TY - JOUR

T1 - Squarefree P-modules and the cd-index

AU - Murai, Satoshi

AU - Yanagawa, Kohji

N1 - Funding Information:
The first author was partially supported by JSPS KAKENHI 25400043 , and the second author was partially supported by JSPS KAKENHI 25400057 . We thank the anonymous referee for careful reading and helpful suggestions.

PY - 2014/11/10

Y1 - 2014/11/10

N2 - In this paper, we introduce a new algebraic concept, which we call squarefree P-modules. This concept is inspired from Karu's proof of the non-negativity of the cd-indices of Gorenstein* posets, and supplies a way to study cd-indices from the viewpoint of commutative algebra. Indeed, by using the theory of squarefree P-modules, we give several new algebraic and combinatorial results on CW-posets. First, we define an analogue of the cd-index for any CW-poset and prove its non-negativity when a CW-poset is Cohen-Macaulay. This result proves that the h-vector of the barycentric subdivision of a Cohen-Macaulay regular CW-complex is unimodal. Second, we prove that the Stanley-Reisner ring of the barycentric subdivision of an odd dimensional Cohen-Macaulay polyhedral complex has the weak Lefschetz property. Third, we obtain sharp upper bounds of the cd-indices of Gorenstein* posets for a fixed rank generating function.

AB - In this paper, we introduce a new algebraic concept, which we call squarefree P-modules. This concept is inspired from Karu's proof of the non-negativity of the cd-indices of Gorenstein* posets, and supplies a way to study cd-indices from the viewpoint of commutative algebra. Indeed, by using the theory of squarefree P-modules, we give several new algebraic and combinatorial results on CW-posets. First, we define an analogue of the cd-index for any CW-poset and prove its non-negativity when a CW-poset is Cohen-Macaulay. This result proves that the h-vector of the barycentric subdivision of a Cohen-Macaulay regular CW-complex is unimodal. Second, we prove that the Stanley-Reisner ring of the barycentric subdivision of an odd dimensional Cohen-Macaulay polyhedral complex has the weak Lefschetz property. Third, we obtain sharp upper bounds of the cd-indices of Gorenstein* posets for a fixed rank generating function.

KW - Barycentric subdivisions

KW - Cd-Index

KW - Flag f-vectors

KW - Regular CW-complexes

KW - Stanley-Reisner rings

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U2 - 10.1016/j.aim.2014.07.037

DO - 10.1016/j.aim.2014.07.037

M3 - Article

AN - SCOPUS:84906344792

SN - 0001-8708

VL - 265

SP - 241

EP - 279

JO - Advances in Mathematics

JF - Advances in Mathematics

ER -