Solution

ID: past-exam-of-the-topology-course-of-the-university-of-oxford/2021/3/b/ii/solution

True. Label the pentagon polygon vertices clockwise from the upper left as
Use for the upward single arrows and for the rightward double arrows in the original diagram. Their endpoint identifications are
All 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.
Its CW cells comprise one vertex, three edges (the pair, the pair, and the free polygon edge), and one face. Therefore
The classification of compact surfaces identifies it as a torus with an open disc removed.

New to topics? Read the docs here!