Discrete Morse theory and the topology of matching complexes of complete graphs

Abstract

We denote the matching complex of the complete graph with $n$ vertices by $M_n$. Bouc first studied the topological properties of $M_n$ in connection with the Quillen complex. Later Bj\"{o}rner, Lov\'{a}sz, Vre\'{c}ica, and \v{Z}ivaljevi\'{c} showed that $M_n$ is homotopically $(\nu_n-1)$-connected, where $\nu_n=\lfloor{\frac{n+1}{3}}\rfloor-1$, but in general the topology of $M_n$ is not very well-understood even for smaller natural numbers. Forman developed discrete Morse theory, which has various applications in diverse fields of studies. In this article, we develop a discrete Morse theoretic technique to capture deeper structural topological properties of $M_n$. We show that $M_n$ is \emph{geometrically} $(\nu_n-1)$-connected, where the notion of geometrical $k$-connectedness as defined in this article, is stronger than that of homotopical $k$-connectedness. Previously, Bj\"{o}rner et al. showed that $M_8$ is simply connected, but not 2-connected. The technique developed here helped us determine that $M_8$ is in fact homotopy equivalent to a wedge of 132 spheres of dimension 2.