A partitioning and related properties for the quotient complex $\Delta (B_{lm})/S_l \wr S_m$

Abstract

We study the quotient complex $\Delta (B_{lm})/S_l\wr S_m$ as a means of deducing facts about the ring $k[x_1,..., x_{lm}]^{S_l\wr S_m}$. It is shown in [He] that this quotient complex is shellable when $l=2$, implying Cohen-Macaulayness of $k[x_1,..., x_{2m}]^{S_2\wr S_m}$ for any field $k$. We now confirm for all pairs $(l,m)$ with $l>2$ and $m>1$ that this quotient complex is not Cohen-Macaulay over $\integ /2\integ $, but it is Cohen-Macaulay over fields of characteristic $p>m$ (independent of $l$). This yields corresponding characteristic-dependent results for the ring of invariants $k[x_1,..., x_{lm}]^{S_l\wr S_m}$. We also prove that this quotient complex and the links of many of its faces are collapsible, and we give a partitioning for this quotient complex.