Fano schemes for generic sums of products of linear forms

Abstract

We study the Fano scheme of [Formula: see text]-planes contained in the hypersurface cut out by a generic sum of products of linear forms. In particular, we show that under certain hypotheses, linear subspaces of sufficiently high dimension must be contained in a coordinate hyperplane. We use our results on these Fano schemes to obtain a lower bound for the product rank of a linear form. This provides a new lower bound for the product ranks of the [Formula: see text] Pfaffian and [Formula: see text] permanent, as well as giving a new proof that the product and tensor ranks of the [Formula: see text] determinant equal five. Based on our results, we formulate several conjectures.