Duality for abelian varieties

Let be an abelian variety over the algebraically closed field . In the previous post, we studied the Picard scheme , or rather its connected component at the identity. The main result was that was itself an abelian variety (in particular, smooth) of the same dimension as , which parametrizes precisely the translation-invariant line bundles on .

We also saw how to construct isogenies between and . Given an ample line bundle on , the map

is an isogeny. Such maps were in fact the basic tool in proving the above result.

The goal of this post is to show that the contravariant functor

from abelian varieties over to abelian varieties over , is a well-behaved duality theory. In particular, any abelian variety is canonically isomorphic to its bidual. (This explains why the double Picard functor on a general variety is the universal abelian variety generated by that variety, the so-called Albanese variety.) In fact, we won’t quite finish the proof in this post, but we will finish the most important step: the computation of the cohomology of the universal line bundle on .

Motivated by this, we set the notation:

Definition 11 We write for .

The main reference for this post is Mumford’s Abelian varieties.

 

The Picard scheme of an abelian variety

Let be an algebraically closed field, and a projective variety over . In the previous two posts, we’ve defined the Picard scheme , stated (without proof) the theorem of Grothendieck giving conditions under which it exists, and discussed the infinitesimal structure of (or equivalently of the connected component at the origin).

We saw in particular that the tangent space to the Picard scheme could be computed via

by studying deformations of a line bundle over the dual numbers. In particular, in characteristic zero, a simply connected smooth variety has trivial . To get interesting ‘s, we should be looking for non-simply connected varieties: abelian varieties are a natural example.

Let be an abelian variety over . The goal in this post is to describe , which we’ll call the dual abelian variety (we’ll see that it is in fact smooth). We’ll in particular identify the line bundles that it parametrizes. Most of this material is from David Mumford’s Abelian varieties and Alexander Polischuk’s Abelian varieties, theta functions, and the Fourier transform. I also learned some of it from a class that Xinwen Zhu taught last spring; my (fairly incomplete) notes from that class are here. 

 

The Picard scheme I

Let be an abelian variety over an algebraically closed field . If , then corresponds to a complex torus: that is, can be expressed complex analytically as where is a complex vector space of dimension and is a lattice (i.e., a -free, discrete submodule of rank ). In this case, one can form the dual abelian variety

At least, as defined is a complex torus, but it turns out to admit the structure of an abelian variety.

The purpose of the next few posts is to describe an algebraic version of this duality: it turns out that can be constructed as a scheme, purely algebraically. I’d like to start with a couple of posts on Picard schemes. A useful reference here is this article of Kleiman.

1. The Picard scheme analytically

Let be a smooth projective variety over the complex numbers . The collection of line bundles is a very interesting invariant of . Usually, it splits into two pieces: the “topological” piece and the “analytic” piece. For instance, there is a first Chern class map

which picks out the topological type of a line bundle. (Topologically, line bundles on a space are classified by their first Chern class.) The admissible topological types are precisely the classes in which project to -classes in under the Hodge decomposition.

 

Deformations of abelian varieties IV: the Serre-Tate theorem

Let be an abelian variety of dimension over a field of characteristic . In the previous posts, we saw that to give a deformation of over a local artinian ring with residue field was the same as giving a continuous morphism of rings : that is, the local deformation space is smooth on parameters.

There is another description of the deformation space in terms of the -divisible group, though. Given , we can form the formal scheme

where each is a finite group scheme of rank over . As a formal scheme, this is smooth: that is, given a small extension in and a morphism , there is an extension

In fact, we start by finding an extension (as is smooth), and then observe that this extension must land in some if landed in .

As a result, we can talk about deformations of this formal (group) scheme.

Definition 16 A deformation of a smooth, formal group scheme over over a ring is a smooth, formal group scheme over which reduces mod the maximal ideal (i.e., when restricted to -algebras) to .

Suppose . Using Schlessinger’s criterion, we find that (under appropriate finiteness hypotheses), must be prorepresentable by a power series ring for some . In other words, corresponds to a formal group over .

Suppose for instance that was a supersingular elliptic curve. By definition, each of the is a thickening of the zero point, and consequently is the formal completion of at zero. This data is equivalent to the formal group of , and as we just saw a deformation of this formal group over a ring is a formal group over which reduces to the formal group of mod the maximal ideal.

The main result is:

Theorem 17 (Serre-Tate) Let . There is an equivalence of categories between abelian schemes over and pairs where is an abelian variety over and a deformation of over .

So, in particular, deformations of over are equivalent to deformations of over .

In this post, I’ll describe an argument due to Drinfeld for this result (which I learned from Katz’s article).

 

Deformations of abelian varieties III: smoothness

(This is the third in a series of posts on deformations of abelian varieties; the first two posts are here and here.)

Let be an abelian variety. In the previous post, we saw that the functor

sending a local artinian algebra with residue field to the set of isomorphism classes of deformations of over was product-preserving, by showing that there were no infinitesimal automorphisms. The next step is to show that it is smooth: that is, given a deformation over and a surjection , it can be lifted to a deformation over . Together with Schlessinger’s theorem, this will yield a description of the local moduli space.

 

Deformations of abelian varieties II: rigidity

This is the second post in a series on deformations of abelian varieties. In the previous post, I described the basic outline of the goals and strategies, as well as a weak version of Schlessinger’s criterion useful in showing prorepresentability of smooth moduli problems without infinitesimal automorphisms. In this post, we’ll see that the deformation problem for abelian varieties satisfies the second condition above: that abelian varieties are rigid. The material here is very classical; I learned it from Oort’s article (from a summer school in the 1970s) and Katz’s article. Most of the material in this post comes from chapter 6 of Mumford’s GIT book, which is surprisingly readable without knowledge of any other parts of it.

Let be an artin ring, and let be an abelian scheme. Consider a morphism of -group schemes

inducing the identity on the special fiber. We would like to show that it is the identity, as in the next proposition:

Proposition 7 Such a morphism is necessarily the identity: that is, an infinitesimal deformation of an abelian variety has no nontrivial infinitesimal automorphisms.

This will imply prorepresentability of the deformation functor, using the general form of Schlessinger’s theorem.

 

Deformations of abelian varieties I: formalism

I’ve been trying to learn something about deformations of abelian varieties lately. One of the big results is:

Theorem 1 The “local moduli space” of an abelian variety is smooth of dimension , if .

Why might you care about this result? Let’s say you’re in the case , so then presumably you’re interested in the stack of elliptic curves. This is a Deligne-Mumford stack: that is, there are enough étale maps from affine schemes, and as a result it makes sense to talk about the strict henselianization at a point (or the completion at a “point” from an algebraically closed field). Then, the point of the above theorem is that you can work out exactly what that is: it’s a one-dimensional thing. This isn’t too surprising, because the isomorphism class of an elliptic curve depends on one parameter (the -invariant). So knowing the deformation theory of elliptic curves lets you say what looks like, very locally.

Let’s make this a bit more precise. An actual formulation of the theorem would specify what “local moduli” really means. For us, it means deformations. A deformation of an abelian variety over an artin local ring with residue field is the data of an abelian scheme (that is, a proper flat group scheme with abelian variety fibers) together with an isomorphism of abelian varieties .

Theorem 2 Let be an algebraically closed field, and let be an abelian variety. Then the functor of deformations of is prorepresentable by for the ring of Witt vectors over .

In other words, to give a deformation of over an artin local with residue field is the same as giving a homomorphism of local rings

The relevance of here essentially comes from the fact that every complete (e.g. artin) local ring with residue field is uniquely a continuous -algebra. If we restricted ourselves to -algebras, we could replace by .

 

Line bundles on abelian varieties II

This, like the previous and probably the next few posts, is an attempt at understanding some of the ideas in Mumford’s Abelian Varieties.

Let be an abelian variety. Last time, I described a formula which allowed us to express the pull-back of a line bundle as

This was a special case of the so-called “theorem of the cube,” which allowed us to express, for three morphisms , the pull-back

in terms of the various pull-backs of partial sums. Namely, we had the formula

In this formula, we take and constant maps at , respectively. Let , etc. denote the translation maps on . Then are the relevant sums, and we have

We get the theorem of the square:

Theorem 1 (Theorem of the square)Let be an abelian variety, , and points. Then

In the rest of this post, I’ll describe some applications of this result, for instance the fact that abelian varieties are projective. 

 

Line bundles on abelian varieties

1. Introduction

Let be an abelian variety over an algebraically closed field , of dimension . One of the basic tools in analyzing the properties of is the study of line bundles on . It’s a little non-intuitive to me why this is the case, so I’m going to try to motivate the topic.

Given , we are interested in questions of the following form: What is the structure of the -torsion points ? To compute , we are reduced to computing the degree of multiplication by ,

(which is a morphism of varieties). In fact, we will show that is a finite flat morphism, and determine the degree of , which is thus the cardinality of the fiber over . The determination will be done by analyzing how acts on line bundles. For a symmetric line bundle over , one can prove the crucial formula

and comparing the Hilbert polynomials of and , one can get as a result

Another way of phrasing this deduction is the following. By the Hirzebruch-Riemann-Roch formula and the parallelizability of an abelian variety, we have

for any line bundle . Consequently, in view of the asserted formula , we find for any line bundle :

Choosing to be a high power of a very ample line bundle, we will have . Now we can appeal to the following result:

Theorem 1 Let be a proper scheme over a field. Let be a finite group scheme, and let be a -torsor. Then if is a coherent sheaf on , we have

It follows from this result that . For prime to the characteristic, the morphism can be seen to be separable, and it follows as a result there are points of -torsion on .