Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ib/paper-4/10g/i/solution

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 let
The 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 is
which 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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