In mathematics, a 3-manifold is a topological space that locally looks like a three-dimensional Euclidean space. A 3-manifold can be thought of as a possible shape of the universe. Just as a sphere looks like a plane (a tangent plane) to a small and close enough observer, all 3-manifolds look like our universe does to a small enough observer. This is made more precise in the definition below.

Principles

Definition

A topological space ${\displaystyle M}$ is a 3-manifold if it is a second-countable Hausdorff space and if every point in ${\displaystyle M}$ has a neighbourhood that is homeomorphic to Euclidean 3-space.

Mathematical theory of 3-manifolds

The topological, piecewise-linear, and smooth categories are all equivalent in three dimensions, so little distinction is made in whether we are dealing with say, topological 3-manifolds, or smooth 3-manifolds.

Phenomena in three dimensions can be strikingly different from phenomena in other dimensions, and so there is a prevalence of very specialized techniques that do not generalize to dimensions greater than three. This special role has led to the discovery of close connections to a diversity of other fields, such as knot theory, geometric group theory, hyperbolic geometry, number theory, Teichmüller theory, topological quantum field theory, gauge theory, Floer homology, and partial differential equations. 3-manifold theory is considered a part of low-dimensional topology or geometric topology.

A key idea in the theory is to study a 3-manifold by considering special surfaces embedded in it. One can choose the surface to be nicely placed in the 3-manifold, which leads to the idea of an incompressible surface and the theory of Haken manifolds, or one can choose the complementary pieces to be as nice as possible, leading to structures such as Heegaard splittings, which are useful even in the non-Haken case.

Thurston's contributions to the theory allow one to also consider, in many cases, the additional structure given by a particular Thurston model geometry (of which there are eight). The most prevalent geometry is hyperbolic geometry. Using a geometry in addition to special surfaces is often fruitful.

The fundamental groups of 3-manifolds strongly reflect the geometric and topological information belonging to a 3-manifold. Thus, there is an interplay between group theory and topological methods.

Invariants describing 3-manifolds

3-manifolds are an interesting special case of low-dimensional topology because their topological invariants give a lot of information about their structure in general. If we let ${\displaystyle M}$ be a 3-manifold and ${\displaystyle \pi =\pi _{1}(M)}$ be its fundamental group, then a lot of information can be derived from them. For example, using Poincare duality and the Hurewicz theorem, we have the following homology groups:

${\displaystyle {\begin{aligned}H_{0}(M)&=H^{3}(M)=&\mathbb {Z} \\H_{1}(M)&=H^{2}(M)=&\pi /\\H_{2}(M)&=H^{1}(M)=&{\text{Hom}}(\pi ,\mathbb {Z} )\\H_{3}(M)&=H^{0}(M)=&\mathbb {Z} \end{aligned}}}$

where the last two groups are isomorphic to the group homology and cohomology of ${\displaystyle \pi }$, respectively; that is,

${\displaystyle {\begin{aligned}H_{1}(\pi ;\mathbb {Z} )&\cong \pi /\\H^{1}(\pi ;\mathbb {Z} )&\cong {\text{Hom}}(\pi ,\mathbb {Z} )\end{aligned}}}$

From this information a basic homotopy theoretic classification of 3-manifolds^[1] can be found. Note from the Postnikov tower there is a canonical map

${\displaystyle q:M\to B\pi }$

If we take the pushforward of the fundamental class ${\displaystyle \in H_{3}(M)}$ into ${\displaystyle H_3(}$Zdroj:https://en.wikipedia.org?pojem=3-manifold
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