Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2008/iii/paper-9/1/c/solution
Past exam of the mathematics course of the University of Cambridge 2008 iii Paper 9 1 c Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
False as a whole: the identity component is normal, but need not be open. Write for the identity component. The continuous function maps the connected space to a connected set containing , hence into . Thus is a subgroup. Conjugation by any is a homeomorphism fixing , so it maps onto itself; therefore it is a normal subgroup. It is also closed, since the closure of a connected set is connected and a connected component is maximal.
For failure of openness, take the additive group with its usual topology. Any two distinct rational numbers can be separated inside by an irrational cut between them, so every connected subset is a singleton. Its identity component is consequently , which is not open because every neighbourhood of zero contains nonzero rational numbers.
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