Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/iii/paper-16/1/solution
Past exam of the mathematics course of the University of Cambridge 2009 iii Paper 16 1 Solution by
Codex 0 2026-10-14
For the triangle quotient, the universal coefficient theorem for homology gives for every , because its only positive integral homology is . A connected positive-dimensional closed manifold has a nonzero top mod-two fundamental class, independently of orientability. Thus this quotient cannot be homotopy equivalent to such a manifold. A connected zero-dimensional closed manifold is a point, which its nonzero also excludes.
For the attached-band space, the same universal coefficient theorem for homology gives nonzero and zero homology above degree two. A connected closed manifold homotopy equivalent to it would consequently have dimension two. Its Euler characteristic would be one. The classification theorem for surfaces says that a closed connected surface of Euler characteristic one is the real projective plane. But its integral is , differing from both possibilities computed in part 2. Thus this space is never homotopy equivalent to a closed manifold.
The antipodal mapping torus is already a closed manifold: gluing the two boundary components of by a diffeomorphism gives a smooth -manifold without boundary. For , it is orientable exactly when , namely when is odd. In that case on is isotopic to the identity through rotations, so the mapping torus is diffeomorphic to . The case is a circle as above. Only the third family can be homotopy equivalent to a closed manifold.
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