Picard sheaves for elliptic curves Hi,
my question concerns the Picard bundles in the special case of an elliptic curve $E$, say over the complex numbers. Let $p$ and $q$ be the projections of $E \times E$.
One defines the $n$-th Picard bundle ($n$ an integer) as follows:
take the line bundle associated to the divisor $n$ times O (the point zero weighted with $n$) on $E$,
pull it back via $p$, tensor it on $E \times E$ with the Poincaré bundle, and then take the direct image of this under $q$.
Does anybody know whether one can say explicitly what the result of this construction is in the special case of an elliptic curve? (one usually defines the Picard sheaves for general curves and uses the jacobian, see the book of Birkenhake Lange, Complex abelian varieties)
Thanks a lot!
 A: Let $J^d$ be the degree $d$ Jacobian variety, parameterizing degree $d$ line bundles. Let $P^0$ be a degree $0$ Poincare bundle on $E \times J^0$.
Let $x$ be a point of $E$. The bundle $p^\ast (\mathcal{O}_E(nx)) \otimes P^0$ on $E \times J^0$ is the pullback of a degree $n$ Poincare bundle $P^n$ on $E \times J^n$ along the map $\operatorname{id}_E \times (- \otimes \mathcal{O}_E(nx)) : E \times J^0 \to E \times J^n$. This map is clearly an isomorphism.
By Proposition 2.1 on page 309 of Arbarello-Cornalba-Griffiths-Harris [for a more comprehensive treatment of this result, see Schwarzenberger's "Jacobians and Symmetric Products"], for $n \geq 1$, (the total space of) the projectivization of the sheaf $q_\ast(P^n)$ is isomorphic to the $n$th symmetric power of $E$, and under this isomorphism, the map from this projective bundle to $J^0 \cong E$ is the Abel-Jacobi map. Hence the same is true for the projectivization of $q_\ast(p^\ast (\mathcal{O}_E(nx)) \otimes P^0)$.
A: Let's just do it with divisor classes. The Poincare divisor (class) on $E\times E$ is $((O)\times E)+(E\times(O))$, or equivalently, $p^*((O))+q^*((O))$. You want to add this divisor class to $np^*((O))$ and then push down via $q_*$. So you're computing
$$q_*((n+1)p^*((O))+q^*((O)),$$ which seems to me should just equal $(O)$, since doesn't $q_*p^*((O))=0$? (I'm also going to add an "elliptic-curve" tag to your question.)
