The difference and ratio of the fractional matching number and the matching number of graphs

Abstract

Given a graph $G$, the matching number of $G$, written $\alpha'(G)$, is the maximum size of a matching in $G$, and the fractional matching number of $G$, written $\alpha'_f(G)$, is the maximum size of a fractional matching of $G$. In this paper, we prove that if $G$ is an $n$-vertex connected graph that is neither $K_1$ nor $K_3$, then $\alpha'_f(G)-\alpha'(G) \le \frac{n-2}6$ and $\frac{\alpha'_f(G)}{\alpha'(G)} \le \frac{3n}{2n+2}$. Both inequalities are sharp, and we characterize the infinite family of graphs where equalities hold.