## Who is Kirszbraun?

Kirszbraun's theorem is one of my favorite theorems in mathematics.

I always wanted to know something about Kirszbraun, or at least to see his picture. Do you have any information about him? (I know only trivial things, like where he was publishing and that he was working in Warsaw.)

P.S. Thanks to Lukasz Grabowski, I fould this entry about Kirszbraun in Polish Biographical Dictionary.

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Curious enough - I wanted to ask a similar question about Dobinski (Dobinski's formula) but I had my doubts whether it's appropriate for MO or not :) – Harun Šiljak Oct 29 2010 at 5:28
Any chance you could comment on what you use Kirszbraun's theorem for? – Deane Yang Nov 30 2010 at 22:12
@Deane, love can not be explained, BUT, assume you have a closed curve in the plane and you map it to the plane on such a way that all distances decrease. Try to prove that the area bounded by this curve decrease without Kirszbraun... – Anton Petrunin Dec 1 2010 at 3:56

Apparently he has an entry written by Edward Marczewski in the polish biographical dictionary , according to the official site of the publisher. Somewhat suprisingly there seem to be no library which has this dictionary where I live so I can't help you any further.

Best of luck! I know the feeling - I once wanted to learn anything about Wegge-Olsen who wrote a nice book on K-theory. I haven't found anything in internet, but in the end I asked Paul Baum accidentally and he told me a bit about him.

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 Oh, Edward Marczewski, the one who proved that HausDim $\ge$ TopDim :) I will find this book; it is just a technical problem. – Anton Petrunin Oct 29 2010 at 2:46 "we want to thank L. Grabowski for bringing to our attention the entry about Kirszbraun in the Polish Biographical Dictionary." arxiv.org/pdf/1012.5636.pdf – Anton Petrunin Mar 23 2011 at 2:34 @Anton, cheers :-) – Łukasz Grabowski Mar 23 2011 at 15:33

It is very, VERY likely that Kirszbraun published only one paper. Namely, if you read carefully that short entry in Polish (written by E.Marczewski = Szpilrajn), you'll notice that in the penultimate sentence it says that upon completing his studies he got a job as an actuary ("aktuariusz", the person who calculates insurance costs) in an insurance company named "Przyszlosc"; therefore, it is quite reasonable to assume that he didn't continue any form of a(n academic) career as a research mathematician. On the other hand, a footnote an the title page of its Fundamenta paper clearly indicates that the paper is an abridged and improved version of his "Magister" (= Master) thesis, defended back in 1930 and prepared in the preceding 4-year period. So, it is rather safe to jump to the conclusion that Über die zusammenziehenden und Lipschitzschen Transformationen is his only publication, and that he didn't pursued any further mathematical research.

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This Banach biography web page in Polish says that Kirszbraun was born in 1903 or 1904 and died in 1942 (he is listed there among other Polish mathematicians who died in the course of WW II). Further googling revealed that his full name was Mojżesz David Kirszbraun. According to the Zentralblatt Math. database, he wrote just one paper in German (just as Mark mentioned in the comments):

Kirszbraun, M.D. Über die zusammenziehenden und Lipschitzschen Transformationen. (German) Fundam. Math. 22, 77-108 (1934).

Here is the link to the review of this paper by Freudental.

Perhaps some Polish colleague(s) could provide further details.

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Maybe he was Jewish? – Yuval Filmus Oct 28 2010 at 1:46
@Yuval: The name suggests he was. Kirszbraun sounds German but spelled Polish way (with sz for sch) + Mojżesz=Moses. – Anton Petrunin Oct 28 2010 at 2:09
@Anton, once you get a bit more information, maybe you could edit the Wikipedia page. – Alex Bartel Oct 28 2010 at 2:28
@Alex: I made a stub en.wikipedia.org/wiki/… but so far there is almost nothing to say – Anton Petrunin Oct 28 2010 at 3:35
@Anton: In which journals? Zentralblatt lists only one paper. I checked all Fundamenta issues from the very beginning. These are available online. – Mark Sapir Oct 28 2010 at 18:32