Combination Theorem for Cauchy Sequences/Inverse Rule
Theorem
Let $\struct {R, \norm {\, \cdot \,} }$ be a normed division ring with zero: $0$.
Let $\sequence {x_n}$ be a Cauchy sequences in $R$.
Suppose $\sequence {x_n}$ does not converge to $0$.
Then:
- $\exists K \in \N: \forall n > K : x_n \ne 0$
and the sequence:
- $\sequence {\paren {x_{K + n} }^{-1} }_{n \mathop \in \N}$ is well-defined and a Cauchy sequence.
Proof
Since $\sequence {x_n}$ does not converge to $0$, by Cauchy Sequence Is Eventually Bounded Away From Non-Limit then:
- $\exists K \in \N$ and $C \in \R_{>0}: \forall n > K: C < \norm {x_n}$
or equivalently:
- $\exists K \in \N$ and $C \in \R_{>0}: \forall n > K: 1 < \dfrac {\norm {x_n} } C$
By Norm Axiom $\text N 1$: Positive Definiteness:
- $\forall n > K : x_n \ne 0$
Let $\sequence {y_n}$ be the subsequence of $\sequence {x_n}$ defined as:
- $y_n = x_{K + n}$
By Subsequence of Cauchy Sequence in Normed Division Ring is Cauchy Sequence:
- $\sequence {y_n}$ is a Cauchy sequence.
So $\sequence { {y_n}^{-1} }$ is well-defined and $\sequence { {y_n}^{-1} } = \sequence {\paren {x_{K + n} }^{-1} }_{n \mathop \in \N}$.
Let $\epsilon > 0$ be given.
Let $\epsilon' = \epsilon C^2$, then $ \epsilon' > 0$.
Similarly, $\sequence {y_n}$ is a Cauchy sequence, we can find $N$ such that:
- $\forall n, m > N_2: \norm {y_n - y_m} < \epsilon'$
Thus $\forall n, m > N$:
- $(1): \quad 1 < \dfrac {\norm {y_n} } C, \dfrac {\norm {y_m} } C$
- $(2): \quad \norm {y_n - y_m} < \epsilon'$
Hence:
\(\ds \norm { {y_n}^{-1} - {y_m}^{-1} }\) | \(<\) | \(\ds \dfrac {\norm {y_n} } C \norm { {y_n }^{-1} - {y_m}^{-1} } \dfrac {\norm {y_m} } C\) | $(1)$ above | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \paren {\norm {y_n} \norm { {y_n }^{-1} - {y_m}^{-1 } } \norm {y_m} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \norm {y_n \paren { {y_n }^{-1 } - {y_m }^{-1} } y_m}\) | Norm Axiom $\text N 2$: Multiplicativity | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \norm {\paren {y_n {y_n }^{-1 } - y_n {y_m }^{-1} } y_m }\) | Ring Axiom $\text D$: Distributivity of Product over Addition | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \norm {y_n {y_n}^{-1} y_m - y_n {y_m}^{-1} y_m}\) | Ring Axiom $\text D$: Distributivity of Product over Addition | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \norm {y_m - y_n}\) | Inverse Property of a Division Ring | |||||||||||
\(\ds \) | \(<\) | \(\ds \dfrac 1 {C^2} \epsilon'\) | $(2)$ above | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {C^2} \paren {\epsilon C^2}\) | Definition of $\epsilon'$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \epsilon\) |
So:
- $\sequence { { {y_n}^{-1} } }$ is a Cauchy sequence in $R$ by definition.
$\blacksquare$
Sources
- 1997: Fernando Q. Gouvea: p-adic Numbers: An Introduction: $\S 3.2$: Completions