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[parent] example of a space that is not semilocally simply connected (Example)

An example of a space that is not semilocally simply connected is the following: Let $$HR=\bigcup_{n\in\BN}\left\{(x,y)\in \BR^2\,\bigg\vert\,\left(x-\frac{1}{2^n}\right)^2+y^2= \left(\frac{1}{2^n}\right)^2\right\}$$ endowed with the subspace topology. Then $(0,0)$ has no simply connected neighborhood. Indeed every neighborhood of $(0,0)$ contains (ever diminshing) homotopically non-trivial loops. Furthermore these loops are homotopically non-trivial even when considered as loops in $HR$ .

Figure: The Hawaiian rings
\begin{figure*}\centering \begin{pspicture}(0,-2)(4,3) \pscircle(2,0){2} \pscirc... ...ircle(.625,0){.625} \pscircle(0.015625,0){0.015625} \end{pspicture}\end{figure*}

It is essential in this example that $HR$ is endowed with the topology induced by its inclusion in the plane. In contrast, the same set endowed with the CW topology is just a bouquet of countably many circles and (as any CW complex) it is semilocaly simply connected.




"example of a space that is not semilocally simply connected" is owned by mathcam. [ full author list (2) | owner history (1) ]
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Also defines:  Hawaiian rings, Hawaiian earrings

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Cross-references: CW complex, circles, plane, inclusion, induced, topology, even, loops, contains, neighborhood, simply connected, subspace topology, semilocally simply connected
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This is version 13 of example of a space that is not semilocally simply connected, born on 2003-02-04, modified 2006-06-06.
Object id is 3967, canonical name is ExampleOfASpaceThatIsNotSemilocallySimplyConnected.
Accessed 4790 times total.

Classification:
AMS MSC54D05 (General topology :: Fairly general properties :: Connected and locally connected spaces )
 57M10 (Manifolds and cell complexes :: Low-dimensional topology :: Covering spaces)

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synonyms by antonio on 2003-02-05 14:29:34
It would be useful to register the names "Hawaiian rings" and "Hawaiian earrings" as synonyms for this object, since they are very commonly used.

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