Locally Trivial Deformations of Toric Varieties

Abstract: We study locally trivial deformations of toric varieties from a combinatorial point of view. For any fan $\Sigma$, we construct a deformation functor $\mathrm{Def}\Sigma$ by considering \v{C}ech zero-cochains on certain simplicial complexes. We show that under appropriate hypotheses, $\mathrm{Def}\Sigma$ is isomorphic to $\mathrm{Def}'{X\Sigma}$, the functor of locally trivial deformations for the toric variety $X_\Sigma$ associated to $\Sigma$. In particular, for any complete toric variety $X$ that is smooth in codimension $2$ and $\mathbb{Q}$-factorial in codimension $3$, there exists a fan $\Sigma$ such that $\mathrm{Def}\Sigma$ is isomorphic to $\mathrm{Def}_X$, the functor of deformations of $X$. We apply these results to give a new criterion for a smooth complete toric variety to have unobstructed deformations, and to compute formulas for higher order obstructions, generalizing a formula of Ilten and Turo for the cup product. We use the functor $\mathrm{Def}\Sigma$ to explicitly compute the deformation spaces for a number of toric varieties, and provide examples exhibiting previously unobserved phenomena. In particular, we classify exactly which toric threefolds arising as iterated $\mathbb{P}^1$-bundles have unobstructed deformation space.