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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.

170 votes

Most memorable titles

The flattering lie You Could Have Invented Spectral Sequences by Timothy Y. Chow.
159 votes

What are some examples of colorful language in serious mathematics papers?

Andre Weil (Oeuvres, vol. 2, page 558) purporting to be R.Lipschitz writing from Hades: "Unfortunately, it appears that there is now in your world a race of vampires, called referees, who clamp down m …
105 votes

Not especially famous, long-open problems which anyone can understand

Is $e+\pi $ rational?
88 votes

Widely accepted mathematical results that were later shown to be wrong?

In 1882 Kronecker proved that every algebraic subset in $\mathbb P^n$ can be cut out by $n+1$ polynomial equations. In 1891 Vahlen asserted that the result was best possible by exhibiting a curve in …
73 votes
Accepted

Cardinalities larger than the continuum in areas besides set theory

The Zariski tangent space at any point of a positive dimensional $C^1$-manifold $X$ has dimension $2^{2^{\aleph_0}}= 2^{\frak c}$. Let me explain in the case when $X=\mathbb R$. Consider the ring $C^ …
Georges Elencwajg's user avatar
70 votes

What elementary problems can you solve with schemes?

If $I,J \subset A$ are comaximal ideals in a commutative ring $A$, i.e. $I+J=A$, then for all $n,m \in \mathbb N$ the ideals $I^n$ and $J^m$ are also comaximal. Proof: $\emptyset= V(A)=V(I+J)=V(I)\ …
53 votes

Pseudonyms of famous mathematicians

Rainich=Rabinowitsch (of trick fame : cf. Nullstellensatz). Here is an anecdote related by Bruce P. Palka, Editor of American Mathematical Monthly in Vol.111 (2004) of that journal (page460). Rai …
52 votes

Fundamental problems whose solution seems completely out of reach

Is every algebraic curve in $\mathbb P^3$ the set-theoretic intersection of two algebraic surfaces ? Not known!
48 votes

Errata for Atiyah–Macdonald

Dear Tim, on page 31 they consider a ring $A$ and two $A$- algebras defined by their structural ring morphisms $f:A\to B$ and $g:A\to C$. They then define the tensor product as a ring $D=B\otimes _A C …
44 votes

Theorems that are 'obvious' but hard to prove

That $\mathbb R^n$ has topological dimension $n$. In a similar vein that affine space $\mathbb A^n_k$ over a field $k$ has Zariski dimension $n$.
43 votes

What should be learned in a first serious schemes course?

Since in 2007-2008 you evoked [ Class 24, §1.8, The problem with locally free sheaves] the equivalence between locally free sheaves and vector bundles on a scheme, the following point, potentially co …
38 votes
Accepted

Justifying a theory by a seemingly unrelated example

[In front of a blackboard, in an office at Real College] Skeptic: And why should I care about holomorphic functions? Holomorphic enthusiast:$\;$ Can you compute $\quad$ $\sum_{n={-\infty}}^{\infty …
34 votes

Theorems for nothing (and the proofs for free)

Wedderburn's theorem: "Every finite division ring is a field." This is really astonishing if you think of quaternions: nothing analogous in the finite case. Then of course the classification of finit …
20 votes

What should be learned in a first serious schemes course?

Dear Ravi, here is a small suggestion. I think one might emphasize as soon as possible that the subschemes of an affine scheme $Spec A$ exactly correspond to the set of ideals of the ring $A$.( I don' …
16 votes

Justifying a theory by a seemingly unrelated example

Let us call "division algebra over $\mathbb R$" a finite-dimensional vector space $A$ equipped with a bilinear map $A \times A \to A: (a,b) \mapsto a \bullet b$ , such that $a\bullet b=0$ implies $a= …

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