points of $C$ is also a club.
$\kappa$ -completeness: For every $A \subseteq \kappa$ , if $A \in \mathcal{U}$ , then the set of accumulation points of $A$ is also in $\mathcal{U}$ .

Consistency Strength of Coherent Systems

The consistency strength of a coherent system of ultrafilters is a measure of the strength of the axioms required to prove the existence of such a system. In this section, we will explore the implications of a coherent system of ultrafilters on the cardinality of $\kappa$ .

Implications for $\kappa$

A coherent system of ultrafilters on $\kappa$ has several implications for the cardinality of $\kappa$ :

$\kappa$ is a strong limit: If $\kappa$ is a coherent system of ultrafilters, then $\kappa$ is a strong limit cardinal.
$\kappa$ is a measurable cardinal: If $\kappa$ is a coherent system of ultrafilters, then $\kappa$ is a measurable cardinal.

Consistency Strength

The consistency strength of a coherent system of ultrafilters is a measure of the strength of the axioms required to prove the existence of such a system. In particular, the consistency strength of a coherent system of ultrafilters on $\kappa$ is at least $\kappa^{++}$ .

Conclusion

In conclusion, a coherent system of ultrafilters on a regular cardinal $\kappa$ is a collection of ultrafilters that satisfy certain properties. The consistency strength of such a system is a measure of the strength of the axioms required to prove the existence of such a system. In this article, we have explored the implications of a coherent system of ultrafilters on the cardinality of $\kappa$ and the consistency strength of such a system.

References

Jech, T. (2003). Set theory. Springer-Verlag.
Kanamori, A. (2009). The higher infinite: Large cardinals in set theory from their beginnings. Springer-Verlag.
Shelah, S. (1998). Cardinal arithmetic. Oxford University Press.

Frequently Asked Questions

In this article, we will address some of the most frequently asked questions about coherent systems of ultrafilters.

Q: What is a coherent system of ultrafilters?

A: A coherent system of ultrafilters on a regular cardinal $\kappa$ is a collection $\mathcal{U}$ of ultrafilters on $\kappa$ that satisfy the following properties:

$\mathcal{U}$ is a filter on $\kappa$
For every $A \subseteq \kappa$ , either $A \in \mathcal{U}$ or $\kappa \setminus A \in \mathcal{U}$
For every $A \subseteq \kappa$ , if $A \in \mathcal{U}$ , then the set of accumulation points of $A$ is also in $\mathcal{U}$

Q: What are the implications of a coherent system of ultrafilters on the cardinality of $\kappa$ ?

A: A coherent system of ultrafilters on $\kappa$ has several implications for the cardinality of $\kappa$ :

$\kappa$ is a strong limit cardinal
$\kappa$ is a measurable cardinal

Q: What is the consistency strength of a coherent system of ultrafilters?

A: The consistency strength of a coherent system of ultrafilters on $\kappa$ is at least $\kappa^{++}$ .

Q: Can a coherent system of ultrafilters exist on a cardinal that is not regular?

A: No, a coherent system of ultrafilters cannot exist on a cardinal that is not regular.

Q: Are there any known examples of coherent systems of ultrafilters?

A: Yes, there are several known examples of coherent systems of ultrafilters. For example, the club filter on $\kappa$ is a coherent system of ultrafilters.

Q: What are the applications of coherent systems of ultrafilters in set theory?

A: Coherent systems of ultrafilters have several applications in set theory, including:

Large cardinal theory: Coherent systems of ultrafilters are used to define large cardinal properties, such as the existence of measurable cardinals and strong cardinals.
Forcing theory: Coherent systems of ultrafilters are used in forcing theory to construct models of set theory with certain properties.

Q: Is there a relationship between coherent systems of ultrafilters and other areas of mathematics?

A: Yes, there is a relationship between coherent systems of ultrafilters and other areas of mathematics, including:

Topology: Coherent systems of ultrafilters are related to the study of topological spaces and their properties.
Measure theory: Coherent systems of ultrafilters are related to the study of measures on sets and their properties.

Conclusion

In conclusion, coherent systems of ultrafilters are a fascinating area of study in set theory, with several implications for the cardinality of $\kappa$ and the consistency strength of such a system. We hope that this Q&A article has provided a helpful introduction to the topic and has sparked further interest in the study of coherent systems of ultrafilters.

References

Jech, T. (2003). Set theory. Springer-Verlag.
Kanamori, A. (2009). The higher infinite: Large cardinals in set theory from their beginnings. Springer-Verlag.
Shelah, S. (1998). Cardinal arithmetic. Oxford University Press.