# Kirszbraun theorem

In mathematics, specifically real analysis and functional analysis, the Kirszbraun theorem states that if U is a subset of some Hilbert space H1, and H2 is another Hilbert space, and

f : UH2

is a Lipschitz-continuous map, then there is a Lipschitz-continuous map

F: H1H2

that extends f and has the same Lipschitz constant as f.

Note that this result in particular applies to Euclidean spaces En and Em, and it was in this form that Kirszbraun originally formulated and proved the theorem.[1] The version for Hilbert spaces can for example be found in (Schwartz 1969, p. 21).[2] If H1 is a separable space (in particular, if it is a Euclidean space) the result is true in Zermelo–Fraenkel set theory; for the fully general case, it appears to need some form of the axiom of choice; the Boolean prime ideal theorem is known to be sufficient.[3]

The proof of the theorem uses geometric features of Hilbert spaces; the corresponding statement for Banach spaces is not true in general, not even for finite-dimensional Banach spaces. It is for instance possible to construct counterexamples where the domain is a subset of Rn with the maximum norm and Rm carries the Euclidean norm.[4] More generally, the theorem fails for $\mathbb{R}^m$ equipped with any $\ell_p$ norm ($p \neq 2$) (Schwartz 1969, p. 20).[2]

For an R-valued function the extension is provided by $\tilde f(x):=\inf_{u\in U}f(u)+\text{Lip}(f)\cdot d(x,u),$ where $\text{Lip}(f)$ is f's Lipschitz constant on U.

## History

The theorem was proved by Mojżesz David Kirszbraun, and later it was reproved by Frederick Valentine,[5] who first proved it for the Euclidean plane.[6] Sometimes this theorem is also called Kirszbraun–Valentine theorem.

## References

1. ^ Kirszbraun, M. D. (1934). "Über die zusammenziehende und Lipschitzsche Transformationen". Fund. Math. 22: 77–108.
2. ^ a b Schwartz, J. T. (1969). Nonlinear functional analysis. New York: Gordon and Breach Science.
3. ^ Fremlin, D. H. (2011). "Kirszbraun's theorem". Preprint.
4. ^ Federer, H. (1969). Geometric Measure Theory. Berlin: Springer. p. 202.
5. ^ Valentine, F. A. (1945). "A Lipschitz Condition Preserving Extension for a Vector Function". American Journal of Mathematics 67 (1): 83–93. doi:10.2307/2371917.
6. ^ Valentine, F. A. (1943). "On the extension of a vector function so as to preserve a Lipschitz condition". Bulletin of the American Mathematical Society 49: 100–108. doi:10.1090/s0002-9904-1943-07859-7. MR 0008251.