Divisibility Relations and $\mathcal{D}$-Extremal ideals

TL;DR

This paper introduces $ ext{D}$-extremal ideals, a class of square-free monomial ideals that encode specific divisibility relations, providing bounds on resolutions and Betti numbers for ideals satisfying these relations.

Contribution

It defines $ ext{D}$-extremal ideals and proves their optimality in bounding resolutions and Betti numbers among ideals with the same divisibility relations.

Findings

$ ext{D}$-extremal ideals encode divisibility relations precisely.

Resolutions of $ ext{D}$-extremal ideals provide bounds for related ideals.

Betti numbers of powers of $ ext{D}$-extremal ideals are maximal among those satisfying the relations.

Abstract

A divisibility relation between the generators of a square-free monomial ideal formally encodes the situation when one generator divides the least common multiple of some other generators. The divisibility relations contribute to the deletion of some parts of the Taylor resolution of the ideal, and therefore lead to finding a resolution closer to the minimal one. Motivated by this observation, for a given set $\mathcal{D}$ of divisibility relations, we study all square-free monomials satisfying the relations in $\mathcal{D}$. We define a class of square-free monomial ideals called $\mathcal{D}$-extremal ideals $\mathcal{E}_\mathcal{D}$ , and show it is optimal in the sense that it is an ideal satisfying exactly those divisibility relations coming from $\mathcal{D}$, and no others. We then show that $\mathcal{E}_\mathcal{D}$ is extremal in the sense that the resolution and betti numbers of the powers of any square-free monomial ideal satisfying the relations in $\mathcal{D}$ are bounded by those of the same powers of $\mathcal{E}_\mathcal{D}$.