# Martin's Blog

## Universal vector extensions of abelian varieties

Posted by Martin Orr on Friday, 09 May 2014 at 18:25

In the last post we showed that extensions of an abelian variety by the additive group are classified by a vector space which is canonically isomorphic to . In this post I will show that there is a so-called universal vector extension of , that is, a vector extension of such that every vector extension of can be obtained in a unique way as a pushout of . The vector group part of is , and an ingredient which we require from last time is that this is finite-dimensional.

Even if you are not interested in vector extensions of abelian varieties for their own sake (and I am not), they still have interesting properties. For example, I will show that they provide examples of algebraic varieties over the complex numbers which are non-isomorphic as algebraic varieties but which are isomorphic as complex manifolds.

### Definition of universal vector extensions

As last time, we will only consider fields of characteristic zero but most of the post remains valid without this assumption.

Let be an abelian variety over . The universal vector extension of is an extension of by a vector group , such that for each vector extension of there exist unique morphisms of algebraic groups and making the following diagram commute:

As defined in the previous post, this says that is the pushout of by .

By the standard universal property argument, if the universal vector extension of exists, then it is unique up to unique isomorphism of vector extensions.

### Construction of universal vector extension

We shall construct the universal vector extension, using the vector group . Last time we showed that this vector space is canonically isomorphic to , which is finite-dimensional.

Choose a basis for and let denote the corresponding extensions of by . We can form the fibre product extension

The choice of basis induces an isomorphism given by We define to be the extension obtained by pushing out the above fibre product extension by . One can show by hand that the extension is independent of the choice of basis, or one can deduce this after proving the universal property.

Now we shall prove that has the required universal property. Every extension of by an arbitrary vector group is isomorphic to a fibre product of extensions of by (via an isomorphism between and ) so it suffices to prove that the universal property holds for extensions of by .

So suppose that is an extension of by , and let be the associated element of . Define a morphism by evaluating at . Considering the definition of the vector space structure on shows that is the pushout of by , as required.

It remains to prove that is unique. Suppose we had some other morphism such that is the pushout of by . Since must be evaluation at some other point . But then is the extension corresponding to , and the definition of implies that .

### Universal extensions over the complex numbers

Let us consider what universal vector extensions look like over .

As remarked above, the vector space part of the universal vector extension is , the vector-space dual of the tangent space to dual abelian variety. Until recently, I imagined that this was naturally isomorphic to , but in fact this is false (of course they are isomorphic because they are vector spaces of the same dimension, but there is no natural isomorphism).

For example, if the ground field is and , then is the complex conjugate space , that is, the vector space with the same underlying set but where the action of is replaced by its complex conjugate.

To construct the universal vector extension complex analytically, take an abelian variety over , and let be a lattice in such that . Then the universal vector extension of is

It turns out that the universal vector extensions of different abelian varieties of the same dimension are all isomorphic to each other as complex Lie groups. To prove this, choose a -basis for . The positivity of a Riemann form for implies that form a basis for as a complex vector space, and hence is isomorphic to as a complex Lie group, for every abelian variety .

On the other hand, the universal vector extensions , of non-isomorphic abelian varieties , are not isomorphic as algebraic varieties. To prove this, we will use the well-known fact (Proposition 3.9 of Milne's notes) that there every morphism of varieties from into an abelian variety is constant. If and are isomorphic, we can consider the square The fact we just mentioned implies that the composition is constant on each fibre of , and so there exists a morphism making the square commute. By also doing the same in the other direction, we get a morphism and conclude that and are isomorphic.

Hence we have constructed examples of smooth complex algebraic varieties which are not isomorphic, but for which the underlying complex manifolds are isomorphic. Note that this contrasts with projective varieties, such as abelian varieties themselves, where Chow's theorem implies that if two smooth projective varieties over are isomorphic as complex manifolds then they are isomorphic as algebraic varieties.

It is interesting to note that there is another algebraic group with the same underlying complex Lie group as the universal vector extension of an abelian variety of dimension , namely . This follows from the exact sequence The algebraic group is of course affine, while universal vector extensions of abelian varieties are not affine because any quotient of an affine algebraic group is also affine.

1. The Hodge filtration and universal vector extensions From Martin's Blog

We will begin this post by looking at the isomorphism between the Hodge filtration H^{0,-1}(A) \subset H_1(A, \mathbb{C}) of a complex abelian variety A and the natural filtration ... on the tangent space to the universal vector extension of A. ...

1. he said on Monday, 02 January 2017 at 21:26 :

'pushing out the above fibre product extension by ''u^{-1}'''?

2. Martin said on Friday, 06 January 2017 at 15:20 :

Fixed. Thank you very much for the correction.