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A new six-dimensional irreducible symplectic variety

Author(s): Kieran G. O'Grady
Journal: J. Algebraic Geom. 12 (2003), 435-505.
Posted: January 14, 2003
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Abstract | References | Additional information

Abstract: We construct a six-dimensional irreducible symplectic variety with $b_2=8$. Since the known examples of irreducible symplectic varieties have $b_2=7$ or $b_2=23$, our variety is in a new deformation class. The example is obtained as follows. Let $J$ be the Jacobian of a genus-two curve with its natural principal polarization: results of another paper of ours give a symplectic desingularization of the moduli space of semistable rank-two sheaves on $J$ with $c_1=0$ and $c_2=2$. Let $\mathcal{M}_{\mathbf{v}}$ be this symplectic desingularization: there is a natural locally trivial fibration $\mathcal{M}_{\mathbf{v}}\rightarrow J\times\widehat{J}$. Our example is the fiber over $(0,\widehat{0})$ of this map, we denote it by $\widetilde{\mathcal{M}}$. The main body of the paper is devoted to the proof that $\widetilde{\mathcal{M}}$is irreducible symplectic and that $b_2(\widetilde{\mathcal{M}})=8$. Applying the generalized Lefschetz Hyperplane Theorem we get that low-dimensional homotopy (or homology) groups of $\widetilde{\mathcal{M}}$are represented by homotopy (or homology) groups of a subset of $\widetilde{\mathcal{M}}$ which has an explicit description. The main problem is to provide the explicit description and to extract the necessary information on homotopy or homology groups.

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Additional Information:

Kieran G. O'Grady
Affiliation: Università La Sapienza, Dipartimento di Matematica G. Castelnuovo, Piazzale A Moro 5, 00185 Rome, Italy
Email: ogrady@mat.uniroma1.it

PII: S 1056-3911(03)00323-0
Received by editor(s): November 9, 2000
Posted: January 14, 2003
Additional Notes: Supported by Cofinanziamento MURST 1999-2001
Dedicated: Dedicato a Riccardino

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