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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.

11 votes

Problems where we can't make a canonical choice, solved by looking at all choices at once

Sard's theorem provides such an example. Given a random smooth map between two manifolds (lets say compact and of the same dimension), there is no canonical way of constructing a regular value. But, S …
7 votes

What is your favorite isomorphism?

The De Rham Isomorphism.
6 votes

Where is number theory used in the rest of mathematics?

If Arakelov geometry counts as number theory, then, http://arxiv.org/pdf/math/0401029v1.pdf demonstrates the computation of the Analytic torsion (a purely analytic object involving the product of dete …
5 votes

What's your favorite equation, formula, identity or inequality?

$(A-\lambda _1) (A-\lambda _2) \ldots = 0$, the Cayley-Hamilton theorem.
5 votes

Why were matrix determinants once such a big deal?

1) The Chern-Weil theory of characteristic classes is built upon determinants of functions of curvature forms of vector bundles. 2) Feynman path integrals require determinants (but typically in infini …
5 votes

Proofs that require fundamentally new ways of thinking

Proving that subgroups of free groups are free requires the knowledge of topology, a completely different field which a priori does not have anything to do with groups.
4 votes

Tweetable Mathematics

Every non-constant complex polynomial has a complex root : If not the inverse is bounded analytic. Use Liouville. #FundamentalTheoremOfAlgebra
3 votes

Why do we need random variables?

Practically everything we measure in real life (for instance the time taken for an apple to fall on Newton's head) is "random" in the sense that if we perform the experiment again, we will not get the …
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3 votes

What should be learned in a first serious schemes course?

Being a differential geometer, it might be nice if you can point out analogies (perhaps even make them rigorous ?) to differential geometry. Like a scheme being flat over another is perhaps akin to a …
3 votes

A good book of functional analysis

I second Reed and Simon's methods of mathematical physics. However, if you are interested primarily in the applications of functional analysis to PDE, for the most part a couple of appendices of Evans …
1 vote

Tweetable Mathematics

There is no smooth surjection from $S^5$ to $S^6$. #Sard