Exercises
[0K7]Let \((Y, σ)\) be a Hausdorff topological space and \(A⊆ Y\). Show that \(x∈ Y\) is an accumulation point for \(A\) if and only if there is a \(J\) filtering set and there is a net \(φ:J→ A⧵\{ x\} \) such that \(\lim _{j∈ J} φ(x) = x\).