Counting graphs admitting a strictly matched involution

Abstract

We study graphs admitting a strictly matched involution, i.e., graphs for which a strictly matched involution exists. A strictly matched involution of a graph $G$ is a self-inverse graph automorphism $f$ with the properties that the fixed point set of $f$ induces a complete subgraph in $G$ and, for the vertices that are no fixed points, every vertex is connected to its image under $f$ by an edge. Andres, Huggan, Mc Inerney, and Nowakowski (2019) determined upper and lower bounds for the number $A(n)$ of graphs admitting a strictly matched involution on $n$ vertices. We improve the upper bound and we compute the exact values for the number $A(n)$ for all $n\leq11$. Furthermore, we use combinatorial enumeration techniques and a decomposition lemma to determine the number $C(n,m)$ of graphs admitting a strictly matched involution with $n$ vertices and $m$ edges for several infinite classes of pairs $(n,m)$.