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Hodge classes on abelian varieties

Posted by Martin Orr on Monday, 25 August 2014 at 18:50

In this post I will define Hodge classes and state the Hodge conjecture. I will restrict my attention to the case of abelian varieties and say the minimum amount necessary to be able to discuss the relationships between the Hodge, Tate and Mumford-Tate conjectures and absolute Hodge classes in subsequent posts. There are many excellent accounts of this material already written, which may give greater detail and generality.

Hodge classes are cohomology classes on a complex variety which are in the intersection of the singular cohomology and the middle component of the Hodge decomposition They can also be defined as rational cohomology classes which are eigenvectors for the Mumford-Tate group. The Hodge conjecture predicts that these classes are precisely the -span of cohomology classes coming from algebraic subvarieties of .

Note: Hodge classes are usually defined as living in the Tate twist rather than in the cohomology group itself. This is because the cycle class maps for singular and for de Rham cohomology differ by factors of , as explained in my post on Tate twists. In this post, I shall only consider the singular cohomology normalisation of the cycle class map and hence omit Tate twists.

Hodge structures and of an abelian variety

So far in this blog, I have talked a lot about the homology group of an abelian variety and its Hodge structure. I also briefly mentioned the cohomology group when talking about Hodge symplectic forms. Now I need to talk about the cohomology groups in general.

Let's start with , which is the dual of . Recall that is a free -module of rank . The Hodge decomposition induces a decomposition where and likewise for .

We could call this type of structure an -Hodge structure. (The precise definition is the case of the definition below.) It is also known as an effective -Hodge structure of weight .

Hodge structures and higher cohomology of abelian varieties

We can generalise the above concept to an effective -Hodge structure of weight . This is defined to be a finite-rank -module together with a decomposition into complex vector spaces satisfying for all and . (The word effective means that we only have pieces where . The words weight mean that for every piece, .)

The importance of this concept comes from the fact that for every smooth projective complex variety (and more generally for every compact Kähler manifold, which includes all complex tori), the cohomology group comes with an effective -Hodge structure of weight . I will not discuss the general construction here, but I will discuss what these Hodge structures are in the case of abelian varieties.

The complex abelian variety is homeomorphic to the product of circles. Basic topology gives us that the cohomology groups are the exterior powers of :

Given an effective Hodge structure of weight , we can make its -th exterior power into an effective Hodge structure of weight by using the decomposition It turns out that is indeed the correct Hodge decomposition for of an abelian variety.

Hodge classes

Let us consider the intersection We have already seen this in the case , where we called elements of this intersection Hodge symplectic forms.

If then this intersection is . This is because each element of is fixed by complex conjugation, so But is zero whenever because and the Hodge decomposition is a direct sum.

On the other hand, when , we have and so may be non-zero. We call elements of integral Hodge classes of weight .

In an earlier post, we essentially defined a Hodge symplectic form to be an integral Hodge class of weight 2. Hence the existence of a polarisation (a Hodge symplectic form satisfying a positivity property) guarantees that an abelian variety possesses a non-zero integral Hodge class of weight 2. The group of integral Hodge classes of weight 2 is known as the Neron-Severi group and its rank, called the Picard number, is an interesting invariant of the abelian variety.

In these blog posts, it will be more convenient to work with We call elements of this group Hodge classes of weight . (Sometimes people call these elements rational Hodge classes, and use the term Hodge class to mean what we call an integral Hodge class.)

Algebraic cycle classes and the Hodge conjecture

The Hodge conjecture is not part of the main things I intend to write about in this blog, but I feel that I should not talk about Hodge classes without mentioning it briefly. It also provides motivation for some of the things we do with Hodge classes, and indeed give one reason why we care about Hodge classes in the first place.

Let be any smooth projective complex variety. If is a complex algebraic subvariety of codimension , then we can use Poincaré duality to define a class in . It turns out that is always an integral Hodge class.

A class in is called an algebraic cycle class if it is in the -span (or sometimes the -span) of those classes of the form . The Hodge conjecture states that every Hodge class is an algebraic cycle class.

Hodge conjecture. The classes of algebraic subvarieties of of codimension span the -vector space .

Hodge classes and the Mumford-Tate group

We can give a characterisation of Hodge classes using the Mumford-Tate group. The key reason why this is important is that there is a sort of converse which I will discuss in a subsequent post, using Hodge classes to characterise the Mumford-Tate group.

Let us recall the definition of the Mumford-Tate group. Associated with the -Hodge structure , there is a group homomorphism such that acts as multiplication by on and by on . The Mumford-Tate group of is defined to be the smallest algebraic subgroup defined over and such that contains the image of .

For each , there is a standard representation of on . We can compose these with from the previous paragraph to get homomorphisms If we look at the eigenspaces of these homomorphisms, we find that acts as multiplication by on .

Hence is the subspace of on which acts as for all .

We would like to state this in terms of the Mumford-Tate group instead of , so we proceed as follows.

The image of (and hence also the Mumford-Tate group) is contained in the group consisting of elements which preserve the symplectic form associated with a chosen polarisation (up to scalars). There is a character defined by

The fact that is a Hodge symplectic form implies that . So acts on as multiplication by .

For each Hodge class of weight , consider the algebraic group This group is defined over , and by the above its real points contain the image of . Hence contains the Mumford-Tate group of , and thus all of acts on as multiplication by .

On the other hand, if we have some which is an eigenvector for all of , then it must in particular be an eigenvector for . We know that the eigenspaces of on are precisely the pieces of the Hodge decomposition, and the only such piece which can intersect is . Thus any eigenvector for must be a Hodge class.

We deduce that

Proposition. A class in is a Hodge class if and only if it is an eigenvector for the action of .

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