The 3-restricted edge-connectivity of the direct product graphs

Abstract

An edge subset \( S \subseteq E(G) \) is called a 3-restricted edge-cut if \( G - S \) is disconnected and each component of \( G - S \) contains at least three vertices. The 3-restricted edge-connectivity of a graph \( G \), denoted by \( λ_3(G) \), is defined as the minimum cardinality among all 3-restricted edge-cuts if there are at least one; otherwise, \( λ_3(G) = +\infty \). It is proved that ${\lambda }_3(G)\le {\xi }_3(G)$ if $G$ has a 3-restricted edge-cut, where ${\xi }_3(G)=\min \left\{|[X,V(G)\setminus X{]}_G|:|X|=3\text{ and }G[X]\text{ is connected}\right\}.$ If \( λ_3(G) = ξ_3(G) \), then \( G \) is said to be maximally 3-restricted edge-connected. The direct product of two graphs $G$ and $H$, denoted by $G\times H$, is defined as the graph with vertex set \( V(G \times H) = V(G) \times V(H) \), where two vertices \( (u_1, v_1) \) and \( (u_2, v_2) \) are adjacent in \( G \times H \) if and only if \( u_1u_2 \in E(G) \) and \( v_1v_2 \in E(H) \). In this paper, we determine, for a regular connected graph \( G\), the 3-restricted edge-connectivity of \( G \times C_n \), \( G \times K_n \) and \( G \times T_n \), where \( C_n \), \( K_n \) and \( T_n \) are the cycle, the complete graph and the total graph with \( n \) vertices, respectively. As corollaries, we establish sufficient conditions for the direct product graphs \( G \times C_n \), \( G \times K_n \) and \( G \times T_n \) to be maximally 3-restricted edge-connected.