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The depth of an ideal with a given Hilbert function
Authors:Satoshi Murai   Takayuki Hibi
Affiliation:Department of Pure and Applied Mathematics, Graduate School of Information Science and Technology, Osaka University, Toyonaka, Osaka, 560-0043, Japan ; Department of Pure and Applied Mathematics, Graduate School of Information Science and Technology, Osaka University, Toyonaka, Osaka, 560-0043, Japan
Abstract:Let $ A = K[x_1, ldots, x_n]$ denote the polynomial ring in $ n$ variables over a field $ K$ with each $ operatorname{deg} x_i = 1$. Let $ I$ be a homogeneous ideal of $ A$ with $ I neq A$ and $ H_{A/I}$ the Hilbert function of the quotient algebra $ A / I$. Given a numerical function $ H : {mathbb{N}} to {mathbb{N}}$ satisfying $ H=H_{A/I}$ for some homogeneous ideal $ I$ of $ A$, we write $ mathcal{A} _H$ for the set of those integers $ 0 leq r leq n$ such that there exists a homogeneous ideal $ I$ of $ A$ with $ H_{A/I} = H$ and with $ operatorname{depth} A / I = r$. It will be proved that one has either $ mathcal{A}_H = { 0, 1, ldots, b }$ for some $ 0 leq b leq n$ or $ vert{mathcal{A}}_Hvert = 1$.

Keywords:Hilbert functions   depth   lexsegment ideals
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