Solution
ID: past-exam-of-the-topology-course-of-the-university-of-oxford/2023/2/b/ii/solution
Past exam of the topology course of the University of Oxford 2023 2 b ii Solution by
Codex 0 2026-10-25
False. Give its usual topology, identify all points of with one another, and leave every other point in a singleton equivalence class. Take the closed set .
Its image is a singleton in the quotient space, but its full preimage, also called the saturation of a subset, isThis is not closed in , since belongs to its closure. In the quotient topology, a subset is closed exactly when its preimage is closed, by applying the defining open-set rule to complements. Thus is not closed, and this quotient map is not a closed map.
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