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The Hawaiian earring is locally path-connected but has no universal cover
Example
The Hawaiian earring is locally path-connected but is not semilocally simply connected at its wedge point. Consequently it has no universal cover.
Facts & Assumptions
Given: The objects, hypotheses, and choice principles stated above.
The loop in is not nullhomotopic. (The projected unit interval is not nullhomotopic in ).
If a space admits a universal covering, then it is semilocally simply connected. No local path-connectedness hypothesis is required. (A space admitting a universal covering is semilocally simply connected).
A space is semilocally simply connected at when there is a neighbourhood of and a basepoint-preserving inclusion whose induced map on fundamental groups is trivial (def-neighbourhood-top, def-induced-homomorphism-on-fundamental-groups, def-based-loops-and-fundamental-group). It is semilocally simply connected when this holds at every point. The neighbourhood need not itself be simply connected. (Semilocally simply connected spaces with explicit basepoint convention).
The quotient topology. Let be a topological space (def-topological-space), let be a set and let be a surjection (def-injection-surjection-bijection). The quotient topology on induced by is the final topology of the one-element family (def-initial-and-final-topology): That this is a topology is discharged in def-initial-and-final-topology, where every final topology is verified to satisfy (T1), (T2) and (T3). Dually, is closed in exactly when is closed in , because . (The quotient topology of a surjection, quotient maps, saturated sets, and the quotient of a space by an equivalence relation with its canonical projection).
Verification
Model the earring in as , where is the circle of radius centered at and all circles meet at . Their diameters tend to zero. Each is homeomorphic to , with the unit loop based at .
Away from the wedge point, sufficiently short open arcs are path-connected neighbourhoods.
At the wedge point, every open neighbourhood contains a smaller metric ball whose intersection with each circle is either an arc through the wedge point or the whole circle. It contains every sufficiently small circle, and all these pieces share the wedge point, so the ball is path-connected.
Every neighbourhood of contains some whole . Define to be the identity on and to send every other circle to . This is continuous away from because each nonzero point has a neighbourhood meeting only its own circle; it is continuous at because . The unit loop in is essential in by [F1], so its composite with the retraction cannot be nullhomotopic in . Thus the inclusion-induced map from to is nontrivial, and semilocal simple connectedness fails at .
The necessity theorem then rules out a universal cover.
The preceding construction and implications establish the assertion.
Depends on
- The projected unit interval is not nullhomotopic in $\mathbb R/\mathbb Z$
- A space admitting a universal covering is semilocally simply connected
- Semilocally simply connected spaces with explicit basepoint convention
- The quotient topology of a surjection, quotient maps, saturated sets, and the quotient of a space by an equivalence relation with its canonical projection
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- Allen Hatcher, Algebraic Topology, §1.3 (standard reference, not scraped)
- Marco Gualtieri, MAT1300 Week 4 Term 2, §1.6 (standard reference, not scraped)
- Omar Antolín Camarena, Proper local homeomorphisms and covering maps (standard reference, not scraped)