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Consider the set of natural numbers $\mathbb N$ with the metric


Describe all convergent sequences and all Cauchy sequences in this metric space. Is the metric space $(\mathbb N, d)$ complete?

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When is $d(m,n)<1/2$? – David Mitra Jan 29 '14 at 21:53
Please show your working so far :) – Shaun Jan 29 '14 at 22:04

Hints: We have

  • $a_n\to a$ in the metric space $(\Bbb N,d)$ iff $d(a_n,a)\to 0$ in $\Bbb R$.
  • For a sequence $c_n\in\Bbb R$ we have $\displaystyle\frac{c_n}{1+c_n}=1-\frac1{1+c_n}$ and its limit is $0$ iff $\displaystyle\frac1{1+c_n}\to 1\ $ iff $\ (1+c_n)\,\to 1\ $ iff $\ c_n\to 0$.
  • Similar considerations can lead to that Cauchy sequences are also Cauchy in $\Bbb R$.
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