Line bundles on abelian varieties

1. Introduction

Let ${A}$ be an abelian variety over an algebraically closed field ${k}$ , of dimension ${g}$ . One of the basic tools in analyzing the properties of ${A}$ is the study of line bundles on ${A}$ . It’s a little non-intuitive to me why this is the case, so I’m going to try to motivate the topic.

Given ${A}$ , we are interested in questions of the following form: What is the structure of the ${n}$ -torsion points ${A[n]}$ ? To compute ${|A[n]|}$ , we are reduced to computing the degree of multiplication by ${n}$ ,

$\displaystyle n_A: A \rightarrow A$

(which is a morphism of varieties). In fact, we will show that ${n_A}$ is a finite flat morphism, and determine the degree of ${n_A}$ , which is thus the cardinality of the fiber over ${0}$ . The determination will be done by analyzing how ${n_A^*}$ acts on line bundles. For a symmetric line bundle ${\mathcal{L}}$ over ${A}$ , one can prove the crucial formula

$\displaystyle n_A^* \mathcal{L} \simeq \mathcal{L}^{n^2},$

and comparing the Hilbert polynomials of ${\mathcal{L}}$ and ${n_A^* \mathcal{L}}$ , one can get as a result

$\displaystyle \deg n_A = n^{2g} .$

Another way of phrasing this deduction is the following. By the Hirzebruch-Riemann-Roch formula and the parallelizability of an abelian variety, we have

$\displaystyle \chi(\mathcal{L}) = \frac{c_1(\mathcal{L})^g}{g!}$

for any line bundle ${\mathcal{L} \in \mathrm{Pic}(A)}$ . Consequently, in view of the asserted formula ${n_A^* \mathcal{L} \simeq \mathcal{L}^{n^2}}$ , we find for any line bundle ${\mathcal{L}}$ :

$\displaystyle \chi(n_A^* \mathcal{L}) = n^{2g} \chi(\mathcal{L}).$

Choosing ${\mathcal{L}}$ to be a high power of a very ample line bundle, we will have ${\chi(\mathcal{L}) \neq 0}$ . Now we can appeal to the following result:

Theorem 1 Let ${X}$ be a proper scheme over a field. Let ${G}$ be a finite group scheme, and let ${\pi: P \rightarrow X}$ be a ${G}$ -torsor. Then if ${\mathcal{F}}$ is a coherent sheaf on ${X}$ , we have

$\displaystyle \chi(\pi^* \mathcal{F}) = (\deg \pi) \chi(\mathcal{F}).$

It follows from this result that ${\deg n_A = n^{2g}}$ . For ${n}$ prime to the characteristic, the morphism ${n_A}$ can be seen to be separable, and it follows as a result there are ${n^{2g}}$ points of ${n}$ -torsion on ${A}$ .

2. The theorem of the cube

As stated above, an important technique in the theory of abelian varieties is the knowledge of how ${n_A}$ acts on the Picard group of ${A}$ . More generally, given a variety ${X}$ , and morphisms ${f, g: X \rightarrow A}$ , one might seek to determine ${(f+g)^* : \mathrm{Pic}(A) \rightarrow \mathrm{Pic}(X)}$ in terms of ${f^*, g^*}$ . There is a fairly deep theorem that allows us to do this:

Theorem 2 (Theorem of the cube) Let ${f, g, h: X \rightarrow A}$ be three morphisms from a variety ${X}$ to the abelian variety ${A}$ . Then for ${\mathcal{L} \in \mathrm{Pic}(A)}$ ,

$\displaystyle (f + g+ h)^* \mathcal{L} \simeq (f+g)^* \mathcal{L} \otimes (f + h)^* \mathcal{L} \otimes (g + h)^* \mathcal{L} \otimes f^* \mathcal{L}^{-1} \otimes g^* \mathcal{L}^{-1} \otimes h^{*} \mathcal{L}^{-1}.$

This is something that deserves a (later) blog post in itself; it’s a fairly non-intuitive result, and I’m not sure at this point I have much to say. For now, I’d like to describe some of the deductions one can make.

Let’s try to inductively determine the action of ${n_A^*}$ on ${\mathrm{Pic}(A)}$ . If we take ${f = n_A, g = 1_A, h = -1_A}$ , we find from the theorem of the cube

$\displaystyle n^*_A \mathcal{L} \simeq (n+1)_A^* \mathcal{L} \otimes (n-1)_A^* \mathcal{L} \otimes n_A^* \mathcal{L}^{-1} \otimes \mathcal{L}^{-1} \otimes (-1)_A^*\mathcal{L}^{-1}.$

If we think of the function ${f: \mathbb{Z} \rightarrow \mathrm{Pic}(A)}$ given by ${f(n) = n_A^* \mathcal{L}}$ , we find then:

$\displaystyle f(n) = f(n+1) + f(n-1) - f(n) - \mathcal{L} - (-1)_A^* \mathcal{L},$

where we write the group operation additively. This allows us to work out ${f}$ inductively, since we know ${f(0), f(1), f(-1)}$ as ${\mathcal{L}, \mathcal{O}_A, (-1)_A^* \mathcal{L}}$ . By induction, we find:

Proposition 3 For any line bundle ${\mathcal{L}}$ on ${A}$ , we have

$\displaystyle n_A^* \mathcal{L} \simeq \mathcal{L}^{(n^2 + n)/2} \otimes (-1)^*\mathcal{L}^{(n^2 - n)/2} .$

In particular, for a symmetric line bundle ${\mathcal{L}}$ , we have

$\displaystyle n_A^* \mathcal{L} \simeq \mathcal{L}^{n^2}.$

Corollary 4 ${n_A}$ is a finite surjective morphism, and the degree of ${n_A: A \rightarrow A}$ is ${n^{2g}}$ .

Proof:

In fact, if one believes the existence of ample bundles on abelian varieties (i.e., that abelian varieties are projective), then we can directly prove it. Let’s assume this, at least for now. Consider a symmetric ample line bundle ${\mathcal{L}}$ on ${A}$ (e.g., choose ${\mathcal{L}_1}$ ample, and consider ${\mathcal{L}_1 \otimes (-1)^* \mathcal{L}_1}$ ). Then

$\displaystyle n_A^* \mathcal{L} \simeq \mathcal{L}^{n^2}$

is ample, and is in particular ample when restricted to the subscheme ${A[n] = \ker n_A}$ . (This is a subscheme as the fiber over zero.) However, ${n_A^* \mathcal{L}}$ is trivial on ${A[n]}$ , which means that the trivial bundle on ${A[n]}$ is ample. This means that ${A[n]}$ is affine, hence finite.

Consequently, the map ${n_A : A \rightarrow A}$ has finite fibers. It is also proper, so by Zariski’s Main Theorem, ${n_A}$ is a finite morphism, necessarily surjective by dimension considerations. (If ${n}$ was prime to the characteristic, we could see this because ${n_A}$ would be étale.)

Finally, we need to compute the degree. Here we observe that ${n_A: A \rightarrow A}$ has now been shown to be a finite surjective morphism; it is flat by generic flatness. One can appeal to the general theory of torsors mentioned previously, together with the Riemann-Roch formula, to get the result. $\Box$

It’s probably worth mentioning why exactly ${n_A: A \rightarrow A}$ makes ${A}$ a torsor over itself (for the group scheme ${A[n]}$ ). There is an action of the group scheme ${A[n]}$ on ${A}$ over ${A}$ . To say that ${A \rightarrow A}$ is an ${A[n]}$ -torsor is to say that we have faithful flatness (already checked) and, after pulling back, we have a splitting:

$\displaystyle A \times_{n_A, A, n_A} A \simeq A \times_k A[n].$

Here the fiber product means the fiber product of the two morphisms ${n_A: A \rightarrow A}$ and ${n_A: A \rightarrow A}$ . This can be checked on the level of functors of points, though, in which case it is just a statement about abelian groups.

Corollary 5 If ${n}$ is prime to the characteristic, then ${A[n] \simeq (\mathbb{Z}/n\mathbb{Z})^{2g}}$ .

Proof: In fact, since ${n_A}$ acts by isomorphisms on the tangent space, ${n_A}$ is an étale morphism. Consequently, the order of ${A[n]}$ is the degree ${n^{2g}}$ (in general, ${A[n]}$ will not be reduced if ${n}$ is not prime to the characteristic). Since this is true for any ${m \mid n}$ , we find that we have an abelian group ${G = A[n]}$ with the following properties:

${G}$ has order ${n^{2g} }$ and is ${n}$ -torsion.
Whenever ${m \mid n}$ , the ${m}$ -torsion in ${G }$ has order ${m^{2g}}$ .

By the structure theory of finite abelian groups, this gives the structure of ${G = A[n]}$ . $\Box$

3. The statement about torsors

To be complete, I’d like to include a proof of the result on torsors.

Theorem 6 Let ${X}$ be a proper scheme over a field ${k}$ . Let ${G}$ be a finite group scheme, and let ${\pi: P \rightarrow X}$ be a ${G}$ -torsor. Then if ${\mathcal{F}}$ is a coherent sheaf on ${X}$ , we have

$\displaystyle \chi(\pi^* \mathcal{F}) = (\deg \pi) \chi(\mathcal{F}). \ \ \ \ \ (1)$

This is a fairly intuitive result: pulling back by a torsor (i.e., the analog of a finite covering space in topology) should correspondingly multiply the Euler characteristic. The actual proof is not too intuitive: it is, however, a relatively common technique. The proof is by dévissage.

Proof: By noetherian induction, we can assume that the result is true whenever ${X}$ is replaced by a proper closed subscheme.

The crucial equation (1) is additive in ${\mathcal{F}}$ , since ${\pi}$ is flat. Consequently, if there is a short exact sequence

$\displaystyle 0 \rightarrow \mathcal{F}' \rightarrow \mathcal{F} \rightarrow \mathcal{F}'' \rightarrow 0$

and the result is true for ${\mathcal{F}', \mathcal{F}''}$ , it is also true for ${\mathcal{F}}$ . Thus, by taking filtrations of the form ${\mathcal{F} \supset \mathcal{N}\mathcal{F} \supset \dots}$ by nilpotent ideals ${\mathcal{N}}$ , we may assume ${X}$ is reduced. Similarly, we may assume ${X}$ is irreducible.

In the end, one reduces to checking it for a single sheaf ${\mathcal{G}}$ on ${X}$ with nonzero support near the generic point. Any other sheaf can be “approximated” (up to taking a direct sum of copies) by a sum of copies of ${\mathcal{G}}$ at the generic point. Since we know the theorem for smaller subschemes, we deduce (1) in general by (1) for ${\mathcal{G}}$ .

In fact, we take ${\pi_* \mathcal{F}}$ where ${\mathcal{F}}$ is some sheaf on ${P}$ (e.g., ${\mathcal{O}_P}$ ). The observation is that we have a cartesian square

since ${\pi}$ is a torsor. Now we can use flat base change to observe:

$\displaystyle \pi^* \pi_* \mathcal{F} \simeq \mathcal{F} \otimes \mathcal{O}_G.$

In particular,

$\displaystyle \chi(\pi^* \pi_*\mathcal{F} ) = (\deg \pi)\chi( \mathcal{F}) = \chi(\pi_* \mathcal{F}),$

where the last equality uses the fact that ${\pi}$ is finite and ${H^\bullet(P, \mathcal{F}) \simeq H^\bullet(X, \pi_* \mathcal{F})}$ . $\Box$

Line bundles on abelian varieties II « Climbing Mount Bourbaki Says:

April 12, 2012 at 8:16 pm

[…] be an abelian variety. Last time, I described a formula which allowed us to express the pull-back of a line bundle […]

plm Says:

April 16, 2012 at 3:28 am

Thank for the post.

Do you know when this fact about the degree of multiplication by n, the number of torsion points of abelian varieties over algebraically closed fields, was discovered? Was it discovered by Weil? I guess this was suspected early from explicit computations.

Also it seems to hint at a deep analogy between algebraically closed fields of 0 and positive characteristics. At what points does the algebraic closure assumption come into play? What is the real part of an algebraically closed field of characteristic p? Over C we can take the fixed field of conjugation. Over fields of positive characteristic we could have to use structures which are not fields, perhaps use strange constructions like in geometry over F_1. We can take the union of fields without square roots of -1 (e.g. F_3, F_27, F_243, etc.), would that be useful? I hope I’ll have the opportunity to understand more about that.

Akhil Mathew Says:

April 16, 2012 at 8:47 pm
I don’t know much about the history of abelian varieties. I would imagine it was known quite a while ago (mid 19th century perhaps) for elliptic curves over the complex numbers.

The algebraic closure is necessary to argue that the number of $k$ -points in a reduced finite scheme (which is the size of the torsion subgroup we want) is the same as the length of a scheme (which is what we get by writing the scheme as a fiber). I’m not really sure the “real part” of a field of characteristic $p$ should mean: it doesn’t even makes sense in characteristic $0$ (except in a few cases)! It’s definitely reasonable to consider elliptic curves over structures which are not fields. I’ve heard that some recent work in homotopy theory has tried to make sense of elliptic curves over more exotic objects (i.e., $latex $E_\infty$-ring spectra), but I’m not familiar with it.