On the Homotopy Type of the Polyhedral Join over the Independence Complex of a Forest

Abstract

We consider a certain class of simplicial complexes which includes the independence complexes of forests. We show that if a simplicial complex $K$ belongs to this class, then the polyhedral join $\mathcal{Z}^*_{K}(\underline{X}, \emptyset)$ is homotopy equivalent to a wedge sum of CW complexes of the form $\Sigma^r X_{i_1} * X_{i_2} * \cdots * X_{i_k}$, where $\underline{X}$ is a family $\{X_i\}_{i \in V(K)}$ of CW complexes and $\Sigma$ denotes the unreduced suspension. This result is applied to study the homotopy type of the independence complex of the lexicographic product $G[H]$ of a graph $H$ over a forest $G$. We denote by $L_m$ a tree on $m$ vertices with no branches. We show that the geometric realization of the independence complex of $L_m [H]$ is homotopy equivalent to a wedge sum of spheres if $m \neq 2,3$ and the geometric realization of the independence complex of $H$ is homotopy equivalent to a wedge sum of same dimensional spheres.