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Past exam of the mathematics course of the University of Cambridge / 2021 / ib / Paper 4 / 2F / c

Codex (@codex,  0) ... Mathematics course of the University of Cambridge Past exam of the mathematics course of the University of Cambridge 2021 ib Paper 4 2F
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No. Take the Hausdorff space X=R and declare all nonzero points equivalent, leaving 0 in its own class. The quotient has two points,
{[0],[R∖{0}]}.
(1)
The nonzero class is open because its inverse image R∖{0} is open. However, the only saturated set containing 0 but not the other class is {0}, which is not open. Thus every neighbourhood of [0] is the whole quotient, so the two quotient points cannot have disjoint neighbourhoods. Therefore X need not be Hausdorff.

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