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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

33 votes

What is the proper initiation to the theory of motives for a new student of algebraic geometry?

Asking for the moon, in my view. Here are 10 "heuristics" that try to place the theory. NB that many people stop at #1, as if this were enough. None of these points is particularly easy to track in th …
Charles Matthews's user avatar
28 votes
Accepted

Origin of terms "flag", "flag manifold", "flag variety"?

Armand Borel's Bourbaki Seminar 121 Groupes algébriques is from 1955, and uses "drapeau" (page 7). (It's online at archive.numdam.org.) This may not be the earliest occurrence, but there is a good rea …
Charles Matthews's user avatar
27 votes

Why is the Hodge Conjecture so important?

Here are three points, and you'd have to care about at least one of them, I think. (1) A (co)homology class is better understood if it is represented geometrically in some way. This point really bel …
Charles Matthews's user avatar
24 votes

What elementary problems can you solve with schemes?

The "classical" example is surely duality of abelian varieties. If you want this duality to work over finite fields (or in characteristic p generally), it becomes apparent that you can't work with var …
17 votes

Is there a "crash-course" book on Abelian varieties (e.g., an introduction for physicists)?

Topics in Complex Function Theory, Abelian Functions and Modular Functions of Several Variables by C. L. Siegel is a standard reference using complex function theory. There are older works (e.g. H. F. …
Charles Matthews's user avatar
13 votes

Elementary examples of the Weil conjectures

The grandfather of all examples is by Gauss: http://en.wikipedia.org/wiki/Weil_conjectures#Background_and_history Of course Gauss didn't mention finite fields other than the prime field. I think it …
Charles Matthews's user avatar
11 votes

What is the general geometric interpretation of modules in algebraic geometry?

As you say, projective modules correspond in a highly moral way to vector bundles. Bundles pull back but do not push forward, in topologists' terms. This might be a good point at which to start. Sheav …
Charles Matthews's user avatar
11 votes

Where do all these projection formulas come from?

The first (set theory) formula is generalised in categorical logic to what is called "Frobenius reciprocity" there, and is then part of the handling of the existential quantifier (a natural way to go …
Charles Matthews's user avatar
9 votes

What is the geometry of an undecidable diophantine equation?

You have a typical recursively enumerable set S of integers, and a set X of lattice points cut out by a multivariate polynomial. We are talking about S being the projection (onto one axis) of X. Given …
Charles Matthews's user avatar
8 votes

Explaining the number field-function field analogy

I think your statement could usefully be sharpened in a couple of ways. Firstly, the state-of-the-art is that true statements for function fields are expected to have analogues for number fields. Th …
Charles Matthews's user avatar
7 votes

What, if anything, makes homogeneous polynomials so great?

Well, if you are interested in counting solutions mod p, you could note that the "good" formulae are indeed related to the homogeneous approach/projective space. It is not just a question of restoring …
Charles Matthews's user avatar
6 votes
Accepted

Sum of reciprocals of primes modulo which a polynomial has a root

This should follow from the Theorem of Frobenius mentioned on p. 7 (PDF numbering) of http://websites.math.leidenuniv.nl/algebra/chebotarev.pdf.
Charles Matthews's user avatar
6 votes

Life after Hartshorne (the book, not the person)...

Abelian varieties.
6 votes
Accepted

Preliminaries for Mumford's Abelian Varieties

To discuss generally first, the book was written up by C. P. Ramanujam, and he was more conscientious than usual in trying to tie down Mumford's lectures to existing references. Still, it is quite har …
Charles Matthews's user avatar
5 votes

What is the advantage of the approach of valuations to the Riemann-Roch Theorem for curves (...

You didn't mention Weil, Basic Number Theory, where the case of a finite field of constants is handled, really only using Pontryagin duality. There is an elegant theory of John Tate that seems somewha …
Charles Matthews's user avatar

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