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Questions about mathematics which don't fall into the other arXiv categories. If you have a general question about mathematics but it is not research level, it's off-topic but it might be welcomed on Mathematics Stack Exchange.

14 votes

A set for which it is hard to determine whether or not it is countable.

Here is an example where it is hard in a proof-theoretic sense to determine whether a set is countable. Jan Reimann and Theodore A. Slaman (in the paper Randomness for continuous measures) study ran …
2 votes

Examples of conjectures whose direct falsity implies different consequences than indirect fa...

Here's an example from computability theory. Statement $A$:"The Turing degrees are linearly ordered". Statement $B$: "The $\Sigma^0_1$ Turing degrees are linearly ordered". Statement $A$ was refut …
Bjørn Kjos-Hanssen's user avatar
13 votes
Accepted

Example of a function that behaves like another function

This may be more than 15 symbols, but $$ f(x)= x + \frac{1}{1+(x+\delta-1)^{2n/(2n-1)}} $$ where $\delta>0$ is small and $n$ is large. For $n=2$ and $\delta=1/10$ it looks like this: (source) ED …
Bjørn Kjos-Hanssen's user avatar
6 votes

What is the term for combining functions $f_1,f_2,\dots,f_n$ into a tuple $(f_1,\dots,f_n)$?

You could call $$\mathbf r(t)=\langle x(t), y(t), z(t)\rangle$$ the vector function (or vector?) of $(x,y,z)$.
Bjørn Kjos-Hanssen's user avatar
1 vote

Are there any organized websites for seminar/conference videos?

You can subscribe to tags on Vimeo, and in principle arXiv tags like pr.probability could be used http://vimeopro.com/staff/tutorials/video/741488
21 votes

Why is the Gaussian so pervasive in mathematics?

(The sort of obvious answer from teaching statistics several times:) The sum of two independent normal random variables is again normal, i.e., the shape of the distribution is unchanged under addition …
Bjørn Kjos-Hanssen's user avatar
10 votes

Still Difficult After All These Years

I would like to believe that by 2110 the Langland's program would be reduced to a 10-page pamphlet (with complete proofs) that I could read over breakfast. Is this belief plausible? In a way, no …