Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/iii/paper-16/5/1/solution
Past exam of the mathematics course of the University of Cambridge 2009 iii Paper 16 5 1 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
False. Take and , with their standard product and connected-sum orientations. They are closed simply connected four-manifolds. For the connected sum, removing a four-ball leaves the fundamental group unchanged, and the Seifert-van Kampen theorem across the separating three-sphere preserves simple connectivity. The Mayer–Vietoris sequence computes integral cohomology in degrees zero, two and four, and zero in other degrees. The Künneth theorem gives the same groups for . Thus their Betti numbers agree and all their integral cohomology is torsion-free.
Their cohomology rings nevertheless differ. In , let come from the two projective summands and let generate degree four. The cohomology ring of the connected sum of two complex projective planes hasThese relations follow by using cycles in the separate summands and the degree-one pinch projections onto them. In , the sphere-factor generators satisfyConsequently every degree-two class of has squarean even multiple of the top generator, whereas in is a primitive generator. An integral graded-ring isomorphism cannot change this property, since it sends a generator of to its positive or negative. A homotopy equivalence induces a graded-ring isomorphism, so none exists. Equivalently, their intersection forms are and , an odd form and an even form respectively.
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