# Combination Theorem for Cauchy Sequences/Quotient Rule

## Theorem

Let $\struct {R, \norm {\, \cdot \,} }$ be a normed division ring with zero $0$.

Let $\sequence {x_n}$ and $\sequence {y_n}$ be Cauchy sequences in $R$.

Suppose $\sequence {y_n}$ does not converge to $0$.

Then:

$\exists K \in \N: \forall n > K : y_n \ne 0$

and the sequences:

$\sequence { {x_{K + n} } \paren {y_{K + n} }^{-1} }_{n \mathop \in \N}$ and $\sequence {\paren {y_{K + n} }^{-1} {x_{K + n} } }_{n \mathop \in \N}$ are well-defined and Cauchy sequences.

## Proof

$\exists K \in \N : \forall n > K : y_n \ne 0$.

and the sequence:

$\sequence {\paren {x_{K + n} }^{-1} }_{n \mathop \in \N}$ is well-defined and a Cauchy sequence.

By Subsequence of Cauchy Sequence in Normed Division Ring is Cauchy Sequence, $\sequence {x_{K + n} }_{n \mathop \in \N}$ is a Cauchy sequence.

the sequences $\sequence { {x_{K + n} } \paren {y_{K + n} }^{-1} }_{n \mathop \in \N}$ and $\sequence {\paren {y_{K + n} }^{-1} {x_{K + n} } }_{n \mathop \in \N}$ are Cauchy sequences.

$\blacksquare$