Sequence is Bounded in Norm iff Bounded in Metric/Sufficient Condition

Theorem
Let $\struct {R, \norm {\,\cdot\,}} $ be a normed division ring.

Let $d$ be the metric induced on $R$ be the norm $\norm {\,\cdot\,}$.

Let $\sequence {x_n}$ be a sequence in $R$.

Let $\sequence {x_n} $ be a bounded sequence in the metric space $\struct {R, d}$

Then:
 * $\sequence {x_n} $ is a bounded sequence in the normed division ring $\struct {R, \norm {\,\cdot\,} }$

Proof
Let $\sequence {x_n} $ be a bounded sequence in the metric space $\struct {R, d}$.

Then:
 * $\exists K \in \R_{> 0} : \forall n, m : \map d {x_n, x_m} \le K$

By the definition of the metric induced by a norm this is equivalent to:
 * $\exists K \in \R_{> 0} : \forall n, m : \norm {x_n - x_m} \le K$

Then $\forall n \in \N$:

Hence the sequence $\sequence {x_n}$ is bounded by $K + \norm {x_1}$ in the normed division ring $\struct {R, \norm {\,\cdot\,} }$.

Also see

 * Definition:Bounded Sequence in Normed Division Ring
 * Definition:Metric Induced by Norm on Division Ring
 * Definition:Bounded Sequence in Metric Space