Interval edge colorings of some products of graphs
An edge coloring of a graph $G$ with colors $1,2,\ldots ,t$ is called an interval $t$-coloring if for each $i\in \{1,2,\ldots,t\}$ there is at least one edge of $G$ colored by $i$, and the colors of edges incident to any vertex of $G$ are distinct and form an interval of integers. A graph $G$ is interval colorable, if there is an integer $t\geq 1$ for which $G$ has an interval $t$-coloring. Let $\mathfrak{N}$ be the set of all interval colorable graphs. In 2004 Kubale and Giaro showed that if $G,H\in \mathfrak{N}$, then the Cartesian product of these graphs belongs to $\mathfrak{N}$. Also, they formulated a similar problem for the lexicographic product as an open problem. In this paper we first show that if $G\in \mathfrak{N}$, then $G[nK_{1}]\in \mathfrak{N}$ for any $n\in \mathbf{N}$. Furthermore, we show that if $G,H\in \mathfrak{N}$ and $H$ is a regular graph, then strong and lexicographic products of graphs $G,H$ belong to $\mathfrak{N}$. We also prove that tensor and strong tensor products of graphs $G,H$ belong to $\mathfrak{N}$ if $G\in \mathfrak{N}$ and $H$ is a regular graph.