Generalized weight properties of resultants and discriminants, and applications to projective enumerative geometry

The goal of this text is to understand and prove a formula stated by Salmon, which gives the first terms of some Taylor expansion of the discriminant of a plane algebraic curve. Salmon uses his formula to derive various enumerative quantities for surfaces in ${\mathbf{P}}^3$. We provide complete proofs of this formula and its enumerative applications, and extend Salmon’s considerations to hypersurfaces in a projective space of arbitrary dimension. To this end, we introduce the concept of reduced discriminant, and provide a thorough study of its weight properties; the latter are deeply linked to projective enumerative geometric properties.