Regularity, singularities and $h$-vector of graded algebras

TL;DR

This paper explores how certain singularities in graded algebras influence their $h$-vector and Cohen-Macaulay properties, establishing bounds and conditions that relate singularity types to algebraic invariants.

Contribution

It provides new bounds on the $h$-vector and regularity of graded algebras with Du Bois or $F$-pure singularities, extending previous results and linking singularity conditions to Cohen-Macaulayness.

Findings

If $R$ satisfies $(S_r)$ and has Du Bois or $F$-pure singularities, then $h_0, ..., h_r \\geq 0$.

The multiplicity of $R$ is at least the sum of $h_0$ through $h_{r-1}$.

Equality cases often imply $R$ is Cohen-Macaulay.

Abstract

Let $R$ be a standard graded algebra over a field. We investigate how the singularities of $R$ affect its $h$-vector, which is the coefficients of the numerator of its Hilbert series. The most concrete consequences of our work asserts that if $R$ satisfies Serre's condition $(S_r)$ and have reasonable singularities (Du Bois on the punctured spectrum or $F$-pure), then $h_0,\dots, h_r\geq 0$. Furthermore the multiplicity of $R$ is at least $h_0+h_1+\dots+h_{r-1}$. We also prove that equality in many cases forces $R$ to be Cohen-Macaulay. The main technical tools are sharp bounds on regularity of certain Ext modules, which can be viewed as Kodaira-type vanishing statements for Du Bois and $F$-singularities. Many corollaries are deduced, for instance that nice singularities of small codimension must be Cohen-Macaulay. Our results build on and extend previous work by de Fernex-Ein, Eisenbud-Goto, Huneke-Smith, Murai-Terai and others.