Sphere with its equator collapsed

ID: sphere-with-its-equator-collapsed

Collapse the equator of a sphere to a single equivalence class and leave every other point in its own class. The two closed hemispheres are disks sharing that equator. A disk with its boundary collapsed is a sphere, so the two hemisphere quotients are spheres sharing just one point: their wedge sum. An explicit map sends height and longitude on the upper hemisphere to , and sends the lower hemisphere to the sphere centred at by reversing the first coordinate. The equator goes to the common origin and the poles go to . This map is continuous and has precisely the collapsed equator as its only nonsingleton fibre. Its induced map is a homeomorphism by the compact-to-Hausdorff continuous bijection theorem.

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