Second order theta divisors on Pryms

Van Geemen and van der Geer, Donagi, Beauville and Debarre proposed characterizations of the locus of jacobians which use the linear system of $2\Theta$-divisors. We give new evidence for these conjectures in the case of Prym varieties.Comment: AMS-Latex, 11 pages, the exposition has been modified, Proposition 5 has been modifie

A Prym construction for the cohomology of a cubic hypersurface

Mumford defined a natural isomorphism between the intermediate jacobian of a conic-bundle over $P^2$ and the Prym variety of a naturally defined \'etale double cover of the discrminant curve of the conic-bundle. Clemens and Griffiths used this isomorphism to give a proof of the irrationality of a smooth cubic threefold and Beauville later generalized the isomorphism to intermediate jacobians of odd-dimensional quadric-bundles over $P^2$. We further generalize the isomorphism to the primitive cohomology of a smooth cubic hypersurface in $P^n$.Comment: AMS-Latex, 39 page

Density and completeness of subvarieties of moduli spaces of curves or abelian varieties

Let $V$ be a subvariety of codimension $\leq g$ of the moduli space \cA_g of principally polarized abelian varieties of dimension $g$ or of the moduli space \tM_g of curves of compact type of genus $g$. We prove that the set $E_1(V)$ of elements of $V$ which map onto an elliptic curve is analytically dense in $V$. From this we deduce that if V \subset \cA_g is complete, then $V$ has codimension equal to $g$ and the set of elements of $V$ isogenous to a product of $g$ elliptic curves is countable and analytically dense in $V$. We also prove a technical property of the conormal sheaf of $V$ if V \subset \tM_g (or \cA_g) is complete of codimension $g$.Comment: AMS-LaTeX, 15 page

Deforming curves in jacobians to non-jacobians I: curves in C(2)C^{(2)}

We introduce deformation theoretic methods for determining when a curve $X$ in a non-hyperelliptic jacobian $JC$ will deform with $JC$ to a non-jacobian. We apply these methods to a particular class of curves in the second symmetric power $C^{(2)}$ of $C$. More precisely, given a pencil $g^1_d$ of degree $d$ on $C$, let $X$ be the curve parametrizing pairs of points in divisors of $g^1_d$ (see the paper for the precise scheme-theoretical definition). We prove that if $X$ deforms infinitesimally out of the jacobian locus with $JC$ then either $d=4$ or $d=5$, dim$H^0 (g^1_5) = 3$ and $C$ has genus 4.Comment: amslatex, 25 page

An inductive approach to the Hodge conjecture for abelian varieties

We describe an inductive approach most appropriate for abelian varieties with an action of an imaginary quadratic field.Comment: amslatex, 11 page

Some properties of second order theta functions on Prym varieties

Let $P \cup P'$ be the two component Prym variety associated to an \'etale double cover $\tilde{C} \to C$ of a non-hyperelliptic curve of genus $g \geq 6$ and let $|2\Xi_0|$ and $|2\Xi_0'|$ be the linear systems of second order theta divisors on $P$ and $P'$ respectively. The component $P'$ contains canonically the Prym curve $\tilde{C}$. We show that the base locus of the subseries of divisors containing $\tilde{C} \subset P'$ is scheme-theoretically the curve $\tilde{C}$. We also prove canonical isomorphisms between some subseries of $|2\Xi_0|$ and $|2\Xi_0'|$ and some subseries of second order theta divisors on the Jacobian of $C$.Comment: references adde

Ampleness of intersections of translates of theta divisors in an abelian fourfold

We prove that the cotangent bundle of a complete intersection of two general translates of the theta divisor of the jacobian of a general curve of genus 4 is ample. From this the same result for a general principally polarized abelian variety of dimension 4 follows.Comment: ams-latex, 8 page

The tangent space to the moduli space of vector bundles on a curve and the singular locus of the theta divisor of the jacobian

We complete the proof of the fact that the moduli space of rank two bundles with trivial determinant embeds into the linear system of divisors on $Pic^{g-1}C$ which are linearly equivalent to $2\Theta$. The embedded tangent space at a semi-stable non-stable bundle $\xi\oplus\xi^{-1}$, where $\xi$ is a degree zero line bundle, is shown to consist of those divisors in $|2\Theta|$ which contain $Sing(\Theta_{\xi})$ where $\Theta_{\xi}$ is the translate of $\Theta$ by $\xi$. We also obtain geometrical results on the structure of this tangent space.Comment: 33 pages, AMS-Late

The primitive cohomology of the theta divisor of an abelian fivefold

The primitive cohomology of the theta divisor of a principally polarized abelian variety of dimension $g$ is a Hodge structure of level $g-3$. The Hodge conjecture predicts that it is contained in the image, under the Abel-Jacobi map, of the cohomology of a family of curves in the theta divisor. In this paper we use the Prym map to show that this version of the Hodge conjecture is true for the theta divisor of a general abelian fivefold.Comment: 59 page

Correspondences with split polynomial equations

We introduce endomorphisms of special jacobians and show that they satisfy polynomial equations with all integer roots which we compute. The eigen-abelian varieties for these endomorphisms are generalizations of Prym-Tjurin varieties and naturally contain special curves representing cohomology classes which are not expected to be represented by curves in generic abelian varieties.Comment: AMS-Latex, 32 pages, minor corrections, computation of dimensions of eigen-abelian varieties adde