Differentials of Cox rings: Jaczewski's theorem revisited

Abstract

A generalized Euler sequence over a complete normal variety $X$ is the unique extension of the trivial bundle $V \otimes {\mathcal O}_X$ by the sheaf of differentials $\Omega_X$, given by the inclusion of a linear space $V\subset {\rm Ext}^1_X({\mathcal O}_X,\Omega_X)$. For $\Lambda$, a lattice of Cartier divisors, let ${\mathcal R}_\Lambda$ denote the corresponding sheaf associated to $V$ spanned by the first Chern classes of divisors in $\Lambda$. We prove that any projective, smooth variety on which the bundle ${\mathcal R}_\Lambda$ splits into a direct sum of line bundles is toric. We describe the bundle ${\mathcal R}_\Lambda$ in terms of the sheaf of differentials on the characteristic space of the Cox ring, provided it is finitely generated. Moreover, we relate the finiteness of the module of sections of ${\mathcal R}_\Lambda$ and of the Cox ring of $\Lambda.$