algebraic geometry

Moduli of Vector Bundles on Elliptic Curves

We’ve been talking about moduli problems, and one notoriously hard type of moduli problem is to “classify” vector bundles on some variety. Even when you restrict yourself to some special case like a specific surface and try to classify only vector bundles of certain rank or Chern class you run into trouble. To this day, these types of problems are a very active area of research.

It is fairly well-known (due to Grothendieck, but a problem in Hartshorne as well) that any finite rank vector bundle over {\mathbb{P}^1} is just a finite direct sum {\oplus_i\mathcal{O}(n_i)}. The next interesting case would be to move up to genus {1} curves. It turns out that Atiyah in the 1957 paper, Vector Bundles over an Elliptic Curve, worked out a classification. I just learned about this a month or two ago, and it is pretty cool so I’d like to briefly describe the idea.

Fix an algebraically closed field {k} of characteristic {0}, and let {E/k} be an elliptic curve. Define {V(r,d)} to be the set of indecomposable vector bundles (up to isomorphism) on {E} of rank {r} and degree {d}, where degree just means the degree of the determinant. It is well known that {V(1,0)} can be identified with {E}, because {V(1,0)=Pic^0(E)} is the set of degree {0} divisors. In particular, this says that the moduli space of degree {0} divisors (line bundles) on {E} is fine and representable by {E}.

Let’s continue with this idea of classifying degree {0} vector bundles. It turns out there is a unique vector bundle {T_r\in V(r,0)} with the property that {\Gamma (E, T_r)\neq 0}, i.e. there are non-trivial global sections. Now, recall how {V(1,0)} works. Let {0\in E} be the origin. Then our isomorphism {E\rightarrow V(1,0)} is given by {P\mapsto \mathcal{O}(-P)\otimes \mathcal{O}([0])}. Our {T_r} is going to play the role of {\mathcal{O}([0])} here. In complete analogy we get a bijection {V(1,0)\rightarrow V(r,0)} by {L\mapsto L\otimes T_r}.

This finishes off the case of degree {0} vector bundles, because we get that {E\simeq V(1,0)\simeq V(r,0)} (roughly speaking, of course there’s a lot more structure we need to know about to say something about the moduli spaces).

In more general situations we can use the same sort of trick. Note that we always have a bijection {V(r,d)\rightarrow V(r, d+nr)} given by {V\mapsto V\otimes \mathcal{O}(n[0])}. Thus one reduction Atiyah makes right away is that (by the Euclidean algorithm) we only need to consider {V(r,d)} for {0\leq d < r}.

Now suppose the rank and degree are non-zero, and {n=\text{gcd}(r,d)}. We can establish a bijection {E\rightarrow V(r,d)}, so that the composed map {E\rightarrow V(r,d)\stackrel{det}{\rightarrow} V(1, d)\rightarrow E} is the map multiplication by {n}. The idea again is that one can find a certain vector bundle {T_{r,d}\in V(r,d)} that is unique up to isomorphism from which all the others can be produced. Most people call this the Atiyah bundle. As a corollary, we see that if {(r,d)=1}, then the moduli space of indecomposable rank {r} and degree {d} vector bundles on {E} is fine and again representable by {E}.

In modern language, we could work out that stable or semi-stable sheaves are the appropriate things to classify if we want a hope for the moduli space to be fine. It turns out that this condition of certain numerical invariants being coprime often implies that the sheaves are stable. For example, on an elliptic curve, a K3 surface, or even when dealing with relative moduli of sheaves on a K3 fibration. We may get to this another day.

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