Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/iii/paper-16/4/2/solution
Past exam of the mathematics course of the University of Cambridge 2009 iii Paper 16 4 2 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Under the intended convention that the finite group is nontrivial, take . It is simply connected by its CW complex structure. The Lefschetz fixed-point theorem says that a continuous self-map of a compact triangulable space with nonzero rational Lefschetz number has a fixed point.
For any continuous , write for the integral degree-two generator, with . Preservation of the cup product gives . Only degrees zero, two and four contribute to the rational Lefschetz number, soConsequently every such map has a fixed point. In a free group action, each nonidentity group element must act by a fixed-point-free homeomorphism. This is impossible here, so no nontrivial finite group can act freely. This is the fixed-point property of even-dimensional complex projective space for complex dimension two. If the trivial group is included, the literal request has no example, since the trivial group acts freely on every space.
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