Generic Projection Methods in Castelnuovo Regularity of Projective Varieties

Abstract

Castelnuovo-Mumford regularity is an important invariant of projective algebraic varieties. A well known conjecture due to Eisenbud and Goto gives a bound for regularity in terms of the codimension and degree,i.e., Castelnuovo-Mumford regularity of a given variety X is less than or equal to deg(X)-codimension(X)+1. Generic projection methods proved to be effective for the study of regularity of smooth projevtive varieties of dimension at most four(cf.[BM},[K2],[L],[Pi] and [R1]) because there are nice vanishing theorems for cohomology of vector bundles (e.g. the Kodaira-Kawamata-Viehweg vanishing theorem) and detailed information about the fibers ofgeneric projections from X to a hypersurface of the same dimension. Here we show by using methods similar to those used in [K2] that $\reg{X}\le(deg(X)-codimension(X)+1)+10$ for any smooth fivefold and $\reg{X}\le(deg(X)-codimension(X)+1)+20$ for any smooth sixfold. Furthermore, using similar methods we give a bound for the regularity of an arbitrary (not necessarily locally Cohen-Macaulay) projective surface X in P^N. To wit, we show that $\reg{X}\le(d-e+1)d-(2e+1)$, where d=deg(X) and e=codimension(X). This is the first bound for surfaces which does not depend on smoothness.