Quotient of Cauchy Sequences is Metric Completion

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

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

Let $\CC$ be the ring of Cauchy sequences over $R$.

Let $\NN$ be the set of null sequences in $R$.

Let $\CC \,\big / \NN$ be the quotient ring of Cauchy sequences of $\CC$ by the maximal ideal $\NN$.

Let $\norm {\, \cdot \,}: \CC \,\big / \NN \to \R_{\ge 0}$ be the norm on the quotient ring $\CC \,\big / \NN$ defined by:
 * $\ds \forall \sequence {x_n} + \NN: \norm {\sequence {x_n} + \NN} = \lim_{n \mathop \to \infty} \norm{x_n}$

Let $d'$ be the metric induced by $\struct {\CC \,\big / \NN, \norm {\, \cdot \,} }$

Let $\phi: R \to \CC \,\big / \NN$ be the mapping from $R$ to the quotient ring $\CC \,\big / \NN$ defined by:
 * $\forall a \in R: \map \phi a = \sequence {a, a, a, \ldots} + \NN$

where $\sequence {a, a, a, \ldots} + \NN$ is the left coset in $\CC \, \big / \NN$ that contains the constant sequence $\sequence {a, a, a, \ldots}$.

Then:
 * $\struct {\CC \,\big / \NN, d'}$ is the metric completion of $\struct {R,d}$

and:
 * $\map \phi R$ is a dense subset of $\CC \,\big / \NN$

Proof
By the definition of the metric induced by a norm then:
 * a sequence $\sequence {x_n}$ is a Cauchy sequence in $\struct {R, \norm {\, \cdot \,} }$ $\sequence {x_n}$ is a Cauchy sequence in $\struct {R, d}$.

So $\CC$ is the set of Cauchy sequences in $\struct {R, d}$.

Let $\sim$ be the equivalence relation on $\CC$ defined by:


 * $\ds \sequence {x_n} \sim \sequence {y_n} \iff \lim_{n \mathop \to \infty} \map d {x_n, y_n} = 0$

Let $\tilde {\CC} = \CC / \sim$ denote the set of equivalence classes under $\sim$.

For $\sequence {x_n} \in \CC$, let $\eqclass {x_n} {}$ denote the equivalence class containing $\sequence {x_n}$.

Lemma 1
Let $\tilde d: \tilde {\CC} \times \tilde {\CC} \to \R_{\ge 0}$ be the metric defined by:
 * $\ds \map {\tilde d} {\eqclass {x_n} {}, \eqclass {y_n} {} } = \lim_{n \mathop \to \infty} \map d {x_n, y_n}$

By Completion of a Metric Space then:
 * $\struct {\tilde {\CC}, \tilde d}$ is the metric completion of $\struct {R, d}$.

Lemma 2
Let $\tilde \phi: R \to \tilde {\CC}$ be the mapping defined by:
 * $\map {\tilde \phi} a = \eqclass {a, a, a, \dotsc} {}$

where $\eqclass {a, a, a, \dotsc} {}$ denotes the equivalence class containing the constant sequence $\sequence {a, a, a, \dotsc}$.

By Completion of a Metric Space then:
 * $\map {\tilde \phi} R$ is a dense subset of $\tilde {\CC}$.

By Lemma 1 then:
 * $\forall \sequence {x_n} \in \CC: \sequence {x_n} + \NN = \eqclass {x_n} {}$

In particular:
 * $\forall a \in R: \sequence {a, a, a, \dotsc} + \NN = \eqclass {a, a, a, \dotsc} {}$

That is:
 * $\forall a \in R: \map \phi a = \map {\tilde \phi} a$

Hence $\phi = \tilde \phi$.

The result follows.