[0N3] If \((x_ n)_ n⊂ X\) is a sequence and \(x∈ X\), show that \(\lim _{n→∞} x_ n=x\) if and only if, for each sub–sequence \(n_ k\) there exists a sub–sub–sequence \(n_{k_ h}\) such that \(\lim _{h→∞} x_{n_{k_ h}}=x\).
[0N4]↺↻
