ON THE DISTANCE TO DISCRIMINANTS AND OSCULATING VARIETIES

Abstract: We study the problem of finding the closest singular hypersurface to a given nonsingular one from algebro-geometric point of view. The set of singular hypersurfaces $Ξ$ of a given degree is a projective variety dual to the Veronese variety. However, as was shown by Raffalli [Raf14], the Bombieri-Weyl (also known as apolar) distance from a general real hypersurface to $Ξ$ could be minimized at a singular point of $Ξ$. We study the singular locus of $Ξ$ and its irreducible component, consisting of hypersurfaces with cuspidal singularities. This variety is projectively dual to the tangential variety of the Veronese variety. We compute its Euclidean Distance Degree using topological tools and deduce a formula for the number of critical points of the distance function to $Ξ$ from a general hypersurface. We also initiate the study of the analogous problem for higher osculating varieties to the Veronese variety. Furthermore, we establish a new extremal property of Chebyshev polynomials (conjectured in [Raf14]) by characterizing real-rooted binary forms that maximize the distance to $Ξ$.