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Lemma: All Convergent Real Sequences Are Cauchy Sequences

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Every convergent real sequence is a Cauchy sequence.

More formally, in the metric space \((\mathbb R,|\cdot|)\) for a sequence \((x_n)_{n\in\mathbb N}\) being “convergent” to a value \(x\in\mathbb R\) means that for a given \(\epsilon > 0\) there is an index \(N(\epsilon)\in\mathbb N\) such that
\[|x_n - x| < \epsilon\text{ for all }n\ge N(\epsilon).\]

Fur such a convergent sequence (in a metric space like \((\mathbb R,|\cdot|)\) it follows then that it is a Cauchy sequence, which means that for a given \(\epsilon > 0\) there exists an index \(N(\epsilon)\in\mathbb N\) with
\[|x_i-x_j| < \epsilon\text{ for all }i,j\ge N(\epsilon).\]

\(N(\epsilon)\) means that the natural number \(N\) depends only on \(\epsilon\).

References

[581] Forster Otto: “Analysis 1, Differential- und Integralrechnung einer Veränderlichen”, Vieweg Studium, 1983


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