Asymptotic regularity of graded families of ideals

TL;DR

This paper investigates the asymptotic behavior of regularity in graded families of ideals, establishing existence in certain cases, providing combinatorial interpretations, and exploring limitations through counterexamples.

Contribution

It proves the existence of the limit of regularity over n for specific classes of ideals and offers a combinatorial interpretation via Newton–Okounkov regions, extending prior results.

Findings

Limit of regularity/n exists for certain ideal families

Counterexamples show limits may not exist for sums and intersections

Explicit Gr"obner basis construction for specific ideal forms

Abstract

We show that the asymptotic regularity of a graded family $(I_n)_{n \ge 0}$ of homogeneous ideals in a reduced standard graded algebra, i.e., the limit $\lim_{n \rightarrow \infty} \text{reg } I_n/n$, exists in several cases; for example, when the family $(I_n)_{n \ge 0}$ consists of artinian ideals, or Cohen-Macaulay ideals of the same codimension over an uncountable base field of characteristic $0$, or when its Rees algebra is Noetherian. Many applications, including simplifications and generalizations of previously known results on symbolic powers and integral closures of powers of homogeneous ideals, are discussed. We provide a combinatorial interpretation of the limit $\lim_{n \rightarrow \infty} \text{reg } I_n/n$ in terms of the associated Newton--Okounkov region in various situations. We give a negative answer to the question of whether the limits $\lim_{n \rightarrow \infty} \text{reg } (I_1^n + \dots + I_p^n)/n$ and $\lim_{n \rightarrow \infty} \text{reg } (I_1^n \cap \cdots \cap I_p^n)/n$ exist, for $p \ge 2$ and homogeneous ideals $I_1, \dots, I_p$. We also examine ample evidence supporting a negative answer to the question of whether the asymptotic regularity of the family of symbolic powers of a homogeneous ideal always exists. Our work presents explicit Gr\"obner basis construction for ideals of the form $Q^n + (f^k)$, where $Q$ is a monomial ideal, $f$ is a polynomial in the polynomial ring in 4 variables over a field of characteristic $2$.