Restricting linear syzygies: algebra and geometry

Abstract

Let $X\subset \mathbb{P}^r$ be a closed scheme in projective space whose homogeneous ideal is generated by quadrics. We say that X (or its ideal I_X) satisfies the condition N_2,p if the syzygies of I_X are linear for p steps. We show that if X satisfies N_2,p then a zero-dimensional or one-dimensional intersection of X with a plane of dimension $\leq p$ is 2-regular. This extends a result of Green and Lazarsfeld. We give conditions when the syzygies of X restrict to the syzygies of the intersection. Many of our results also work for ideals generated by forms of higher degree. As applications, we bound the p for which some well-known projective varieties satisfy N_2,p. Another application, carried out by us in a different paper, is a step in the classification of 2-regular reduced projective schemes. Extending a result of Fröberg, we determine which monomial ideals satisfy N_2,p. We also apply Green's ‘linear syzygy theorem’ to deduce a relation between the resolutions of I_X and $I_{X\cup \Gamma}$ for a scheme Γ, and apply the result to bound the number of intersection points of certain pairs of varieties such as rational normal scrolls.