English

Splittings of Ideals of Points in $\mathbb{P}^{1}\times\mathbb{P}^{1}$

Commutative Algebra 2025-10-08 v1 Algebraic Geometry

Abstract

Let IXI_\mathbb{X} be the bihomogeneous ideal of a finite set of points XP1×P1\mathbb{X} \subseteq \mathbb{P}^1 \times \mathbb{P}^1. The purpose of this note is to consider ``splittings'' of the ideal IXI_\mathbb{X}, that is, finding ideals JJ and KK such that IX=J+KI_\mathbb{X} = J+K, where JJ and KK have prescribed algebraic or geometric properties. We show that for any set of points X\mathbb{X}, we cannot partition the generators of IXI_\mathbb{X} into two ideals of points. The best case scenario is where at most one of JJ or KK is an ideal of points. To remedy this we introduce the notion of unions of lines and ACM (Arithmetically Cohen-Macaulay) points which allows us to say more about splittings. For a set W\mathbb{W} of unions of lines and ACM sets of points, we can write IW=J+KI_\mathbb{W} = J + K where both JJ and KK are ideals of unions of lines and ACM points as well. When W\mathbb{W} is a union of lines and ACM points, we discuss some consequences for the graded Betti numbers of IWI_{\mathbb{W}} in terms of these splittings.

Keywords

Cite

@article{arxiv.2510.05628,
  title  = {Splittings of Ideals of Points in $\mathbb{P}^{1}\times\mathbb{P}^{1}$},
  author = {Elena Guardo and Graham Keiper and Adam Van Tuyl},
  journal= {arXiv preprint arXiv:2510.05628},
  year   = {2025}
}