Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/iii/paper-16/5/1/solution

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 has
These relations follow by using cycles in the separate summands and the degree-one pinch projections onto them. In , the sphere-factor generators satisfy
Consequently every degree-two class of has square
an 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.

New to topics? Read the docs here!