Extremal results for graphs with binding number strictly less than $1/r$

Abstract

The binding number $b(G)$ of a graph, introduced by Woodall [J. Combin. Theory, Ser. B, 1973], is a central topic of both structural and extremal graph theory. It is closely related to fundamental combinatorial and structural properties of graphs. The graphs with $b(G)\geq1$ exhibit strong expansion properties and a highly connected global structure. In contrast, the structure for graphs with $b(G)<1$ remains far less well understood. Kane et al. [J. Graph Theory, 1981] proved that if $b(G)<1$, then every binding set of $G$ is independent. Goddard and Swart [Quaest. Math., 1990] showed that if $b(G)\leq1$, then the toughness $\tau(G)\leq b(G).$ This makes it particularly interesting to investigate extremal problems for graphs with \(b(G)<1\). For any integer $r\geq1,$ we completely characterize the unique extremal graph that maximizes the size (spectral radius) among all graphs of order $n$ satisfying $b(G)<\frac{1}{r}.$ For any bipartite graph $G=(X,Y)$ on $n$ vertices, it is readily seen that $b(G)\leq\min\{|X|/|Y|,|Y|/|X|\}\leq1.$ Notably, the complete balanced bipartite graph $K_{\frac{n}{2}, \frac{n}{2}}$ achieves the maximum size (spectral radius) among all bipartite graphs with $b(G)=1$. In this paper, we completely determine the extremal graphs maximizing the size or the spectral radius among all bipartite graphs with $b(G)<\frac{1}{r}$, where $r\geq1$ is an integer.