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Questions designed to generate a "big list" of certain results, examples, conjectures, etc. via many individual answers, each contributing one or a few instances. Such a question should typically be in Community Wiki mode (CW); after asking, please, flag for moderators attention requesting the question to be made CW.

26 votes

Why is a topology made up of 'open' sets?

Not sure if anyone has mentioned this article, but Gregory Moore discussed the development of the notion of open sets vs other historical approaches, in the paper "The emergence of open sets, closed s …