Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2006/ii/paper-1/21h/solution
Past exam of the mathematics course of the University of Cambridge 2006 ii Paper 1 21H Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-14
Use the usual cell structure on the torus: one vertex, two one-cells , and one two-cell attached by the word . Take the meridian being collapsed as . The quotient has one vertex, the surviving circle , and a two-cell whose attaching word reduces to and is null-homotopic. Attaching a two-cell to a circle by a null-homotopic loop gives a space homotopy equivalent to .
Alternatively its cellular chain groups are , and both boundary maps vanish: the two-cell traverses once in each direction, and the one-cell's endpoints coincide. Thus the integral homology groups areThe pinched torus is connected, so reduced is zero.
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