Posted by Martin Orr on
Friday, 19 June 2015 at 19:30

Tate twists in singular cohomology are a device for dealing with factors of

which come up whenever we compare singular and de Rham cohomology of complex projective varieties.
In this post I will explain the problem, including calculating the

in the case of

, and define Tate twists to solve it.

In the case of singular cohomology, Tate twists are largely a matter of normalising things conveniently.
Without them, we could just write out factors of everywhere.
On the other hand, there is also a notion of Tate twists for -adic cohomology, which cannot be omitted so easily, and which I will discuss in a subsequent post.

Tags
abelian-varieties, alg-geom, hodge, maths
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Posted by Martin Orr on
Thursday, 22 January 2015 at 11:10

As I explained last time, one of the key steps in the the proof of Deligne's theorem on absolute Hodge classes is Principle B.
This allows us to take an absolute Hodge classes on one fibre in a family of varieties, and deduce that certain classes are absolute Hodge on other fibres of the same family.
In this post I shall explain a proof of Principle B due to Blasius, which I think is simpler than Deligne's original proof.

As I also mentioned last time, one can state Principle B in a number of slightly different forms.
I have chosen to use the following version instead of the one I gave last time (where there was a lot hidden in the mention of the Gauss-Manin connection, which I am happy not to have to talk about).
This version can be applied to Shimura varieties just as easily, or even more easily, than the previous one.

**Theorem.** Let

be a family of abelian varieties over

, with connected base

.
Let

be a global section of

.

If there is a point

such that

is an absolute Hodge class on

, then for every

,

is an absolute Hodge class on

.

Tags
abelian-varieties, alg-geom, hodge, maths
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Posted by Martin Orr on
Wednesday, 17 December 2014 at 19:00

Today I will outline the proof of Deligne's theorem that Hodge classes on an abelian variety are absolute Hodge.
The proof goes through three steps of reducing to increasingly special types of abelian varieties, until finally one reaches a case where it is easy to finish off.
This post has ended up longer than usual, but I don't think it is worth splitting into two.

A key ingredient is Deligne's Principle B, which is used for two of the three reduction steps.
Principle B says that if we have a family of varieties and a flat section of the relative de Rham cohomology bundle

, such that the section specialises to an absolute Hodge class at one point of , then in fact it is absolute Hodge everywhere.
This means that, if we have a method for constructing suitable families of abelian varieties and sections of their relative de Rham cohomology, then we only have to prove that Hodge classes are absolute Hodge at one point of each relevant family.
We use Shimura varieties to construct these families of abelian varieties on which to apply Principle B.

The outline of the proof looks like this:

- Reduce to Hodge classes on abelian varieties of CM type (using Principle B)
- Reduce to a special type of Hodge classes, called
*Weil classes*, on a special type of abelian variety, called *abelian varieties of split Weil type* (using linear algebra)
- Reduce to Hodge classes on abelian varieties which are isogenous to a power of an elliptic curve (using Principle B)
- Observe that it is easy to prove Deligne's theorem (and indeed the Hodge conjecture) for abelian varieties which are isogenous to a power of an elliptic curve

Tags
abelian-varieties, alg-geom, hodge, maths, shimura-varieties
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Posted by Martin Orr on
Wednesday, 10 September 2014 at 11:20

In this post I will fill in a missing detail from two weeks ago, where I mentioned that the Mumford-Tate group is determined by the Hodge classes.
More precisely, I will show that an element

of

is in the Mumford-Tate group if and only if every Hodge class on every Cartesian power is an eigenvector of .
In the context of Deligne's theorem on absolute Hodge classes, this is known as Principle A.

We will also see that a version of this statement holds for the -adic monodromy group and Tate classes.
This implies a link between the Hodge, Tate and Mumford-Tate conjectures.

Tags
abelian-varieties, alg-geom, alg-groups, hodge, maths, number-theory
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