English

Non-level semi-standard graded Cohen-Macaulay domain with $h$-vector $(h_0,h_1,h_2)$

Commutative Algebra 2016-10-12 v1 Combinatorics

Abstract

Let kk be an algebraically closed field of characteristic 0, and AA a Cohen-Macaulay graded domain with A0=kA_0=k. If AA is semi-standard graded (i.e., AA is finitely generated as a k[A1]k[A_1]-module), it has the hh-vector (h0,h1,...,hs)(h_0, h_1, ..., h_s), which encodes the Hilbert function of AA. From now on, assume that s=2s=2. It is known that if AA is standard graded (i.e., A=k[A1]A=k[A_1]), then AA is level. We will show that, in the semi-standard case, if AA is not level, then h1+1h_1+1 divides h2h_2. Conversely, for any positive integers hh and nn, there is a non-level AA with the hh-vector (1,h,(h+1)n)(1, h, (h+1)n). Moreover, such examples can be constructed as Ehrhart rings (equivalently, normal toric rings).

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Cite

@article{arxiv.1610.03157,
  title  = {Non-level semi-standard graded Cohen-Macaulay domain with $h$-vector $(h_0,h_1,h_2)$},
  author = {Akihiro Higashitani and Kohji Yanagawa},
  journal= {arXiv preprint arXiv:1610.03157},
  year   = {2016}
}

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12 pages