The 3-restricted edge-connectivity of the direct product graphs
Wenxin Wang, Yingzhi Tian
TL;DR
The paper addresses the problem of determining the 3-restricted edge-connectivity, $λ_3$, of direct product graphs formed from a regular connected graph $G$ and standard graphs $C_n$, $K_n$, and $T_n$. It derives exact formulas for $λ_3(G×C_n)$, $λ_3(G×K_n)$, and $λ_3(G×T_n)$ in terms of the second restricted edge-connectivity $λ_2(G)$, degree $k$, and girth $g(G)$, with parity considerations for $n$. The results yield corollaries showing that if $G$ is maximally 3-restricted edge-connected, then the corresponding product graphs are also maximally 3-restricted edge-connected under specified $n$–values. These findings extend restricted-edge-connectivity theory to direct product graphs and have implications for reliability assessments in network design.
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 $λ_3(G)\leqξ_3(G)$ if $G$ has a 3-restricted edge-cut, where $ξ_3(G) = \min \left\{ |[X, V(G) \setminus X]_G|:|X| = 3 \text{ and } G[X] \text{ is connected} \right\}.$ 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.