# Equivalence of Definitions of Equivalent Division Ring Norms/Topologically Equivalent implies Convergently Equivalent

## Theorem

Let $R$ be a division ring.

Let $\norm {\,\cdot\,}_1: R \to \R_{\ge 0}$ and $\norm {\,\cdot\,}_2: R \to \R_{\ge 0}$ be norms on $R$.

Let $d_1$ and $d_2$ be the metrics induced by the norms $\norm {\,\cdot\,}_1$ and $\norm {\,\cdot\,}_2$ respectively.

Let $d_1$ and $d_2$ be topologically equivalent metrics.

Then:

- $d_1$ and $d_2$ are convergently equivalent metrics.

## Proof

Let $\sequence {x_n}$ converge to $l$ in $\norm {\,\cdot\,}_1$.

Let $\epsilon \in \R_{\gt 0}$ be given.

Let $\map {B_\epsilon^2} i$ denote the open ball centered on $l$ of radius $\epsilon$ in $\struct {R, \norm{\,\cdot\,}_2}$.

By Open Ball is Open Set then $\map {B_\epsilon^2} l$ is open set in $\struct{R, d_2}$.

Since $d_1$ and $d_2$ are topologically equivalent metrics then $\map {B_\epsilon^2} l$ is open set in $\struct {R, d_1}$.

By the definition of an open set in a metric space then:

- $\exists \delta \in \R_{\gt 0}: \map {B_\delta^1} l \subseteq \map {B_\epsilon^2} l$

Hence:

- $\forall x \in R: \norm {x - l}_1 < \delta \implies \norm {x - l}_2 < \epsilon$

Since $\sequence {x_n}$ converges to $l$ in $\norm{\,\cdot\,}_1$ then:

- $\exists N \in \N: \forall n \ge N: \norm {x_n - l}_1 < \delta$

Hence:

- $\exists N \in \N: \forall n \ge N: \norm {x_n - l}_2 < \epsilon$

Since $\sequence {x_n}$ and $\epsilon \gt 0$ were arbitrary then it has been shown that for all sequences $\sequence {x_n}$ in $R$:

- $\sequence {x_n}$ converges to $l$ in $\norm {\,\cdot\,}_1 \implies \sequence {x_n}$ converges to $l$ in $\norm {\,\cdot\,}_2$.

By a similar argument it is shown that for all sequences $\sequence {x_n}$ in $R$:

- $\sequence {x_n}$ converges to $l$ in $\norm {\,\cdot\,}_2 \implies \sequence {x_n}$ converges to $l$ in $\norm {\,\cdot\,}_1$.

The result follows.

$\blacksquare$

## Sources

- 1997: Fernando Q. Gouvea:
*p-adic Numbers: An Introduction*: $\S 3.1$ Absolute Values on $\Q$, Lemma $3.1.2$ and Problem $66$