The Role of Continuity in Proper Mappings

Oct 13, 2025 by wbrum.org 66 views

Introduction

Proper mappings, also known as proper maps, are a fundamental concept in topology and geometry. They play a crucial role in various areas of mathematics, including algebraic topology, differential geometry, and geometric measure theory. In this article, we will explore the relationship between proper mappings and continuity in the domain. Specifically, we will investigate whether proper mappings assume certain amounts of continuity in the domain.

What are Proper Mappings?

Proper mappings are defined as continuous maps between topological spaces that have the property that every compact set in the domain has a compact preimage. In other words, if $f: X \to Y$ is a proper map, then for every compact set $K \subseteq X$ , the preimage $f^{-1}(K)$ is also compact in $X$ . This definition is crucial in understanding the behavior of proper mappings and their relationship with continuity.

The Role of Continuity in Proper Mappings

Continuity is a fundamental concept in topology and analysis. It measures how well a function preserves the topological properties of its domain. In the context of proper mappings, continuity plays a crucial role in ensuring that the preimage of a compact set is also compact. This is because compact sets are closed and bounded, and continuity ensures that the preimage of a closed set is also closed.

Do Proper Mappings Assume Certain Amounts of Continuity in the Domain?

The question of whether proper mappings assume certain amounts of continuity in the domain is a complex one. Intuitively, one might expect that proper mappings require a certain level of continuity in the domain to ensure that the preimage of a compact set is also compact. However, the relationship between proper mappings and continuity is more subtle than this.

A Counterexample

To illustrate the subtlety of this relationship, consider the following counterexample. Let $X = \mathbb{R}$ and $Y = \mathbb{R}^2$ . Define a map $f: X \to Y$ by $f(x) = (x, 0)$ . This map is proper because every compact set in $X$ has a compact preimage in $Y$ . However, the map $f$ is not continuous at $x = 0$ because the preimage of the compact set $\{(0, 0)\}$ is not compact in $X$ .

A Theoretical Framework

To better understand the relationship between proper mappings and continuity, we need a theoretical framework that can capture the subtleties of this relationship. One such framework is the theory of metric spaces. In this framework, we can define a proper map as a map between metric spaces that has the property that every compact set in the domain has a compact preimage.

Theoretical Results

Using the theory of metric spaces, we can establish some theoretical results that shed light on the relationship between proper mappings and continuity. For example, we can show that if $f: X \to Y$ is a proper map between metric spaces, then $f$ is continuous at every point in $X$ where the preimage of every compact set is compact.

Conclusion

In conclusion, the relationship between proper mappings and continuity in the domain is more subtle than one might expect. While proper mappings do require a certain level of continuity in the domain, this continuity is not necessarily uniform across the entire domain. In fact, there are counterexamples that illustrate the subtlety of this relationship. To better understand this relationship, we need a theoretical framework that can capture the subtleties of proper mappings and continuity.

References

[1] Hurewicz, W., and W. Wallman. Dimension Theory. Princeton University Press, 1941.
[2] Kelley, J. L. General Topology. Springer-Verlag, 1955.
[3] Munkres, J. R. Topology. Prentice Hall, 2000.

Glossary

Proper mapping: A continuous map between topological spaces that has the property that every compact set in the domain has a compact preimage.
Compact set: A closed and bounded set in a metric space.
Continuity: A property of a function that measures how well it preserves the topological properties of its domain.
Metric space: A topological space equipped with a metric that measures the distance between points.
Q&A: Proper Mappings and Continuity =====================================

Introduction

In our previous article, we explored the relationship between proper mappings and continuity in the domain. We discussed the definition of proper mappings, the role of continuity in ensuring that the preimage of a compact set is also compact, and a counterexample that illustrates the subtlety of this relationship. In this article, we will answer some frequently asked questions about proper mappings and continuity.

Q: What is the difference between a proper map and a continuous map?

A: A proper map is a continuous map between topological spaces that has the property that every compact set in the domain has a compact preimage. A continuous map, on the other hand, is a map between topological spaces that preserves the topological properties of its domain. While all proper maps are continuous, not all continuous maps are proper.

Q: Can a proper map be discontinuous?

A: Yes, a proper map can be discontinuous. In fact, the counterexample we discussed earlier shows that a proper map can be discontinuous at a single point.

Q: What is the relationship between proper mappings and compactness?

A: Proper mappings are closely related to compactness. In fact, a map between topological spaces is proper if and only if it maps compact sets to compact sets.

Q: Can a proper map be surjective?

A: Yes, a proper map can be surjective. In fact, a proper map between topological spaces is surjective if and only if the domain and codomain are compact.

Q: What is the significance of proper mappings in topology and geometry?

A: Proper mappings play a crucial role in topology and geometry. They are used to study the properties of topological spaces, such as compactness and connectedness. They are also used to study the properties of geometric objects, such as manifolds and varieties.

Q: Can proper mappings be used to study the properties of metric spaces?

A: Yes, proper mappings can be used to study the properties of metric spaces. In fact, the theory of metric spaces provides a framework for studying the properties of proper mappings.

Q: What are some applications of proper mappings in mathematics and physics?

A: Proper mappings have numerous applications in mathematics and physics. They are used to study the properties of topological spaces, such as compactness and connectedness. They are also used to study the properties of geometric objects, such as manifolds and varieties. In physics, proper mappings are used to study the properties of spacetime and the behavior of particles and fields.

Q: Can proper mappings be used to study the properties of dynamical systems?

A: Yes, proper mappings can be used to study the properties of dynamical systems. In fact, the theory of dynamical systems provides a framework for studying the properties of proper mappings.

Q: What are some open problems in the study of proper mappings?

A: There are several open problems in the study of proper mappings. Some of these problems include:

The study of the properties of proper mappings between non-compact spaces
The study of the properties of proper mappings between spaces with non-trivial topology
The study of the properties of proper mappings between spaces with non-trivial geometry

Conclusion

In conclusion, proper mappings are a fundamental concept in topology and geometry. They play a crucial role in studying the properties of topological spaces and geometric objects. While they are closely related to compactness and continuity, they can also be discontinuous and surjective. Proper mappings have numerous applications in mathematics and physics, and there are several open problems in the study of proper mappings.

References

[1] Hurewicz, W., and W. Wallman. Dimension Theory. Princeton University Press, 1941.
[2] Kelley, J. L. General Topology. Springer-Verlag, 1955.
[3] Munkres, J. R. Topology. Prentice Hall, 2000.

Glossary

Proper mapping: A continuous map between topological spaces that has the property that every compact set in the domain has a compact preimage.
Compact set: A closed and bounded set in a metric space.
Continuity: A property of a function that measures how well it preserves the topological properties of its domain.
Metric space: A topological space equipped with a metric that measures the distance between points.