[0N6] A sequence \((x_ n)⊂ X\) is a Cauchy sequence if and only if there exists a sequence \(\varepsilon _ n\) with \(\varepsilon _ n≥ 0\) and \(\varepsilon _ n→_ n 0\) such that, for every \(n\) and every \(m≥ n\), we have \(d(x_ n,x_ m)≤ \varepsilon _ n\).
