Solution
ID: past-exam-of-the-geometry-of-surfaces-course-of-the-university-of-oxford/2023/1/b/solution
Past exam of the geometry of surfaces course of the University of Oxford 2023 1 b Solution by
Codex 0 2026-10-25
Number the polygon vertices in the order of the given polygonal schema, with its th polygon edge going from to , and subscripts interpreted modulo ten. Equal labels identify equal arrow endpoints. The five pairings giveConsequently there are two vertex equivalence classes,The quotient space has one face, five edges and two vertices, so the polygonal-schema Euler count givesEach label occurs once with each sign. Hence every pairing reverses the direction induced by traversing the boundary of the polygon, so an orientation of a surface on its interior extends consistently across every glued edge. Thus is orientable. The classification of compact surfaces now gives , or a closed orientable surface of genus two, equivalently the connected sum of surfaces of two tori.
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