Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2004/iii/paper-16/3/c/solution

This sequence cannot occur. The same duality and coefficient calculation gives when . More generally the third homology of a closed oriented four-manifold is torsion-free, whereas the proposed nonzero third group is torsion. Either observation rules it out.

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