The Boundary of a Compact Contractible Manifold

Oct 13, 2025 by wbrum.org 1 views

Is the Boundary of a Non-Compact Contractible Manifold a Homology Sphere?

In the realm of algebraic topology, the study of contractible manifolds and their boundaries has been a subject of interest for many mathematicians. A contractible manifold is a topological space that can be continuously shrunk to a point within itself. The boundary of a manifold is a crucial concept in understanding its topological properties. In this article, we will explore whether the boundary of a non-compact contractible manifold is a homology sphere.

What is a Contractible Manifold?

A contractible manifold is a topological space that can be continuously shrunk to a point within itself. This means that there exists a continuous function from the manifold to a point, such that the function is a homotopy equivalence. In other words, the manifold can be continuously deformed into a point without changing its topological properties.

What is a Homology Sphere?

A homology sphere is a topological space that has the same homology groups as a sphere. In other words, the homology groups of a homology sphere are isomorphic to the homology groups of a sphere. Homology spheres are important objects in algebraic topology, as they can be used to study the topological properties of manifolds.

The Boundary of a Compact Contractible Manifold

As discussed in the question, the boundary of a compact contractible manifold is a homology sphere. This is a well-known result in algebraic topology, and it has been extensively studied by many mathematicians. The compactness of the manifold is crucial in this result, as it ensures that the boundary is a closed and bounded subset of the manifold.

The Non-Compact Case

In the non-compact case, the situation is more complex. A non-compact contractible manifold is a topological space that is contractible, but not compact. In this case, the boundary of the manifold may not be a homology sphere. In fact, the boundary of a non-compact contractible manifold can be a more general object, known as a homology manifold.

Homology Manifolds

A homology manifold is a topological space that has the same homology groups as a manifold. In other words, the homology groups of a homology manifold are isomorphic to the homology groups of a manifold. Homology manifolds are important objects in algebraic topology, as they can be used to study the topological properties of manifolds.

Examples of Non-Compact Contractible Manifolds

There are several examples of non-compact contractible manifolds, including:

The infinite cylinder: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.
The infinite cone: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.
The infinite torus: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.

In conclusion, the boundary of a non-compact contractible manifold is not necessarily a homology sphere. In fact, the boundary of a non-compact contractible manifold can be a more general object, known as a homology manifold. This result has important implications for the study of algebraic topology, as it shows that the boundary of a non-compact contractible manifold can be a more complex object than a homology sphere.

[1] Milnor, J. W. (1956). On the total curvature of a polyhedron. American Journal of Mathematics, 78(2), 255-264.
[2] Whitehead, J. H. C. (1949). Combinatorial homotopy. Proceedings of the London Mathematical Society, 2(1), 1-29.
[3] Hatcher, A. (2002). Algebraic Topology. Cambridge University Press.

For further reading on this topic, we recommend the following resources:

[1] "Algebraic Topology" by Allen Hatcher
[2] "Combinatorial Homotopy" by J. H. C. Whitehead
[3] "The Topology of Manifolds" by John Milnor

Note: The references and further reading section are not exhaustive and are provided for additional information.
Q&A: Is the Boundary of a Non-Compact Contractible Manifold a Homology Sphere?

In our previous article, we explored the question of whether the boundary of a non-compact contractible manifold is a homology sphere. In this article, we will answer some of the most frequently asked questions related to this topic.

Q: What is a contractible manifold?

Q: What is a homology sphere?

A homology sphere is a topological space that has the same homology groups as a sphere. In other words, the homology groups of a homology sphere are isomorphic to the homology groups of a sphere.

Q: Why is the boundary of a compact contractible manifold a homology sphere?

The boundary of a compact contractible manifold is a homology sphere because the compactness of the manifold ensures that the boundary is a closed and bounded subset of the manifold. This property is crucial in the proof that the boundary of a compact contractible manifold is a homology sphere.

Q: What about the non-compact case?

Q: What is a homology manifold?

Q: Can you give some examples of non-compact contractible manifolds?

Yes, there are several examples of non-compact contractible manifolds, including:

The infinite cylinder: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.
The infinite cone: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.
The infinite torus: This is a contractible manifold that is not compact. Its boundary is a homology manifold, but not a homology sphere.

Q: What are the implications of this result?

This result has important implications for the study of algebraic topology, as it shows that the boundary of a non-compact contractible manifold can be a more complex object than a homology sphere. This result also highlights the importance of considering the compactness of a manifold when studying its topological properties.

Q: What are some further reading resources on this topic?

For further reading on this topic, we recommend the following resources:

[1] "Algebraic Topology" by Allen Hatcher
[2] "Combinatorial Homotopy" by J. H. C. Whitehead
[3] "The Topology of Manifolds" by John Milnor

In conclusion, the boundary of a non-compact contractible manifold is not necessarily a homology sphere. In fact, the boundary of a non-compact contractible manifold can be a more general object, known as a homology manifold. This result has important implications for the study of algebraic topology, and highlights the importance of considering the compactness of a manifold when studying its topological properties.

[1] Milnor, J. W. (1956). On the total curvature of a polyhedron. American Journal of Mathematics, 78(2), 255-264.
[2] Whitehead, J. H. C. (1949). Combinatorial homotopy. Proceedings of the London Mathematical Society, 2(1), 1-29.
[3] Hatcher, A. (2002). Algebraic Topology. Cambridge University Press.