Math 2.0

General topology Connectedness Counterexample

Comb space

Builds on Path-connected spaceLocally connected spaceLocally path-connected space

Counterexample

Let K={0}∪{1/n:n≥1}⊆ℝK = \set{0} \cup \set{1/n : n \ge 1} \subseteq \R. The comb space is the subspace of ℝ2\R^2 C=([0,1]×{0})∪(K×[0,1]).C = ([0, 1] \times \set{0}) \cup (K \times [0, 1]). It consists of a base segment, the tooth {0}×[0,1]\set{0} \times [0, 1], and the teeth {1/n}×[0,1]\set{1/n} \times [0, 1].

The space CC is path-connected. Each point can be joined to (0,0)(0, 0) by going down its tooth and then along the base.

The space CC is not locally connected. Take p=(0,1)p = (0, 1) and the open set U=C∩B(p,1/2)U = C \cap B(p, 1/2). For large nn, the set UU contains a piece of the tooth {1/n}×[0,1]\set{1/n} \times [0, 1] near height 11. Any open set VV with p∈V⊆Up \in V \subseteq U contains such pieces, but UU contains no part of the base. The vertical line x=cx = c with cc strictly between two teeth splits VV into two disjoint open parts, one of them containing pp. So no open neighbourhood of pp inside UU is connected.

Remarks

So connectedness, even path-connectedness, does not imply local connectedness. For the same reason, CC is not locally path-connected. The comb space is contractible, so it also shows that contractible spaces need not be locally connected.

Reading path

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

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  1. Locally connected space concept
  2. Locally path-connected space concept
  3. Path-connected space concept
  4. Comb space counterexample

Further reading

  1. Web https://en.wikipedia.org/wiki/Comb_space
About this page
TagA2Q4RXD162
URIm2://A2Q4RXD162
Kindcounterexample
WritingW0 stub
MSC54G20, 54D05
UsesPath-connected space, Locally connected space, Locally path-connected space
Same as
Changed2 November 2026

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