Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2010/ii/paper-2/21h/solution
Past exam of the mathematics course of the University of Cambridge 2010 ii Paper 2 21H Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-14
Take a CW complex with one vertex, loops labelled , and one two-cell attached along . The Seifert-van Kampen theorem gives the required fundamental group. Connected double covering spaces correspond to index-two subgroups, equivalently kernels of surjective homomorphisms . The relation imposes modulo two, so the unique nonzero possibility sends both to one. Thus there is a unique connected double cover up to covering isomorphism.
Construct it with two vertices; both labelled edges switch sheets, and attach the two lifts of the two-cell. Contract one of the edges to a spanning tree. With coset representatives , the remaining Schreier generators can be taken as , and . Reading the two lifts of the attaching word, starting on the two respective sheets, gives and . ThusFor example, a letter contributes nothing on the first sheet and on the second; a letter contributes on the first and on the second. Both letters then switch sheets, which directly verifies the two relators.
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