What is a closed set ?

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:    • Can a ball be a sphere?  
Open Sets:    • Taste of topology: Open Sets  
Topology Playlist:    • Topology  
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