Asymptotic behavior of invariants of syzygies of maximal Cohen-Macaulay modules

TL;DR

This paper studies the asymptotic behavior of invariants of syzygy modules over complete intersection rings, revealing quasi-polynomial patterns and bounds related to their complexity and regularity.

Contribution

It establishes the quasi-polynomial nature of Hilbert coefficients of syzygies and links their asymptotic growth to the modules' complexity and regularity.

Findings

Hilbert coefficients of syzygies follow a quasi-polynomial pattern with period 2.

Asymptotic inequalities relate Hilbert coefficients and module invariants.

Boundedness of Castelnuovo-Mumford regularity when certain limits are equal.

Abstract

Let $(A,\mathfrak{m})$ be a complete intersection ring of codimension $c\geq 2$ and dimension $d\geq 1$. Let $M$ be a finitely generated maximal Cohen-Macaulay $A$-module. Set $M_i=\text{Syz}^A_{i}(M)$. Let $e^{\mathfrak{m}}_i(M)$ be the $i$-th Hilbert coefficient of $M$ with respect to $\mathfrak{m}$. We prove for all $i\gg0$, the function $i\mapsto e^{\mathfrak{m}}_j(M_i)$ is a quasi-polynomial type with period $2$ and degree $\text{cx}(M)-1$ for $j=0,1$, where $\text{cx}(M)$ is the complexity of $M.$ For $\text{cx}(M)=2,$ we prove $$\lim_{n\to \infty}\dfrac{e^{\mathfrak{m}}_1(M_{2n+j})}{n}\geq \lim_{n\to \infty}\dfrac{e^{\mathfrak{m}}_0(M_{2n+j})}{n}-\lim_{n\to \infty}\dfrac{\mu(M_{2n+j})}{n}$$ for $j=0,1$. When equality holds, we prove that the Castelnuovo-Mumford regularity of the associated graded ring of $M_i$ with respect to the maximal ideal $\mathfrak{m}$ is bounded for all $i\geq 0$.