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# Journal of the European Mathematical Society

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**Volume 16, Issue 9, 2014, pp. 1817–1848**

**DOI: 10.4171/JEMS/476**

Published online: 2014-10-22

Singularities of theta divisors and the geometry of $\mathcal A_5$

Gavril Farkas^{[1]}, Samuel Grushevsky

^{[2]}, R. Salvati Manni

^{[3]}and Alessandro Verra (1) Humboldt-Universität zu Berlin, Germany

(2) Stony Brook University, USA

(3) Università di Roma La Sapienza, Italy

We study the codimension two locus $H$ in $\mathcal A_g$ consisting of principally polarized abelian varieties whose theta divisor has a singularity that is not an ordinary double point. We compute the class $[H]\in CH^2(\mathcal A_g)$ for every $g$. For $g=4$, this turns out to be the locus of Jacobians with a vanishing theta-null. For $g=5$, via the Prym map we show that $H\subset \mathcal A_5$ has two components, both unirational, which we describe completely. We then determine the slope of the effective cone of $\overline{\mathcal A_5}$ and show that the component $\overline{N_0'}$ of the Andreotti-Mayer divisor has minimal slope and the Iitaka dimension $\kappa(\overline{\mathcal A_5}, \overline{N_0'})$ is equal to zero.

*Keywords: *Theta divisor, moduli space of principally polarized abelian varieties, effective cone, Prym variety

Farkas Gavril, Grushevsky Samuel, Salvati Manni R., Verra Alessandro: Singularities of theta divisors and the geometry of $\mathcal A_5$. *J. Eur. Math. Soc.* 16 (2014), 1817-1848. doi: 10.4171/JEMS/476