I define closed sets, an important notion in topology and analysis. It is defined in terms of limit points, and has a priori nothing to do with open sets. Yet I show the important result that a set is closed if and only if its complement is open. More topology videos can be found on my playlist below.
Boundary of sphere: https://youtu.be/68H4joYPeP8
Open Sets: https://youtu.be/WcxBy4cUzWo
Topology Playlist: https://www.youtube.com/playlist?list=PLJb1qAQIrmmA13vj9xkHGGBXRMV32EKVI
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