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arXiv · 1901.01116

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

Hailong Dao·Linquan Ma·Matteo Varbaro
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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.

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Hailong Dao, Linquan Ma, Matteo Varbaro. 2024-08-23. Regularity, singularities and $h$-vector of graded algebras. https://arxiv.org/abs/1901.01116

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