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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.

61 votes

Is amateur research in mathematics viable?

This is possible. I have at least two friends who studied mathematics (in the graduate school), did not defend their PhD, and found some jobs not related to mathematics. Still they do research, and pu …
9 votes

Conic sections in high dimensions

The answer is no. A spherical cone is $$x_0^2=x_1^2+\ldots+x_n^2.$$ Intersecting it with a hyperplane $x_0=c^Tx$, where $c$ is a (column) vector we obtain a quadratic form with martix $$I-cc^T,$$ that …
Alexandre Eremenko's user avatar
8 votes

A search for theorems which appear to have very few, if any hypotheses

For every holomorphic map from the complex plane to the Riemann sphere, and every $R<\arccos(1/3)$ there exists a disk of radius $R$ in the image in which an inverse holomorphic branch exists. (The co …
1 vote

A search for theorems which appear to have very few, if any hypotheses

Every bounded analytic function in the unit disk has radial limits almost everywhere.
12 votes

Proof or citation?

On my opinion, the main criterion for a reference is that it must be AVAILABLE. Either on Internet or in most university libraries. An unpublished thesis in Ukrainian which is not available on Interne …
23 votes

Fascinating moments: equivalent mathematical discoveries

An example which always puzzled me is J. Milnor's paper entitled Eigenvalues of the Laplace operator on certain manifolds, Proc. Nat. Acad. Sci. U.S.A. 51 1964, 542. The whole paper occupies about ha …
16 votes

A search for theorems which appear to have very few, if any hypotheses

There are infinitely many prime numbers. Every integer is a product of primes, in essentially unique way. (Theorems with NO hypotheses:-)
16 votes

Can pure mathematics harness citizen science?

I have no proposal, but only want to mention a historical example of what can be called "Citizen science" in mathematics. http://www.computer.org/portal/web/csdl/doi/10.1109/85.707573. This is how th …
4 votes

Golden ratio in contemporary mathematics

Yes, it does: Lyubich, Mikhail; Milnor, John The Fibonacci unimodal map. J. Amer. Math. Soc. 6 (1993), no. 2, 425–457. and two more papers of the same authors studying what they call Fibonacci map.
16 votes

Special rational numbers that appear as answers to natural questions

Rational number 1/4 occurs as a universal constant in several problems of Analysis. The most famous is the "Koebe 1/4 Theorem". Let $f(z)=z+\ldots$ be an injective holomorphic function in the unit dis …
12 votes

When is 2 qualitatively different from 3?

Complex numbers exist only in dimension 2. That is the only multiplication laws on $R^n$ which satisfy all field axioms exist for $n=1$ (real numbers) and $n=2$ (complex numbers).
0 votes

Idempotent solutions to the implict function theorem other than the identity?

Examples are abundant. If $x_0$ is a point such that $g(x_0,x_0)=0$ and $g_y(x_0,x_0)\neq 0$ then $g_x(x_0,x_0)\neq 0$ the Implicit Function Theorem, there is a unique function $y=\phi(x), \phi(x_0)=x …
Alexandre Eremenko's user avatar
29 votes

Where can square roots come from when they are not distances?

$i=\sqrt{-1}$ has no apparent relation with any distance. Also $\int_{-\infty}^\infty e^{-x^2}dx=\sqrt{\pi}.$
11 votes

Negative impact of wrong or non-rigorous proofs

There are several examples of wrong proofs which were believed to be correct for some time, but I would not say that they "did damage to mathematics". One of the most famous examples is Dulac's proof …
9 votes

On similar concepts in mathematics whose similarity is a non-trivial fact.

In my example, the similarity did not require a hard proof but it was not seen for many years for the reasons which I would call "social". In 1928 Weil (and simultaneously Siegel) defined and studied …

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