A Riemann-Hurwitz-Plucker formula

Abstract

We prove a simultaneous generalization of the classical Riemann-Hurwitz and Plucker formulas, addressing the total inflection of a morphism from a (smooth, projective) curve to an arbitrary (smooth, projective) higher-dimensional variety. Our definition of ramification is relative to an algebraic family of divisors on the target variety, and our formula is obtained using the theory of refined top Chern classes. In assigning multiplicities to ramification points, we frequently have to consider excess degeneracy loci, but we are able to show nonetheless that the multiplicities are always nonnegative, and are positive under very mild hypotheses.