# “Every scheme as a sheaf” references?

I have sometimes hard time reading papers that are written in the language of schemes being replaced by the functors they represent (I have especially homotopy scheme theory in mind).

I think the topic is connected to topoi and Grothendieck topologies, but for now I'm looking for something simple, just the working overview of the language of representable functors, what is a scheme, etc.

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This is called the "functor of points" approach, which you might want to look up. –  Harry Gindi Dec 31 '09 at 20:00
Yes, this is algebraic geometry based on presheave view point –  Shizhuo Zhang Dec 31 '09 at 22:12
I think I got the link of the paper by Kontsevich-Rosenberg:) webdoc.sub.gwdg.de/ebook/serien/e/mpi_mathematik/2004/35.ps. –  Shizhuo Zhang Jan 1 '10 at 6:56

You can start with these notes by Vistoli, which talk about that stuff in the direction of doing stacks and descent theory. The other articles in FGA explained might be useful, as they do a lot of moduli space construction (ie, prove that a given functor is representable). Another place to look would be in Eisenbud and Harris's "Geometry of Schemes" where they characterize schemes among sheaves on CommRing, towards the end of the book.

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I feel like Vistoli's notes don't address the question above. It's a great reference for sheaves, grothendieck topologies, and descent, but it doesn't address the "functor of points" approach to schemes, since it doesn't really talk about ringed and locally ringed spaces from a category-theoretic viewpoint. It's more about generalizing QCoh than Sch. –  Harry Gindi Jan 1 '10 at 2:24
Yeah, which is part of why I mentioned the other stuff. Though Vistoli's notes do talk about what it means for a functor to be a sheaf, which, though not explicitly in his question, is implicitly something he's curious about from the title, which is why it leaped to mind. –  Charles Siegel Jan 1 '10 at 2:46
Yeah, I gave you a +1 anyway for the other stuff. –  Harry Gindi Jan 1 '10 at 14:24
Yeah, I read the paper anyway and I like it, thanks! –  Ilya Nikokoshev Jan 1 '10 at 18:55

Chapter I of Demazure and Gabriel's Introduction to Algebraic Geometry and Algebraic Groups develops the basic notions of algebraic geometry from the functor of points approach.

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This one is nice, but I wonder if there is an online equivalent? –  Ilya Nikokoshev Jan 1 '10 at 18:53

It is a very nice question. Functor view point algebraic geometry was proposed by Gabriel and later developed by Grothendieck.

Actually, Kontsevich and Rosenberg developed noncommutaive algebraic geometry completely based on this point of view explicitly. They take the presheaf $\text{Alg}^{op} \rightarrow \text{Set}$ as a noncommutative space and developed flat descent theory, the theory of noncommutative smooth space, theory of noncommutative stack. As an interesting example, they defined noncommutative grassmannian, group scheme, general flag vairety as a presheaves and using descent theory of quasi coherent sheaves to glue affine presheaves together according to the so called "smooth topology".

I attended a lecture course last semester, he proved a theorem which "shows" that "One can do algebraic geometry only using presheaves rather than sheaves, if one need sheaves, just take sheafification and all the properties will hold".

There are the following references:

All these papers are available in Max Plank preprint series (search Kontsevich or Rosenberg in "author" and leave other blank empty).

If you take a look at the first paper, just disregard the notion of "Q-category" which is a technique tool to generalize grothendieck topologies because in noncommutative case, flat morphism does not respect to base change in general.

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Question about these papers: Is there a more elementary version of these papers for the case of CommRing, or are these papers the first papers to actually construct Sch completely functorially? –  Harry Gindi Jan 4 '10 at 4:02
1. Read the excellent proof of Yoneda's lemma in wikipedia.

2. Read the short section on "functor of points" in Mumford's "Lectures on curves on an algebraic surface. For reviewing the basics of schemes etc., Mumford's Red book will be useful.

I think this is the simplest and most compact way of getting started. Then one can move on with the sources suggested by other people in other answers to this question.

I hope my answer is not "too introductory".

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I think Eisenbud-Harris is probably the best place to get started. Alternatively, I think these notes by Brian Osserman are nice.

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You may also find the notes "Introduction to Functorial Algebraic Geometry" useful. They are based on a course given by Groethendieck - and can be found here. Unfortunately they're a little rough around the edges sometimes and slightly dizzying to read because of the scanning.

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Yes, I took a look at that scanned version. But it is hard to read because of scanning. I wonder where I could get the copy of this book. I mean buy this book. –  Shizhuo Zhang Jan 12 '10 at 20:11