Supporting rank and the intersection of all Hassett Divisors

Abstract

We prove that the dimension of the intersection $\mathcal Z$ of all Hassett divisors of special cubic fourfolds is sixteen. We do this by studying which subsets of the natural numbers $\mathbb N$ can be obtained as the image of a positive-definite integral quadratic form and what the minimal possible rank of such a form is. In particular, for the subset of $\mathbb N$ consisting of all possible discriminants of special cubic fourfolds, we show this rank is four and that this is the codimension of $\mathcal Z$ in $\mathcal C$, the twenty-dimensional moduli space of cubic fourfolds.