180

108

I'm a graduate student who's been learning about schemes this year from the usual sources (e.g. Hartshorne, Eisenbud-Harris, Ravi Vakil's notes). I'm looking for some examples of elementary self-contained problems that scheme theory answers - ideally something that I could explain to a fellow grad student in another field when they ask "What can you do with schemes?"

Let me give an example of what I'm looking for: In finite group theory, a well known theorem of Burnside's is that a group of order $p^a q^b$ is solvable. It turns out an easy way to prove this theorem is by using fairly basic character theory (a later proof using only 'elementary' group theory is now known, but is much more intricate). Then, if another graduate student asks me "What can you do with character theory?", I can give them this example, even if they don't know what a character is.

Moreover, the statement of Burnside's theorem doesn't depend on character theory, and so this is also an example of character theory proving something external (e.g. character theory isn't just proving theorems about character theory).

I'm very interested in learning about similar examples from scheme theory.

What are some elementary problems (ideally not depending on schemes) that have nice proofs using schemes?

Please note that I'm **not** asking for large-scale justification of scheme theoretic algebraic geometry (e.g. studying the Weil conjectures, etc). The goal is to be able to give some concrete notion of what you can do with schemes to, say, a beginning graduate student or someone not studying algebraic geometry.

My gut feeling is that this question has appeared on MO at least two times. But anyway, 1+, since I also wonder if there are

elementaryproblems. – Martin Brandenburg – 2011-03-21T16:32:23.8777Once you have enough scheme-theoretic machinery, there's an almost trivial proof that the nonsingular points of a variety form a dense open subset; see III.4 Prop. 3 of Mumford's Red Book, or Hartshorne Corollary II.8.16. The basic idea is to show that, in an appropriate sense, the variety is nonsingular at its generic point. – Charles Staats – 2011-03-21T18:18:59.680

37Fermat's Last Theorem is a completely elementary problem, whose very nice proof uses schemes in an essential way... – Kevin Buzzard – 2011-03-21T19:09:39.533

see also the following challenge (not elementary though)

http://math.columbia.edu/~dejong/wordpress/?s=challenge

4Community wiki? – Yemon Choi – 2011-03-22T05:25:41.460

9

I'm not sure how many of these actually need schemes, but you might like this:

http://math.stanford.edu/~vakil/725/funprobs.pdf

Possibly related: http://mathoverflow.net/questions/76942/what-motivates-modern-algebraic-geometry-for-a-combinatorial-constructive-algebra

– darij grinberg – 2013-04-04T00:20:16.610