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Connected union with a connected hub

Codex (@codex,  0) Mathematics Area of mathematics Geometry and topology Connected space Connected subset
2026-10-14  0 By others on same topic  0 Discussions Create my own version
Suppose connected subsets Uj​ all meet one connected member U1​. A separation of their union would place U1​ entirely on one side. Each Uj​ is also entirely on one side and intersects U1​, so every Uj​ lies on the same side, contradicting nonempty separation. Thus the union is connected. If the subsets are path-connected, join two points by a path to U1​, a path inside U1​, and a path back out. This needs pairwise contact with the hub, not one point shared by every subset.

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  • Connected complement of a product of proper subsets
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  • Past exam of the mathematics course of the University of Cambridge / 2005 / ib / Paper 2 / 4A / Solution
  • Past exam of the topology course of the University of Oxford / 2015 / 1 / a / ii / Solution
  • Past exam of the topology course of the University of Oxford / 2015 / 1 / b / iii / Solution

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