Maximal and maximum induced matchings in connected graphs

Abstract

An induced matching in a graph is a set of edges whose endpoints induce a $1$-regular subgraph. Gupta et al. (2012,\cite{Gupta}) showed that every $n$-vertex graph has at most $10^{\frac{n}{5}}\approx 1.5849^n$ maximal induced matchings, which is attained by the disjoint union of copies of the complete graph $K_5$. In this paper, we show that the maximum number of maximal and maximum induced matchings in a connected graph of order $n$ is \begin{align*} \begin{cases} {n\choose 2} &~ {\rm if}~ 1\leq n\le 8; \\ {{\lfloor \frac{n}{2} \rfloor}\choose 2}\cdot {{\lceil \frac{n}{2} \rceil}\choose 2} -(\lfloor \frac{n}{2} \rfloor-1)\cdot (\lceil \frac{n}{2} \rceil-1)+1 &~ {\rm if}~ 9\leq n\le 13; \\ 10^{\frac{n-1}{5}}+\frac{n+144}{30}\cdot 6^{\frac{n-6}{5}} &~ {\rm if}~ 14\leq n\le 30;\\ 10^{\frac{n-1}{5}}+\frac{n-1}{5}\cdot 6^{\frac{n-6}{5}} & ~ {\rm if}~ n\geq 31, \\ \end{cases} \end{align*} and also show that this bound is tight. This result implies that we can enumerate all maximal induced matchings of an $n$-vertex connected graph in time $O(1.5849^n)$. Moreover, our result provides an estimate on the number of maximal dissociation sets of an $n$-vertex connected graph.