Sharp effective Nullstellensatz for projective Cohen-Macaulay schemes

Abstract

In this paper, we prove a sharp effective Nullstellensatz for projective Cohen-Macaulay schemes of pure dimension over an arbitrary field. In particular, if L is an ample line bundle on a smooth complex projective variety X and J is a coherent ideal sheaf with J\otimes L globally generated, then (\sqrt{J})^{°_L X}\subseteq J. This establishes the sharp bound conjectured by Ein and Lazarsfeld. More generally, we obtain sharp bounds in terms of mixed intersection numbers and a refinement involving products of powers of prime ideal sheaves associated to the irreducible components of the base locus. We also establish sharp radical power bounds for homogeneous ideals in finitely generated standard graded Cohen-Macaulay domains, extending Kollár's sharp effective Nullstellensatz for polynomial rings. The proofs combine purity and cycle preservation for leading Ext sheaves with Koszul evaluation maps compatible with successive divisor cuts.