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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.

17 votes

Different ways of thinking about the derivative

Algebraic: a derivation on a ring $R$ is an additive map $R \rightarrow R$ that satisfies the product rule (with suitable generalizations allowing modules, etc.) This is related to the Symbolic way o …
22 votes

Notable mathematics during World War II

Eilenberg and Mac Lane's papers on category theory started appearing: "Natural Isomorphisms in Group Theory" in the Proc. National Acad. Sci. USA in 1942 and "General Theory of Natural Equivalences" i …
35 votes

Fields of mathematics that were dormant for a long time until someone revitalized them

Modular forms were actively studied by number theorists Hecke and Siegel in the 1930s, but it was not widely appreciated. Around the same time Hardy, in a series of lectures on Ramanujan's work deliv …
32 votes

What are the worst notations, in your opinion?

Writing a finite field of size $q$ as $\mathrm{GF}(q)$ instead of as $\mathbf{F}_q$ always rubbed me the wrong way. I know where it comes from (Galois Field), and I think it is still widely used in c …
29 votes

Demonstrating that rigour is important

Nonexistence theorems can not be demonstrated with numerical evidence. For example, the impossibility of classical geometric construction problems (trisecting the angle, doubling the cube) could only …
17 votes

Special rational numbers that appear as answers to natural questions

For a prime number $p$, the number of nonisomorphic groups of order $p^n$ is $p^{(2/27)n^3 + O(n^{8/3})}$. I was surprised when I first saw this formula with leading coefficient $2/27$ in the exponent …
33 votes

Examples of common false beliefs in mathematics

After learning that the Witt vectors of a finite field of size $p^n$ is the ring of integers of the unramified extension of ${\mathbf Q}_p$ of degree $n$, I think lots of people then think that the Wi …
11 votes

Magic trick based on deep mathematics

Here is a card trick from Edwin Connell's Elements of Abstract and Linear Algebra, page 18 (it can be found online). I always do this trick to my undergraduate number theory class in the first minute …
9 votes

Ways to prove the fundamental theorem of algebra

Here is the proof by Pukhlikov (1997) at http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=mp&paperid=6&option_lang=eng which Ilya mentioned as being only in Russian so far. What I present …
12 votes

Applications of Group Theory Which Motivate Theoretical Questions?

I had the same question before I taught a course that was largely group theory. Here is the webpage I created to address the issue: http://www.math.uconn.edu/~kconrad/math216/whygroups.html
15 votes

What would you want to see at the Museum of Mathematics?

There are many interesting films at the site http://www.etudes.ru/ (not in English): curves of constant width, Pick's theorem, geometry of polyhedra, an infinite staircase with the harmonic series, me …
6 votes

What well known results with countability assumptions can be naturally extended to uncountab...

Here are some examples from algebra where finiteness assumptions can be removed. In the first two, the statement of the more general result is unchanged, but the third result has to be expressed in a …
14 votes

Algebraic number theory and applications to properties of the natural numbers.

The truncated exponential polynomial $1 + x + x^2/2! + ... + x^n/n!$ is irreducible for all positive integers $n$. This result is due to Schur and the proof uses prime ideal factorizations in the numb …
13 votes

Counterexamples in algebra?

If $f$ and $g$ are relatively prime in ${\mathbf Q}[X]$ then the mapping ${\mathbf Q}[X]/(fg) \rightarrow {\mathbf Q}[X]/(f) \times {\mathbf Q}[X]/(g)$ given by $h \bmod fg \mapsto (h \bmod f, h \bmod …
15 votes

Ways to prove the fundamental theorem of algebra

Here is a translation into English of a second "real" proof from the journal Ilya mentioned in his answer. This proof is due to Petya Pushkar', in the 1997 paper titled О некоторых топологических док …

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