Weddle loci of linear systems of quadrics and the rank of partially symmetric tensors

Abstract

We establish a connection between properties of partially symmetric tensors (i.e. tensors associated to linear systems of quadric hypersurfaces) and the geometry of some related loci, generalization of the Weddle loci introduced in \cite{CFF+22} for their role in the study of configurations of points and interpolation problems. In particular, we consider linear systems of plane conics and linear systems of quadric surfaces, and show that when the associated tensors have low rank, then the singularities of the corresponding Weddle loci satisfy a (sharp) lower bound. Thus, we obtain a criterion to exclude that the rank of some partially symmetric tensors is too low. In the final section, devoted to partially symmetric $n\times n\times n$ tensors which lie in one component $M$ of a standard decomposition of the space of $3$-dimensional tensors (\cite{IR22}), we prove that the number of singular points of the Weddle locus associated to a general tensor in $M$ equals the (recursively defined) $n$-th Jacobsthal number.