Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/ii/paper-3/20h/c/solution
Past exam of the mathematics course of the University of Cambridge 2018 ii Paper 3 20H c Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-03
Let . Define a surjectionIts kernel determines a connected regular four-sheeted cover of a wedge of circles and a real projective plane, say .
Use the CW structure on with one vertex, one edge for every and for , and one two-cell attached along . In the regular cover, the degree-two cellular chain group isThe cellular boundary formula gives the lifted attaching word the boundaryin the -edge summand of . ThereforeThere are no three-cells, so
If were homotopy equivalent to a compact connected surface , the subgroup corresponding to this cover would produce a connected four-sheeted cover homotopy equivalent to . A finite cover of a compact surface is again a compact connected surface. But the integral top homology of a compact connected surface has rank at most one, contradicting the displayed rank two. Hence is not homotopy equivalent to a compact connected surface for any .
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