Hawaiian earring
Builds on CircleCompact spacePath-connected space
Example
For let be the circle with centre and radius . The Hawaiian earring is with the subspace topology from . All circles pass through the origin . The space is compact and path-connected, but it is not semilocally simply connected at .
Check. is closed and bounded in , so it is compact. Each is path-connected and contains , so is path-connected. Fix and let be the identity on and send every other circle to . This map is continuous, and it is a retraction onto . Let be the loop that goes once around . The map sends it to a generator of the fundamental group , so is not null-homotopic in . Every neighbourhood of contains for all large , so it contains loops at that are not null-homotopic in .
Remarks
Since is not semilocally simply connected, it has no universal covering space. The retractions together give a homomorphism from onto the product . It is onto: a loop can run times around during the time interval . This loop is continuous at time because the circles shrink. So is uncountable. The wedge sum of countably many circles, with the CW topology, has a countable free fundamental group and is not homeomorphic to .
Reading path
To understand this page from the ground up, read these 4 pages in order. The foundations come first.
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- no evidence level
- Circle concept
- Compact space concept
- Path-connected space concept
- Hawaiian earring example
Further reading
About this page
| Tag | 73EBKQPP8Q |
|---|---|
| URI | m2://73EBKQPP8Q |
| Kind | example |
| Writing | W0 stub |
| MSC | 55Q52, 54G20 |
| Uses | Circle, Compact space, Path-connected space |
| Changed | 2 November 2026 |
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