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Non-level semi-standard graded Cohen–Macaulay domain with h-vector (h0,h1,h2)
Authors:Akihiro Higashitani  Kohji Yanagawa
Institution:1. Department of Mathematics, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-Ku, Kyoto, 603-8555, Japan;2. Department of Mathematics, Kansai University, Suita, Osaka 564-8680, Japan
Abstract:Let k be an algebraically closed field of characteristic 0, and A=?iNAi a Cohen–Macaulay graded domain with A0=k. If A is semi-standard graded (i.e., A is finitely generated as a kA1]-module), it has the h-vector(h0,h1,,hs), which encodes the Hilbert function of A. From now on, assume that s=2. It is known that if A is standard graded (i.e., A=kA1]), then A is level. We will show that, in the semi-standard case, if A is not level, then h1+1 divides h2. Conversely, for any positive integers h and n, there is a non-level A with the h-vector (1,h,(h+1)n). Moreover, such examples can be constructed as Ehrhart rings (equivalently, normal toric rings).
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