Math 2.0

Algebraic topology Example

Hawaiian earring

Builds on CircleCompact spacePath-connected space

Example

For n≥1n \ge 1 let Cn⊆ℝ2C_n \subseteq \R^2 be the circle with centre (1/n,0)(1/n, 0) and radius 1/n1/n. The Hawaiian earring is H=⋃n≥1CnH = \bigcup_{n \ge 1} C_n with the subspace topology from ℝ2\R^2. All circles pass through the origin 00. The space HH is compact and path-connected, but it is not semilocally simply connected at 00.

Check. HH is closed and bounded in ℝ2\R^2, so it is compact. Each CnC_n is path-connected and contains 00, so HH is path-connected. Fix nn and let rn ⁣:H→Cnr_n \colon H \to C_n be the identity on CnC_n and send every other circle to 00. This map is continuous, and it is a retraction onto CnC_n. Let γn\gamma_n be the loop that goes once around CnC_n. The map rnr_n sends it to a generator of the fundamental group π1(Cn)≅ℤ\pi_1(C_n) \cong \Z, so γn\gamma_n is not null-homotopic in HH. Every neighbourhood of 00 contains CnC_n for all large nn, so it contains loops at 00 that are not null-homotopic in HH.

Remarks

Since HH is not semilocally simply connected, it has no universal covering space. The retractions rnr_n together give a homomorphism from π1(H,0)\pi_1(H, 0) onto the product ∏n≥1ℤ\prod_{n \ge 1} \Z. It is onto: a loop can run knk_n times around CnC_n during the time interval [1−21−n,1−2−n][1 - 2^{1-n}, 1 - 2^{-n}]. This loop is continuous at time 11 because the circles shrink. So π1(H,0)\pi_1(H, 0) is uncountable. The wedge sum of countably many circles, with the CW topology, has a countable free fundamental group and is not homeomorphic to HH.

Reading path

To understand this page from the ground up, read these 4 pages in order. The foundations come first.

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Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
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  1. Circle concept
  2. Compact space concept
  3. Path-connected space concept
  4. Hawaiian earring example

Further reading

  1. Web https://en.wikipedia.org/wiki/Hawaiian_earring
About this page
Tag73EBKQPP8Q
URIm2://73EBKQPP8Q
Kindexample
WritingW0 stub
MSC55Q52, 54G20
UsesCircle, Compact space, Path-connected space
Changed2 November 2026

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