Linear Configurations of Complete Graphs $K_4$ and $K_5$ in $\mathbb R^3$, and Higher Dimensional Analogs

Abstract

We investigate the space $C(X)$ of images of linearly embedded skeleta of simplices $X$ in $\mathbb R^n$, for two families of codimension 2 complexes, each ranging over $n$. In the first family, $X=K$ is the $(n-2)$-skeleton of the $n$-simplex. In the second family, $X=L$ is the $(n-2)$-skeleton of the $(n+1)$-simplex. The main result is that for $n>2$, $C(X)$ (for either $X=K,L$) deformation retracts to a subspace homeomorphic to the double mapping cylinder \[SO(n)/A_{n+1}\leftarrow SO(n)/A_n\rightarrow SO(n)/S_n,\] where $A_n$ is the alternating group and $S_n$ the symmetric group. The resulting fundamental group provides an example of a generalization of the braid group, which is the fundamental group of a configuration of points in the plane. This group is presented, for the case $n=3$, and its action on $F_3$ is presented.