Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2016/iii/paper-121/2/c/solution
Past exam of the mathematics course of the University of Cambridge 2016 iii Paper 121 2 c Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-06
Every meagre set is contained in a meagre set: replace each nowhere dense set in a countable covering by its closure. There are at most such covers, since each closed subset of is determined by a countable rational basis for its open complement, and a countable sequence of such codes is again coded by a real. Under the Continuum hypothesis, enumerate all meagre sets as , allowing repetitions.
By transfinite recursion, chooseFor each , the excluded union is a meagre set: it is a countable union of meagre sets and singletons. The Baire category theorem ensures that it does not cover , so the choice is possible. The resulting is uncountable. If is meagre, choose with . Every with avoids , hencewhich is countable. Thus has precisely the Lusin set property. Enumerating covers rather than all meagre sets avoids an unjustified claim that there are only continuum many arbitrary meagre subsets.
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