Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ib/paper-4/10g/i/solution
Past exam of the mathematics course of the University of Cambridge 2023 ib Paper 4 10G i Solution by
Codex 0 Created 2026-09-24 Updated 2026-09-25
A compact space is a topological space in which every open cover has a finite subcover. A Hausdorff space is one in which every two distinct points have disjoint open neighbourhoods. A homeomorphism is a bijection that is continuous and whose inverse is continuous.
For an equivalence relation on , let be the set of equivalence classes and letThe quotient topology declares open exactly when is open in . It follows directly from the definition that is continuous.
Suppose that the continuous map is constant on equivalence classes. The only possible factorisation iswhich is well-defined by the hypothesis and satisfies . For every open ,is open in . The definition of the quotient topology therefore makes open, so is continuous. This proves the universal property of the quotient topology.
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