Calculating Discriminants by Higher Direct Images

Abstract

The author uses the homological algebra to construct for any line bundle $\mathcal {L}$ on a nonsingular projective variety X the complex $\mathbb {F}(\mathcal {L})$ whose determinant is equal to the equation of the dual variety ${X^{\text {V}}}$. This generalizes the Cayley-Koszul complexes defined by Gelfand, Kapranov and Zelevinski. The formulas for the codimension and degree of ${X^{\text {V}}}$ in terms of complexes $\mathbb {F}(\mathcal {L})$ are given. In the second part of the article the general technique is applied to classical discriminants and hyperdeterminants.