Linearity Defects of Face Rings
Abstract
Description
Let $S = K[x_1, ..., x_n ]$ be a polynomial ring over a field $K$, and $E = K < y_1, ..., y_n >$ an exterior algebra. The "linearity defect" $ld_E(N)$ of a finitely generated graded $E$-module $N$ measures how far $N$ departs from "componentwise linear". It is known that $ld_E(N) < \infty$ for all $N$. But the value can be arbitrary large, while the similar invariant $ld_S(M)$ for an $S$-module $M$ is alway at most $n$.
We show that if $I_Δ$ (resp. $J_Δ$) is the squarefree monomial ideal of $S$ (resp. $E$) corresponding to a simplicial complex $Δ$ on ${1, >..., n}$, then $ld_E(E/J_Δ) = ld_S(S/I_Δ)$. Moreover, except some extremal cases, $ld$ is a topological invariant of the Alexander dual $Δ^\vee$ of $Δ$.
We also show that, when $n > 3$, $ld_E(E/J_Δ) = n-2$ (this is the largest possible value) if and only if $Δ$ is an $n$-gon.
19pages. Section 5 is largely revised; particularly, the proof of Theorem 5.1 is simplified. To appear in J. Algebra
19pages. Section 5 is largely revised; particularly, the proof of Theorem 5.1 is simplified. To appear in J. Algebra