Hierarchies of simplicial complexes via the BGG-correspondence

Details Hierarchies of simplicial complexes via the BGG-correspondence Hierarchies of simplicial complexes via the BGG-correspondence

2003-02-25

arxiv.org/abs/math/0302313v2

Mathematics

Combinatorics

23 pages, additional examples and also added references to k-Cohen-Macaulay complexes as defined by Baclawski

Scientific paper

Via the BGG-correspondence a simplicial complex D on [n] is transformed into a complex of coherent sheaves L(D) on the projective space n-1-space. In general we compute the support of each of its cohomology sheaves. When the Alexander dual D* is Cohen-Macaulay there is only one such non-zero cohomology sheaf. We investigate when this sheaf can be an a'th syzygy sheaf in a locally free resolution and show that this corresponds exactly to the case of D* being a+1-Cohen-Macaulay as defined by K.Baclawski. By putting further conditions on the sheaves we get nice subclasses of a+1- Cohen-Macaulay simplicial complexes whose f-vector depends only on a and the invariants n,d, and c. When a=0 these are the bi-Cohen-Macaulay simplicial complexes, when a=1 and d=2c cyclic polytopes are examples, and when a=c we get Alexander duals of the Steiner systems S(c,d,n). We also show that D* is Gorenstein* iff the associated coherent sheaf of D is an ideal sheaf.

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