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Questions designed to generate a "big list" of certain results, examples, conjectures, etc. via many individual answers, each contributing one or a few instances. Such a question should typically be in Community Wiki mode (CW); after asking, please, flag for moderators attention requesting the question to be made CW.

9 votes

Fixed point theorems

I forgot who proved it, but the statement is nice and very easy to prove: A function $f:X\to X$ is fixed point free if and only if there is a partition of $X$ into three subsets s.t. $f$ maps each of …
5 votes

Interesting examples of generic behavior of mathematical objects being either unreasonably s...

Their are many such examples in the theory of finite dimensional Banach spaces. Suppose that $X$ is an $n$ dimensional Banach space. If you take a random subspace of dimension $k$, then for some value …
19 votes

Theorems first published in textbooks?

It happened to me once. While visiting the Institute for Advanced Studies at the Hebrew University of Jerusalem in 1976-77 I answered a question from the preliminary manuscript of volume 1 of Lindens …
7 votes

Mathematicians who were late learners?-list

R. H. Bing taught high school for several years before entering graduate school.
4 votes

Prominent non-mathematical work of mathematicians

Per Enflo is famous for solving Banach's basis problem, Grothendieck's approximation problem, and the invariant subspace problem for general Banach spaces, and has other fundamental research in linear …
17 votes

Are there proofs that you feel you did not "understand" for a long time?

The proof that the trace is well defined for square matrices looked like symbol pushing to me. Many years later I realized that the proof is nonsense if you live in certain infinite dimensional world …
23 votes

Quick proofs of hard theorems

Lomonosov's 1973 proof that every compact operator $T$ has a hyperinvariant subspace (i.e., a subspace that is invariant for every operator that commutes with $T$) was much simpler than proofs existin …
2 votes

Individual mathematical objects whose study amounts to a (sub)discipline?

$C[0,1]$. Since every separable metric space embeds isometrically into $C[0,1]$ and every separable Banach space embeds isometrically isomorphically into $C[0,1]$, the study of $C[0,1]$ includes the …
21 votes
Accepted

A book you would like to write

Gosh, what a question, Gil. What is your answer? I have written many books in my head, but I am much too lazy actually to write a book. I guess my first choice would be Geometric nonlinear function …
3 votes

Applications of Brouwer's fixed point theorem

One standard consequence of Brouwer's theorem is Borsuk's antipodal mapping theorem, which in turn is used to prove that if $E$, $F$ are subspaces of a normed space and the dimension of $E$ is strictl …
6 votes

Fundamental problems whose solution seems completely out of reach

Is every complemented subspace of $C[0.1]$ isomorphic to $C(K)$ for some compact metric space $K$? Is every infinite dimensional complemented subspace of $L_1[0.1]$ isomorphic either to $L_1[0.1]$ or …
10 votes

What are some results in mathematics that have snappy proofs using model theory?

There are many results in Banach space theory that are proved via ultraproducts or non standard hulls, and most books on the subject contain a few. One nice one that is easy to state is that a Banach …
Bill Johnson's user avatar
  • 31.5k
12 votes

Your favorite surprising connections in mathematics

I agree with Zavosh that Jones' linking of Von Neumann algebras to knot theory is one of the great connections in modern times. Closer to home for me is Pisier's use of a theorem of Beurling on holom …
30 votes

How helpful is non-standard analysis?

Nonstandard hulls of spaces are used all the time in Banach space theory, so much so that books devote sections to the construction of ultraproducts of Banach spaces (e.g. Absolutely summing operators …
Bill Johnson's user avatar
  • 31.5k
15 votes

Famous mathematical quotes

Jean Bourgain, in response to the question, "Have you ever proved a theorem that you did not know was true until you made a computation?" Answer: "No, but nevertheless it is important to do the comp …

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