The Hessian correspondence of hypersurfaces of degree 3 and 4

Abstract

Let $X$ be a hypersurface, of degree $d$, in an $n$--dimensional projective space. The Hessian map is a rational map from $X$ to the projective space of symmetric matrices that sends a point $p\in X$ to the Hessian matrix of the defining polynomial of $X$ evaluated at $p$. The Hessian correspondence is the map that sends a hypersurface to its Hessian variety; i.e. the Zariski closure of its image via the Hessian map. In this paper, we study this correspondence for hypersurfaces with Waring rank at most $n+1$ and for hypersurfaces of degree $3$ and $4$. We prove that, for hypersurfaces with Waring rank $k\le n+1$, the map is birational onto its image for $d$ even, and it is generically finite of degree $2^{k-1}$ for $d$ odd. We prove that, for degree $3$ and $n=1$, the map is two to one, and that, for degree $3$ and $n\ge 2$, and for degree $4$, the Hessian correspondence is birational. In this study, we introduce the $k$--gradients varieties and analyze their main properties. We provide effective algorithms for recovering a hypersurface from its Hessian variety, for degree $3$ and $n\ge 1$, and for degree $4$ and $n$ even.