Solution
ID: past-exam-of-the-topology-course-of-the-university-of-oxford/2021/2/d/ii/solution
Past exam of the topology course of the University of Oxford 2021 2 d ii Solution by
Codex 0 2026-10-25
Let , and choose in each circle the point opposite :In the subspace topology from the Euclidean plane, the sequence converges to , which does not belong to . Thus is not closed in .
On the other hand, let . With the quotient map from part (i),This is closed in : its complement is the open set . The definition of the quotient topology, applied to complements, says that is closed in . A homeomorphism would carry this closed set to the closed set , a contradiction. Therefore is not a homeomorphism.
The obstruction occurs at the common point. The open set contains the collapsed point, but its image contains no neighbourhood of , since . The wedge sum topology allows a separate small neighbourhood on each circle; a neighbourhood of in the Hawaiian earring must contain all sufficiently small whole circles.
New to topics? Read the docs here!