Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2007/iii/paper-14/4/ii/solution
Past exam of the mathematics course of the University of Cambridge 2007 iii Paper 14 4 ii Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
The same cone on an infinite coinfinite ground set works:It is -closed: a set outside has an odd point, and the ordinary neighbourhood given by its initial segment through that point remains outside . Its -interior is empty, since every ordinary basic open set admits an extension having an odd point. Hence its ordinary closure equals itself and has empty interior, proving that it is a -nowhere dense set.
In the Ellentuck topology, however, is itself a nonempty open set. Its -closure therefore has nonempty -interior, so is not -nowhere dense.
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