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[2DY] A topological space is a pair \((X,π)\) where \(X\) is a non-empty set with associated the family \(π\) of the open sets, which is called topology.
[2DY] A topological space is a pair \((X,π)\) where \(X\) is a non-empty set with associated the family \(π\) of the open sets, which is called topology.