Note that this embedding depends on the chosen point x0 and is therefore not entirely canonical.
The Kuratowski–Wojdysławski theorem states that every bounded metric space X is isometric to a closed subset of a convex subset of some Banach space. (N.B. the image of this embedding is closed in the convex subset, not necessarily in the Banach space.) Here we use the isometry
The convex set mentioned above is the convex hull of Ψ(X).
In both of these embedding theorems, we may replace Cb(X) by the Banach space ℓ ∞(X) of all bounded functions X → R, again with the supremum norm, since Cb(X) is a closed linear subspace of ℓ ∞(X).
These embedding results are useful because Banach spaces have a number of useful properties not shared by all metric spaces: they are vector spaces which allows one to add points and do elementary geometry involving lines and planes etc.; and they are complete. Given a function with codomain X, it is frequently desirable to extend this function to a larger domain, and this often requires simultaneously enlarging the codomain to a Banach space containing X.
Formally speaking, this embedding was first introduced by Kuratowski, but a very close variation of this embedding appears already in the paper of Fréchet where he first introduces the notion of metric space.
- Tight span, an embedding of any metric space into an injective metric space defined similarly to the Kuratowski embedding
- Juha Heinonen (January 2003), Geometric embeddings of metric spaces, retrieved 6 January 2009
- Karol Borsuk (1967), Theory of retracts, Warsaw. Theorem III.8.1
- Kuratowski, C. (1935) "Quelques problèmes concernant les espaces métriques non-separables" (Some problems concerning non-separable metric spaces), Fundamenta Mathematicae 25: pp. 534–545.
- Fréchet M. (1906) "Sur quelques points du calcul fonctionnel", Rendiconti del Circolo Matematico di Palermo 22: 1–74.