I’m supposed to give a talk on this subject for one of my courses, so I consider this post as a “pre-exposition.” I learned from and heavily used the great exposition “*Vector bundles on curves*” by *Montserrat Teixidor I Bigas* in this post. I wrote up the pre-requisites here.

In 1957, *Atiyah* in this famous paper “**Vector bundles over an elliptic curve**” classified indecomposable vector bundles of arbitrary rank and degree. Briefly, every vector bundle (locally free sheaf) is decomposed uniquely (up to the order) to the direct sum of indecomposable vector bundles and *the set of isomorphism classes of indecomposable vector bundles of a fixed rank and degree is isomorphic to the Jacobian of the curve which the latter is isomorphic to the curve itself.* The isomorphisms are *canonical* for vector bundles of degree zero and for higher degree, they *depend on the choice* of a *line bundle of degree one* (a base point on the curve) [“*Vector bundles on curves” by Montserrat Teixidor I Bigas, section 4.*]

An elliptic curve is a smooth projective curve of genus one over an algebraically closed field One may assume that Throughout, the words vector bundle and locally free sheaf are used interchangeably.

Denote by the set of isomorphism classes of indecomposable vector bundles of rank and degree where the degree of a vector bundle of rank is defined as the degree of the associated locally free sheaf which is

One can also show that is equal to the degree of its *determinant*.

**Case 1**: Vector bundles of degree zero

For every positive integer there exists a unique (self-dual) indecomposable vector bundle of rank and degree zero with only one section. It turns out that any indecomposable vector bundle of rank and degree zero is isomorphic to for a unique line bundle of degree zero. Therefore, the set (moduli space) of indecomposable vector bundles of degree zero on i.e. is (canonically) isomorphic to the moduli space of degree zero line bundles on which in turn by definition is the Jacobian of

**Case 2**: Vector bundles of non-negative degree (general case)

In general, let be a fixed line bundle of *degree one*, (which corresponds to fixing a base point) on then there is an isomorphism sending Note that

If there is an isomorphism sending to where is given by the following extension with

By these two operations, we can assume that and Moreover, if we will have and if then Note that non of these operations changes Thus, we can construct a sequence of isomorphisms with s.t. and This is the sequence of positive numbers, so it will terminate when and Hence, we have established the isomorphism

and by *case 1*, is isomorphic to the *Jacobian* of (and is isomorphic to the curve itself.)

That being said, the above isomorphism is *completely determined* up to the choice of a line bundle of *degree one*.

So far, we have achieved what we wanted. Let’s now, try to dig more. Denote by the element in corresponding to in

Proposition 1: a) Every vector bundle of rank and degree can be written as for some line bundle of degree zero.b) If then where run over the set of line bundles of order (in the Picard group)

c) If then

d) If then

e) If then

Here is some results involving stability and semi-stability of vector bundles on

Proposition 2: a) An indecomposable vector bundle of degree zero is semi-stable not stable.b) Indecomposable vector bundles are semi-stable and they are stable if and only if

For detailed and complete description of vector bundles on an elliptic curve, especially when the ground field is of look at Atiyah’s original paper.