On Domatic and Total Domatic Numbers of Product Graphs

A \emph{domatic} (\emph{total domatic}) \emph{$k$-coloring} of a graph $G$ is an assignment of $k$ colors to the vertices of $G$ such that each vertex contains vertices of all $k$ colors in its closed neighborhood (neighborhood). The \emph{domatic} (\emph{total domatic}) \emph{number} of $G$, denoted $d(G)$ ($d_t (G)$), is the maximum $k$ for which $G$ has a domatic (total domatic) $k$-coloring. In this paper, we show that for two non-trivial graphs $G$ and $H$, the domatic and total domatic numbers of their Cartesian product $G \cart H$ is bounded above by $\max\{|V(G)|, |V(H)|\}$ and below by $\max\{d(G), d(H)\}$. Both these bounds are tight for an infinite family of graphs. Further, we show that if $H$ is bipartite, then $d_t(G \cart H)$ is bounded below by $2\min\{d_t(G),d_t(H)\}$ and $d(G \cart H)$ is bounded below by $2\min\{d(G),d_t(H)\}$. These bounds give easy proofs for many of the known bounds on the domatic and total domatic numbers of hypercubes \cite{chen,zel4} and the domination and total domination numbers of hypercubes \cite{har,joh} and also give new bounds for Hamming graphs. We also obtain the domatic (total domatic) number and domination (total domination) number of $n$-dimensional torus $\mathop{\cart}\limits_{i=1}^{n} C_{k_i}$ with some suitable conditions to each $k_i$, which turns out to be a generalization of a result due to Gravier \cite{grav2} %[\emph{Total domination number of grid graphs}, Discrete Appl. Math. 121 (2002) 119-128] and give easy proof of a result due to Klav\v{z}ar and Seifter \cite{sand}.