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

e g X l e (d e g (X) - c o d i m e n s i o n (X) + 1) + 10

for any smooth fivefold and

e g X l e (d e g (X) - c o d i m e n s i o n (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

e g X l e (d - e + 1) d - (2 e + 1)

, where d=deg(X) and e=codimension(X). This is the first bound for surfaces which does not depend on smoothness.

Mathematics Subject Classification ID

Projective techniques in algebraic geometry (14N05) Low codimension problems in algebraic geometry (14M07)

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