Symmetric iterated Betti numbers

Abstract

We define a set of invariants of a homogeneous ideal $I$ in a polynomial ring called the symmetric iterated Betti numbers of $I$. For $I_Γ$, the Stanley-Reisner ideal of a simplicial complex $Γ$, these numbers are the symmetric counterparts of the exterior iterated Betti numbers of $Γ$ introduced by Duval and Rose. We show that the symmetric iterated Betti numbers of an ideal $I$ coincide with those of a particular reverse lexicographic generic initial ideal $\Gin(I)$ of $I$, and interpret these invariants in terms of the associated primes and standard pairs of $\Gin(I)$. We verify that for an ideal $I=I_Γ$ the extremal Betti numbers of $I_Γ$ are precisely the extremal (symmetric or exterior) iterated Betti numbers of $Γ$. We close with some results and conjectures about the relationship between symmetric and exterior iterated Betti numbers of a simplicial complex.

20 pages, 2 figures