Definition:Completion (Metric Space)

This page or section has statements made on it that ought to be extracted and proved in a Theorem page. In particular: There's a lot of stuff on this page which should be extracted into other places and linked to or transcluded. You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by creating any appropriate Theorem pages that may be needed. To discuss this page in more detail, feel free to use the talk page.

Definition

Let $M_1 = \struct {A, d}$ and $M_2 = \struct {\tilde A, \tilde d}$ be metric spaces.

Then $M_2$ is a completion of $M_1$, or $M_2$ completes $M_1$, if and only if:

$(1): \quad M_2$ is a complete metric space

$(2): \quad A \subseteq \tilde A$

$(3): \quad A$ is dense in $M_2$

$(4): \quad \forall x, y \in A : \map {\tilde d} {x, y} = \map d {x, y}$. In terms of restriction of functions, this says that $\map {\tilde d {\restriction_A} } = d$.

It is immediate from this definition that a completion of a metric space $M_1$ consists of:

A complete metric space $M_2$

An isometry $\phi : A \to \tilde A$

such that $\map \phi A = \set {\map \phi x: x \in A}$ is dense in $M_2$.

An isometry is often required to be bijective, so here one should consider $\phi$ as a mapping from $A$ to the image of $\phi$.

Therefore to insist that $\phi$ be an isometry, in this context, is to say that $\phi$ must be an injection that preserves the metric of $M_1$.

Sources

• 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology (2nd ed.) ... (previous) ... (next): Part $\text I$: Basic Definitions: Section $5$: Metric Spaces: Complete Metric Spaces