Splitting Algebras: Koszul, Cohen-Macaulay and Numerically Koszul (1207.1330v2)

Abstract: We study a finite dimensional quadratic graded algebra R defined from a finite ranked poset. This algebra has been central to the study of the splitting algebra of the poset, A, as introduced by Gelfand, Retakh, Serconek and Wilson . The algebra A is known to be quadratic when the poset satisfies a combinatorial condition known as uniform, and R is the quadratic dual of an associated graded algebra of A. We prove that R is Koszul and the poset is uniform if and only if the poset is Cohen-Macaulay. Koszulity of R implies Koszulity of A. We also show that when R is Koszul, the cohomology of the order complex of the poset can be identified with certain cohomology groups defined internally to the ring R. Finally, we settle in the negative the long-standing question: Does numerically Koszul imply Koszul for algebras of the form R?