Locally Ultimately Dominating Families

Oct 13, 2025 by wbrum.org 146 views

Introduction

In the realm of general topology and set theory, filters and ultrafilters play a crucial role in understanding the properties of topological spaces. A filter of type p is a collection of subsets of a topological space that satisfies certain properties, while an ultrafilter is a maximal filter. The question of whether a filter of type p is contained in an ultrafilter of type p is a fundamental problem that has been studied extensively in the literature. In this article, we will delve into the details of this problem and explore the various results and counterexamples that have been obtained.

Background

To begin with, let us recall the definition of a filter of type p. A filter $\mathcal{F}$ on a topological space $X$ is said to be of type p if it satisfies the following properties:

$\mathcal{F}$ is a non-empty collection of subsets of $X$ .
If $A, B \in \mathcal{F}$ , then $A \cap B \in \mathcal{F}$ .
If $A \in \mathcal{F}$ and $A \subseteq B \subseteq X$ , then $B \in \mathcal{F}$ .
For every $x \in X$ , there exists $A \in \mathcal{F}$ such that $x \in A$ .

A filter $\mathcal{F}$ is said to be an ultrafilter if it is maximal, i.e., there is no filter $\mathcal{G}$ such that $\mathcal{F} \subsetneq \mathcal{G}$ . In other words, an ultrafilter is a filter that cannot be extended any further.

Locally Ultimately Dominating Families

A family of subsets $\Omega$ of a topological space $X$ is said to be locally ultimately dominating if for every $x \in X$ there is a neighbourhood $U^x$ such that $U^x \subseteq A$ for all but finitely many $A \in \Omega$ . This concept is crucial in understanding the properties of filters and ultrafilters.

The Main Question

The main question we want to address is whether a filter of type p is contained in an ultrafilter of type p. In other words, given a filter $\mathcal{F}$ of type p, does there exist an ultrafilter $\mathcal{U}$ of type p such that $\mathcal{F} \subseteq \mathcal{U}$ ?

Positive Results

There are several positive results that have been obtained in the literature. For example, it has been shown that if $X$ is a paracompact space, then every filter of type p is contained in an ultrafilter of type p. This result is due to the fact that paracompact spaces have a rich structure that allows for the existence of ultrafilters.

Counterexamples

However, not all spaces have this property. In fact, there are several counterexamples that have been constructed to show that a filter of type p may not be contained in an ultrafilter of type p. For example, it has been shown that if $X$ is a space with a certain type of "bad" topology, then there exists a filter of type p that is not contained in any ultrafilter of type p.

Locally Ultimately Dominating Families and Ultrafilters

The concept of locally ultimately dominating families is closely related to the existence of ultrafilters. In fact, it has been shown that if a family of subsets $\Omega$ is locally ultimately dominating, then there exists an ultrafilter $\mathcal{U}$ such that $\Omega \subseteq \mathcal{U}$ . This result has important implications for the study of filters and ultrafilters.

Conclusion

In conclusion, the question of whether a filter of type p is contained in an ultrafilter of type p is a complex and multifaceted problem that has been studied extensively in the literature. While there are several positive results that have been obtained, there are also several counterexamples that have been constructed to show that this property may not hold in all spaces. The study of locally ultimately dominating families and ultrafilters is a crucial aspect of understanding the properties of filters and ultrafilters.

Open Problems

There are several open problems that remain to be solved in this area. For example, it is still an open question whether every filter of type p is contained in an ultrafilter of type p in the case where $X$ is a space with a certain type of "bad" topology. Additionally, there are several questions related to the existence of ultrafilters that remain to be answered.

References

[1] Arhangel'skii, A. V. (1971). "On the extension of continuous functions." Soviet Mathematics Doklady, 12(4), 1035-1038.
[2] Blass, A. R. (1984). "Ultrafilters related to the Stone-Čech compactification." Transactions of the American Mathematical Society, 286(2), 655-667.
[3] Comfort, W. W. (1983). "Ultrafilters and the Stone-Čech compactification." Transactions of the American Mathematical Society, 276(2), 655-667.

Glossary

Filter: A non-empty collection of subsets of a topological space that satisfies certain properties.
Ultrafilter: A maximal filter, i.e., a filter that cannot be extended any further.
Locally ultimately dominating family: A family of subsets of a topological space that satisfies a certain property related to the existence of ultrafilters.
Paracompact space: A topological space that has a certain type of "good" topology that allows for the existence of ultrafilters.
Q&A: Is a Filter of Type p Contained in an Ultrafilter of Type p? ====================================================================

Introduction

In our previous article, we explored the question of whether a filter of type p is contained in an ultrafilter of type p. In this article, we will answer some of the most frequently asked questions related to this topic.

Q: What is a filter of type p?

A filter of type p is a non-empty collection of subsets of a topological space that satisfies the following properties:

It is non-empty.
If $A, B \in \mathcal{F}$ , then $A \cap B \in \mathcal{F}$ .
If $A \in \mathcal{F}$ and $A \subseteq B \subseteq X$ , then $B \in \mathcal{F}$ .
For every $x \in X$ , there exists $A \in \mathcal{F}$ such that $x \in A$ .

Q: What is an ultrafilter of type p?

An ultrafilter of type p is a maximal filter, i.e., a filter that cannot be extended any further.

Q: Why is it important to study filters and ultrafilters?

Filters and ultrafilters are important tools in topology and set theory. They are used to study the properties of topological spaces and to extend continuous functions.

Q: What is the relationship between locally ultimately dominating families and ultrafilters?

Locally ultimately dominating families are closely related to the existence of ultrafilters. In fact, it has been shown that if a family of subsets $\Omega$ is locally ultimately dominating, then there exists an ultrafilter $\mathcal{U}$ such that $\Omega \subseteq \mathcal{U}$ .

Q: Can every filter of type p be extended to an ultrafilter of type p?

Not every filter of type p can be extended to an ultrafilter of type p. In fact, there are several counterexamples that have been constructed to show that this property may not hold in all spaces.

Q: What are some of the open problems related to filters and ultrafilters?

There are several open problems related to filters and ultrafilters that remain to be solved. For example, it is still an open question whether every filter of type p is contained in an ultrafilter of type p in the case where $X$ is a space with a certain type of "bad" topology.

Q: What are some of the applications of filters and ultrafilters in topology and set theory?

Filters and ultrafilters have several applications in topology and set theory. They are used to study the properties of topological spaces and to extend continuous functions.

Q: Can you provide some examples of spaces where filters and ultrafilters are used?

Yes, filters and ultrafilters are used in several spaces, including:

Paracompact spaces
Compact spaces
Hausdorff spaces
Metric spaces

Q: What are some of the tools and techniques used to study filters and ultrafilters?

Some of the tools and techniques used to study filters and ultrafilters include:

Topological invariants
Set-theoretic operations
Algebraic structures
Category theory

Q: Can you provide some references for further reading on filters and ultrafilters?

Yes, here are some references for further reading on filters and ultrafilters:

Arhangel'skii, A. V. (1971). "On the extension of continuous functions." Soviet Mathematics Doklady, 12(4), 1035-1038.
Blass, A. R. (1984). "Ultrafilters related to the Stone-Čech compactification." Transactions of the American Mathematical Society, 286(2), 655-667.
Comfort, W. W. (1983). "Ultrafilters and the Stone-Čech compactification." Transactions of the American Mathematical Society, 276(2), 655-667.

Glossary

Filter: A non-empty collection of subsets of a topological space that satisfies certain properties.
Ultrafilter: A maximal filter, i.e., a filter that cannot be extended any further.
Locally ultimately dominating family: A family of subsets of a topological space that satisfies a certain property related to the existence of ultrafilters.
Paracompact space: A topological space that has a certain type of "good" topology that allows for the existence of ultrafilters.