The generalized 3-connectivity of Cartesian product graphs

The generalized connectivity of a graph, which was introduced recently by Chartrand et al., is a generalization of the concept of vertex connectivity. Let $S$ be a nonempty set of vertices of $G$, a collection $\{T_1,T_2,...,T_r\}$ of trees in $G$ is said to be internally disjoint trees connecting $S$ if $E(T_i)\cap E(T_j)=\emptyset$ and $V(T_i)\cap V(T_j)=S$ for any pair of distinct integers $i,j$, where $1\leq i,j\leq r$. For an integer $k$ with $2\leq k\leq n$, the $k$-connectivity $κ_k(G)$ of $G$ is the greatest positive integer $r$ for which $G$ contains at least $r$ internally disjoint trees connecting $S$ for any set $S$ of $k$ vertices of $G$. Obviously, $κ_2(G)=κ(G)$ is the connectivity of $G$. Sabidussi showed that $κ(G\Box H) \geq κ(G)+κ(H)$ for any two connected graphs $G$ and $H$. In this paper, we first study the 3-connectivity of the Cartesian product of a graph $G$ and a tree $T$, and show that $(i)$ if $κ_3(G)=κ(G)\geq 1$, then $κ_3(G\Box T)\geq κ_3(G)$; $(ii)$ if $1\leq κ_3(G)< κ(G)$, then $κ_3(G\Box T)\geq κ_3(G)+1$. Furthermore, for any two connected graphs $G$ and $H$ with $κ_3(G)\geqκ_3(H)$, if $κ(G)>κ_3(G)$, then $κ_3(G\Box H)\geq κ_3(G)+κ_3(H)$; if $κ(G)=κ_3(G)$, then $κ_3(G\Box H)\geq κ_3(G)+κ_3(H)-1$. Our result could be seen as a generalization of Sabidussi's result. Moreover, all the bounds are sharp.

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