Alphabeta Math
ExampleConstruction: Literature-sourcedVerification: AI-adaptedverified 2026-09-26 (gpt-6-sol)
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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.

[F1]

The loop t↦[t] in R/Z is not nullhomotopic. (The projected unit interval is not nullhomotopic in R/Z).

[F2]

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).

[F3]

A space X is semilocally simply connected at x∈X when there is a neighbourhood U of x and a basepoint-preserving inclusion (U,x)↪(X,x) 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).

[F4]

The quotient topology. Let (X,T) be a topological space (def-topological-space), let Y be a set and let q:X→Y be a surjection (def-injection-surjection-bijection). The quotient topology on Y induced by q is the final topology of the one-element family (q) (def-initial-and-final-topology): Tq  :=  { V⊆Y:q−1[V]∈T }. 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, C⊆Y is closed in Tq exactly when q−1[C] is closed in X, because q−1[Y∖V]=X∖q−1[V]. (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

technique · direct
1.1givenF1

Model the earring in R2 as H=⋃n≥1Cn, where Cn is the circle of radius 1/n centered at (1/n,0) and all circles meet at o=(0,0). Their diameters tend to zero. Each Cn is homeomorphic to R/Z, with the unit loop based at o.

2.1step 1.1F3F2

Away from the wedge point, sufficiently short open arcs are path-connected neighbourhoods.

3.1step 1.1step 2.1

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.

4.1step 1.1step 3.1F1F3

Every neighbourhood U of o contains some whole Cn. Define rn:H→Cn to be the identity on Cn and to send every other circle to o. This is continuous away from o because each nonzero point has a neighbourhood meeting only its own circle; it is continuous at o because ∣rn(z)∣≤∣z∣. The unit loop in Cn⊂U is essential in Cn by [F1], so its composite with the retraction cannot be nullhomotopic in H. Thus the inclusion-induced map from U to H is nontrivial, and semilocal simple connectedness fails at o.

5.1step 4.1F2

The necessity theorem then rules out a universal cover.

6.1step 5.1∎

The preceding construction and implications establish the assertion.

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

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Sources