Pseudo-effective cones of projective bundles and weak Zariski decomposition

Abstract

We consider the projective bundle $\mathbb {P}_X(E)$ over a smooth complex projective variety X, where E is a semistable bundle on X with $c_2(\mathrm{End}(E)) =0$. We give a necessary and sufficient condition to get the equality $\mathrm{Nef}^{\,1}(\mathbb {P}_X(E)) = \overline{\mathrm{Eff}}{}^{\,1}(\mathbb {P}_X(E))$ of nef cone and pseudo-effective cone of divisors in $\mathbb {P}_X(E)$. As an application of our result, we show the equality of nef and pseudo-effective cones of divisors of projective bundles over some special varieties. In particular, we show that weak Zariski decomposition exists on these projective bundles. We also show that weak Zariski decomposition exists for fibre product $\mathbb {P}_C(E)\,{\times }_C\,\mathbb {P}_C(E')$ over a smooth projective curve C. Finally, we show that a semistable bundle E of rank $r\geqslant 2$ with $c_2(\mathrm{End}(E)) = 0$ on a smooth complex projective surface of Picard number 1 is k-homogeneous, i.e., $\overline{\mathrm{Eff}}{}^{\,k}(\mathbb {P}_X(E)) = \mathrm{Nef}^{\,k}(\mathbb {P}_{X}(E))$ for all $1 \leqslant k < r$.