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Type of topological space
In topology, a branch of mathematics, a topological manifold is a topological space that locally resembles real n dimensional Euclidean space. Topological manifolds are an important class of topological spaces, with applications throughout mathematics. All manifolds are topological manifolds by definition. Other types of manifolds are formed by adding structure to a topological manifold (e. g. differentiable manifolds are topological manifolds equipped with a differential structure). Every manifold has an "underlying" topological manifold, obtained by simply "forgetting" the added structure. However, not every topological manifold can be endowed with a particular additional structure. For example, the E8 manifold is a topological manifold which cannot be endowed with a differentiable structure.

A topological space X is called locally Euclidean if there is a non negative integer n such that every point in X has a neighborhood which is homeomorphic to real n space Rn. A topological manifold is a locally Euclidean Hausdorff space. It is common to place additional requirements on topological manifolds. In particular, many authors define them to be paracompact or second countable. Assume such a space X is σ compact. Then it is Lindelöf, and because Lindelöf + regular implies paracompact, X is metrizable. But in a metrizable space, second countability coincides with being Lindelöf, so X is second countable. Conversely, if X is a Hausdorff second countable manifold, it must be σ compact. A manifold need not be connected, but every manifold M is a disjoint union of connected manifolds. These are just the connected components of M, which are open sets since manifolds are locally connected. Being locally path connected, a manifold is path connected if and only if it is connected. It follows that the path components are the same as the components.

The Hausdorff property is not a local one; so even though Euclidean space is Hausdorff, a locally Euclidean space need not be. It is true, however, that every locally Euclidean space is T1. An example of a non Hausdorff locally Euclidean space is the line with two origins. This space is created by replacing the origin of the real line with two points, an open neighborhood of either of which includes all nonzero numbers in some open interval centered at zero. This space is not Hausdorff because the two origins cannot be separated.

A manifold is metrizable if and only if it is paracompact. The long line is an example a normal Hausdorff 1 dimensional topological manifold that is not metrizable nor paracompact. Since metrizability is such a desirable property for a topological space, it is common to add paracompactness to the definition of a manifold. In any case, non paracompact manifolds are generally regarded as pathological. An example of a non paracompact manifold is given by the long line. Paracompact manifolds have all the topological properties of metric spaces. In particular, they are perfectly normal Hausdorff spaces. Manifolds are also commonly required to be second countable. This is precisely the condition required to ensure that the manifold embeds in some finite dimensional Euclidean space. For any manifold the properties of being second countable, Lindelöf, and σ compact are all equivalent. Every second countable manifold is paracompact, but not vice versa. However, the converse is nearly true: a paracompact manifold is second countable if and only if it has a countable number of connected components. In particular, a connected manifold is paracompact if and only if it is second countable. Every second countable manifold is separable and paracompact. Moreover, if a manifold is separable and paracompact then it is also second countable. Every compact manifold is second countable and paracompact.

By invariance of domain, a non empty n manifold cannot be an m manifold for n ≠ m. The dimension of a non empty n manifold is n. Being an n manifold is a topological property, meaning that any topological space homeomorphic to an n manifold is also an n manifold.

The full classification of n manifolds for n greater than three is known to be impossible; it is at least as hard as the word problem in group theory, which is known to be algorithmically undecidable. In fact, there is no algorithm for deciding whether a given manifold is simply connected. There is, however, a classification of simply connected manifolds of dimension ≥ 5.

A slightly more general concept is sometimes useful. A topological manifold with boundary is a Hausdorff space in which every point has a neighborhood homeomorphic to an open subset of Euclidean half space (for a fixed n):

Every topological manifold is a topological manifold with boundary, but not vice versa.

The disjoint union of a countable family of n manifolds is a n manifold (the pieces must all have the same dimension).

The connected sum of two n manifolds is defined by removing an open ball from each manifold and taking the quotient of the disjoint union of the resulting manifolds with boundary, with the quotient taken with regards to a homeomorphism between the boundary spheres of the removed balls. This results in another n manifold.

Any open subset of an n manifold is an n manifold with the subspace topology.