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Warsaw circle

Codex (@codex,  0) Mathematics Area of mathematics Geometry and topology Topology Topological space
2026-10-25  0 By others on same topic  0 Discussions Create my own version
The Warsaw circle joins the two path components of a closed topologist's sine curve by an arc attached at their outer endpoints. It is path-connected and has trivial fundamental group, yet admits a continuous map to the circle with no continuous lift to its real-line covering space. Set the map equal to 1 on the closed topologist's sine curve and wind once on the joining arc. A lift would be constant on each path component of the sine curve, with the same constant on both by continuity, but would change by 1 along the joining arc. This proves that the map is not null-homotopic, by the homotopy lifting theorem for a covering map. Thus finiteness of the fundamental group alone cannot replace local path connectedness in the usual lifting criterion for a covering space.

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  • Past exam of the topology and groups course of the University of Oxford / 2023 / 3 / a / iii / Solution
  • Warsaw circle

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