Editing Kirszbraun theorem (section)

==History==

The theorem was proved by [[Mojżesz David Kirszbraun]], and later it was reproved by [[Frederick Valentine]],<ref>{{cite journal |first=F. A. |last=Valentine |title=A Lipschitz Condition Preserving Extension for a Vector Function |journal=[[American Journal of Mathematics]] |volume=67 |issue=1 |year=1945 |pages=83–93 |doi=10.2307/2371917 |jstor=2371917 }}</ref> who first proved it for the Euclidean plane.<ref>{{cite journal |first=F. A. |last=Valentine |title=On the extension of a vector function so as to preserve a Lipschitz condition |journal=Bulletin of the American Mathematical Society |volume=49 |pages=100–108 |year=1943 |issue=2 |mr=0008251 |doi=10.1090/s0002-9904-1943-07859-7|doi-access=free }}</ref> Sometimes this theorem is also called '''Kirszbraun–Valentine theorem'''.