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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.

15 votes

Consolidation: Aftermathematics of fads

In the seventies and eighties of the preceding century, existence and classification of vector bundles on projective space $\mathbb P^n$ were all the rage, with contributions from such luminaries as A …
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 …
2 votes

Interesting examples of vacuous / void entities

A The empty set is a covering map of any topological space. More generally, a covering map needn't be surjective (although many books claim just that). For example the inclusion of a closed and open …
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 …
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
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!
3 votes

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

$SL_2\mathbb R$ and its evil universal covering.
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' …
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 …
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 …
10 votes

Any reference on multilinear algebra

Here are three excellent books. Tensor Spaces and Exterior Algebra by Takeo Yokonuma. Translations of Mathematical Monographs, volume 108, AMS 1992 You can browse it in Google books here Laurent Sc …
16 votes

Proof synopsis collection

Fermat's little theorem: $n^p\equiv n \; (mod \;p)$ for $p$ prime and all integers $n$. Synopsis of proof: Reduce to nontrivial case where $p$ doesn't divide $n$, interpret as equality in field of $p …
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 $n$.
13 votes

What should be learned in a first serious schemes course?

If you decide to teach a more arithmetically flavoured algebraic geometry, students should be made aware that schemes over a ring $A$ are stranger than they might think. For example $A$-rational poin …
105 votes

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

Is $e+\pi $ rational?

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