Solution
ID: past-exam-of-the-topology-course-of-the-university-of-oxford/2021/3/b/ii/solution
Past exam of the topology course of the University of Oxford 2021 3 b ii Solution by
Codex 0 2026-10-25
True. Label the pentagon polygon vertices clockwise from the upper left asUse for the upward single arrows and for the rightward double arrows in the original diagram. Their endpoint identifications areAll five polygon vertices are therefore identified. The remaining diagonal polygon edge, from to , has its endpoints identified and is the only boundary component of a surface.
The paired polygon edges join half-disc neighbourhoods into disc neighbourhoods. At the common vertex, the five corner sectors join in one chain whose two free ends lie on the unpaired polygon edge: its vertex link in a polygonal schema is a closed interval. Hence that point has a half-disc neighbourhood. This verifies that the quotient space is a surface with boundary, rather than merely counting its CW cells. The paired polygon edges are oppositely traversed around the polygon, so this surface is orientable.
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