# Manifold

This article defines a property of topological space that is pivotal (viz important) among currently studied properties of topological spaces

The article on this topic in the Differential Geometry Wiki can be found at: topological manifold

## Definition

A topological space is said to be a manifold if it satisfies all of the following conditions:

• It is Hausdorff: any two distinct points can be separated by disjoint open subsets.
• It is second-countable: it has a countable basis.
• It is locally Euclidean, viz., every point has a neighborhood that is homeomorphic to some open set in Euclidean space. If the topological space is connected, the dimension of the Euclidean space is the same for all points (we usually assume that the same Euclidean space is used for all points, viz., that the dimension is the same at all points)

If the dimension of the Euclidean space at each point is $m$, then we call the manifold a $m$-manifold.

## Significance of three parts of the definition

### Significance of local Euclideanness

Locally Euclidean is the most important property of manifolds, since this means that all the nice properties that we know about Euclidean spaces, are applicable locally. Thus, manifolds are locally contractible, locally path-connected, locally metrizable, and so on. Also, many properties of the manifold that are essentially local in nature can be proved using local Euclideanness.

### Significance of Hausdorffness

If we do not assume Hausdorffness, we get pathologies like the line with two origins.

More pertinently, the important way in which we use Hausdorffness is as follows: in a Hausdorff space, any compact subset is closed. Thus, in particular, the images of closed discs of Euclidean space, inside the manifold, continue to remain closed in the whole manifold. This is crucial to applying the gluing lemma for closed subsets, for proofs like those of the fact that any connected manifold is homogeneous or that the inclusion of any point in a manifold is a cofibration.

### Significance of second-countability

The assumption of second-countability can be dispensed with for a number of purposes, but is crucial for some applications. The standard example of something that is a manifold but for the second-countability assumption, is the long line.

## Relation with other properties

### Stronger properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
connected manifold manifold that is connected as a topological space; equivalently, it is path-connected |FULL LIST, MORE INFO
compact manifold manifold that is compact as a topological space |FULL LIST, MORE INFO
compact connected manifold manifold that is both compact and connected Connected manifold|FULL LIST, MORE INFO
differentiable manifold manifold that occurs as the underlying topological manifold of a differential manifold the E8 manifold is a 4-manifold that is not differentiable. For dimensions 1, 2, and 3, all manifolds are differentiable Triangulable manifold|FULL LIST, MORE INFO
triangulable manifold manifold that admits a triangulation, i.e., is homeomorphic to the geometric realization of a simplicial complex |FULL LIST, MORE INFO
manifold admitting a PL structure underlying topological manifold of a PL manifold Triangulable manifold|FULL LIST, MORE INFO
Lie group manifold that occurs as the underlying topological manifold of a Lie group |FULL LIST, MORE INFO

### Weaker properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
homology manifold locally compact space whose homology groups with respect to the exclusion of any point look like those of a manifold |FULL LIST, MORE INFO
manifold with boundary Hausdorff, second-countable, and every point is contained in an open subset that is homeomorphic to an open subset of Euclidean half-space |FULL LIST, MORE INFO
sub-Euclidean space homeomorphic to a closed subset of Euclidean space obvious counterexamples, such as a closed unit disk |FULL LIST, MORE INFO
metrizable space underlying topological space of a metric space |FULL LIST, MORE INFO
paracompact Hausdorff space paracompact and Hausdorff (via metrizable) Elastic space, Metrizable space|FULL LIST, MORE INFO
normal space any two disjoint closed subsets can be separated by disjoint open subsets Elastic space, Metrizable space, Monotonically normal space, Paracompact Hausdorff space, Protometrizable space|FULL LIST, MORE INFO
regular space any point and closed subset not containing it can be separated by disjoint open subsets Metrizable space, Monotonically normal space|FULL LIST, MORE INFO
Hausdorff space any two distinct points can be separated by disjoint open subsets Metrizable space, Monotonically normal space, Submetrizable space|FULL LIST, MORE INFO
locally Euclidean space every point is contained in an open subset that is homeomorphic to an open subset of Euclidean space |FULL LIST, MORE INFO
locally contractible space it has a basis of open subsets that are all contractible Locally Euclidean space|FULL LIST, MORE INFO
locally metrizable space it has a basis of open subsets that are all metrizable Locally Euclidean space|FULL LIST, MORE INFO
locally compact space every point is contained in an open subset whose closure is compact Homology manifold|FULL LIST, MORE INFO
nondegenerate space the inclusion of any point in it is a cofibration manifold implies nondegenerate |FULL LIST, MORE INFO
compactly nondegenerate space every point is contained in an open subset whose closure is compact, and the inclusion of the point in the closure is a cofibration. |FULL LIST, MORE INFO

### Potentially weaker properties

It is most likely true that every manifold is a CW-space, i.e., it can be given the structure of a CW-complex. However, the case of dimension four is still open. See for instance this Math Stack Exchange question.

### Incomparable properties

A topological space that occurs as the geometric realization of a simplicial complex is termed a polyhedron. Not every manifold is a polyhedron. For instance, the E8 manifold in 4 dimensions is not a polyhedron. Conversely, not every polyhedron is a manifold. For instance, the geometric realization of any graph with a vertex of degree more than two is not a manifold.

A manifold that is homeomorphic to the geometric realization of a simplicial complex is termed a triangulable manifold.

## Metaproperties

### Products

This property of topological spaces is closed under taking finite products

A direct product of manifolds is again a manifold. Fill this in later

### Covering spaces

This property of topological spaces is closed under passing to covering spaces; viz if a topological space has this property, so does any covering space of it

Any covering space of a manifold naturally gets the structure of a manifold.

### Fiber bundles

This property of topological spaces is a fiber bundle-closed property of topological spaces: it is closed under taking fiber bundles, viz if the base space and fiber both satisfy the given property, so does the total space.
Manifold, Orientable manifold

If $E$ is a fiber bundle with base space $B$ and fiber space $F$, and both $B$ and $F$ are manifolds, then $E$ is also a manifold. Note that this covers the particular cases of direct products and covering spaces.

## References

• Topology (2nd edition) by James R. MunkresMore info, Page 225, Chapter 4, Section 36 (formal definition, as definition of $m$-manifold, where $m$ is the dimension)