## The Hodge filtration and universal vector extensions

Posted by Martin Orr on Friday, 13 June 2014 at 20:10

We will begin this post by looking at the isomorphism between the Hodge filtration
```
of a complex abelian variety
```

```
and the natural filtration
```

```
on the tangent space to the universal vector extension of
```

`.`

The significance of this isomorphism is that the Hodge filtration, as we defined it before, is constructed by transcendental methods, valid only over ```
,
but the universal vector extension is an object of algebraic geometry.
So this gives us an analogue for the Hodge filtration for abelian varieties over any base field.
Furthermore, in the usual way of algebraic geometry, the construction of the universal vector extension can be carried out uniformly in families of abelian varieties.
```

We will use the construction of the universal vector extension in families to show that “the Hodge filtration varies algebraically in families.”
We will first have to explain what this statement means.
We will also mention briefly why ` does not vary algebraically.`

A note on the general philosophy of this post: the usual construction of an algebraic-geometric object isomorphic to the Hodge filtration uses de Rham cohomology, which works for ```
of an arbitrary smooth projective variety.
My aim in using universal vector extensions is to give an
```

*ad hoc* construction of de Rham (co)homology, valid only for ```
of an abelian variety, requiring less sophisticated technology than the general construction.
This fits with previous discussion on this blog of the Hodge structure on
```

`, constructed via the exponential map from the tangent space of `

`, and of the `

`-adic `

`, constructed as the Tate module.`