Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/iii/paper-16/5/2/solution
Past exam of the mathematics course of the University of Cambridge 2009 iii Paper 16 5 2 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
False. Take and . Both are closed oriented simply connected manifolds and . A cohomological obstruction rules out even a topological fiber bundle.
Suppose a fiber bundle existed. The long exact sequence of homotopy groups of a fibration, including its component terms, makes path-connected. A local trivialization over a small open ball in identifies its preimage, an open subset of , with . Projection onto gives a retraction. An open smooth four-manifold has the homotopy type of a CW complex of dimension at most four, so its rational cohomology vanishes in degrees above four. A retract's cohomology injects into that of the ambient space. Therefore
Use the multiplicative cohomological Serre spectral sequence with rational coefficients. Since is simply connected, there is no nontrivial coefficient action, and only columns zero and three occur on . Thus , and the only possibly nonzero differential is . Let be the degree-three base generator. It cannot survive, because the sequence converges to and that ring has zero degree-three part. It has no outgoing differential and its only possible incoming differential is from . Hence there is withThe class is nonzero and, by the degree bound above, has a first vanishing power for some . Multiplicativity says that is a graded derivation. Since is even,But , in which by minimality of ; also is nonzero over . This is a contradiction. Thus no such fiber bundle exists. This is the special case of a sphere cannot fibre over a lower-dimensional odd sphere required here.
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