Past exam of the mathematics course of the University of Cambridge 2012 iii Paper 10 3 ii Solution Created 2026-10-03 Updated 2026-10-07
Yes. Use the sequence , . Every nonempty finite sum has only digits and in decimal notation, with leading digit , so none is a power of , including .
We still need an idempotent ultrafilter containing these sums; merely exhibiting an infinite set is insufficient. PutThe nonempty clopen sets are nested, so compactness gives a nonempty compact . This is the tail finite-sums semigroup. To verify closure under addition, take and fix . For any , choose a finite representation of and then an index beyond every index in that representation. Disjointness of supports gives , so . Consequentlyand . This holds for every , proving . The Ellis–Numakura lemma now supplies an idempotent ultrafilter in . It contains , a subset of the non-powers of , so by upward closure it contains the whole set of non-powers of .