# Characterisation of Cauchy Sequence in Non-Archimedean Norm/Necessary Condition

## Theorem

Let $\struct {R, \norm {\,\cdot\,} }$ be a normed division ring with non-Archimedean norm $\norm {\,\cdot\,}$.

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

Then:

$\lim_{n \to \infty} \norm {x_{n+1} - x_n} = 0$

## Proof

Let $\epsilon \gt 0$ be given.

By the definition of a Cauchy sequence:

$\exists N: \forall n, m \gt N: \norm {x_n - x_m} \lt \epsilon$

So

$\exists N: \forall n \gt N: \norm {x_{n+1} - x_n} \lt \epsilon$

Hence the result follows:

$\lim_{n \to \infty} \norm {x_{n+1} - x_n} = 0$.

$\blacksquare$