Editing Poincaré conjecture (section)

== Overview ==
{{Dark mode invert|[[File:Torus cycles.svg|thumb|upright|Neither of the two colored loops on this [[torus]] can be continuously tightened to a point. A torus is not homeomorphic to a sphere.]]}}

The Poincaré conjecture was a mathematical problem in the field of [[geometric topology]]. In terms of the vocabulary of that field, it says the following:
<blockquote>'''Poincaré conjecture'''.{{br}}Every three-dimensional [[topological manifold]] which is [[closed manifold|closed]], [[connected space|connected]], and [[simply connected|has trivial fundamental group]] is [[homeomorphic]] to the [[3-sphere|three-dimensional sphere]].</blockquote>
Familiar shapes, such as the surface of a ball (which is known in mathematics as the ''two''-dimensional sphere) or of a [[torus]], are two-dimensional. The surface of a ball has trivial fundamental group, meaning that any loop drawn on the surface can be continuously deformed to a single point. By contrast, the surface of a torus has nontrivial fundamental group, as there are loops on the surface which cannot be so deformed. Both are topological manifolds which are closed (meaning that they have no boundary and take up a finite region of space) and connected (meaning that they consist of a single piece). Two closed manifolds are said to be homeomorphic when it is possible for the points of one to be reallocated to the other in a continuous way. Because the (non)triviality of the fundamental group is known to be invariant under homeomorphism, it follows that the two-dimensional sphere and torus are not homeomorphic.

The two-dimensional analogue of the Poincaré conjecture says that any two-dimensional topological manifold which is closed and connected but non-homeomorphic to the two-dimensional sphere must possess a loop which cannot be continuously contracted to a point. (This is illustrated by the example of the torus, as above.) This analogue is known to be true via the classification of closed and connected two-dimensional topological manifolds, which was understood in various forms since the 1860s. In higher dimensions, the closed and connected topological manifolds do not have a straightforward classification, precluding an easy resolution of the Poincaré conjecture.