March 21, 2013

Duality for abelian varieties

Posted by Akhil Mathew under algebraic geometry | Tags: , , |
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Let {X} be an abelian variety over the algebraically closed field {k}. In the previous post, we studied the Picard scheme {\mathrm{Pic}_X}, or rather its connected component {\mathrm{Pic}^0_X} at the identity. The main result was that {\mathrm{Pic}^0_X} was itself an abelian variety (in particular, smooth) of the same dimension as {X}, which parametrizes precisely the translation-invariant line bundles on {X}.

We also saw how to construct isogenies between {X} and {\mathrm{Pic}^0_X}. Given an ample line bundle {\mathcal{L}} on {X}, the map

\displaystyle X \rightarrow \mathrm{Pic}^0_X, \quad x \mapsto t_x^* \mathcal{L} \otimes \mathcal{L}^{-1}

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

\displaystyle X \mapsto \mathrm{Pic}^0_X

from abelian varieties over {k} to abelian varieties over {k}, 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 X \times \mathrm{Pic}^0_X.

Motivated by this, we set the notation:

Definition 11 We write {\hat{X}} for {\mathrm{Pic}^0_X}.

The main reference for this post is Mumford’s Abelian varieties. (more…)

 

March 20, 2013

The Picard scheme of an abelian variety

Posted by Akhil Mathew under algebraic geometry | Tags: , , , |
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Let {k} be an algebraically closed field, and {X} a projective variety over {k}. In the previous two posts, we’ve defined the Picard scheme {\mathrm{Pic}_X}, stated (without proof) the theorem of Grothendieck giving conditions under which it exists, and discussed the infinitesimal structure of {\mathrm{Pic}_X} (or equivalently of the connected component {\mathrm{Pic}^0_X} at the origin).

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

\displaystyle T \mathrm{Pic}^0_X = H^1(X, \mathcal{O}_X),

by studying deformations of a line bundle over the dual numbers. In particular, in characteristic zero, a simply connected smooth variety has trivial {\mathrm{Pic}^0_X}. To get interesting {\mathrm{Pic}_X^0}‘s, we should be looking for non-simply connected varieties: abelian varieties are a natural example.

Let {X} be an abelian variety over {k}. The goal in this post is to describe {\mathrm{Pic}^0_X}, 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.  (more…)

 

March 19, 2013

The Picard scheme II: deformation theory

Posted by Akhil Mathew under algebraic geometry | Tags: , , , , |
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Let {X} be a projective variety over the algebraically closed field {k}, endowed with a basepoint {\ast}. In the previous post, we saw how to define the Picard scheme {\mathrm{Pic}_X} of {X}: a map from a {k}-scheme {Y} into {\mathrm{Pic}_X} is the same thing as a line bundle on {Y \times_k X} together with a trivialization on {Y \times \ast}. Equivalently, {\mathrm{Pic}_X} is the sheafification (in the Zariski topology, even) of the functor

\displaystyle Y \mapsto \mathrm{Pic}(X \times_k Y)/\mathrm{Pic}(Y),

so we could have defined the functor without a basepoint.

We’d like to understand the local structure of {\mathrm{Pic}_X} (or, equivalently, of {\mathrm{Pic}^0_X}), and, as with moduli schemes in general, deformation theory is a basic tool. For example, we’d like to understand the tangent space to {\mathrm{Pic}_X} at the origin {0 \in \mathrm{Pic}_X}. The tangent space (this works for any scheme) can be identified with

\displaystyle \hom_{0}( \mathrm{Spec} k[\epsilon]/\epsilon^2, \mathrm{Pic}_X). (more…)

 

March 19, 2013

The Picard scheme I

Posted by Akhil Mathew under algebraic geometry | Tags: , , , |
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Let {X} be an abelian variety over an algebraically closed field {k}. If {k = \mathbb{C}}, then {X} corresponds to a complex torus: that is, {X} can be expressed complex analytically as {V/\Lambda} where {V} is a complex vector space of dimension {\dim X} and {\Lambda \subset V} is a lattice (i.e., a {\mathbb{Z}}-free, discrete submodule of rank {2g}). In this case, one can form the dual abelian variety

\displaystyle X^{\vee} = \hom(X, S^1) = \hom_{\mathrm{cont}}(V/\Lambda, \mathbb{R}/\mathbb{Z}) \simeq \hom_{\mathbb{R}}(V, \mathbb{R})/2\pi i \hom(\Lambda, \mathbb{Z}).

At least, {X^{\vee}} 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 {X^{\vee}} 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 {X } be a smooth projective variety over the complex numbers {\mathbb{C}}. The collection of line bundles {\mathrm{Pic}(X)} is a very interesting invariant of {X}. Usually, it splits into two pieces: the “topological” piece and the “analytic” piece. For instance, there is a first Chern class map

\displaystyle c_1: \mathrm{Pic}(X) \rightarrow H^2(X; \mathbb{Z}) ,

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 {H^2(X; \mathbb{Z})} which project to {(1,1)}-classes in {H^2(X; \mathbb{C})} under the Hodge decomposition. (more…)