On the domination number of the cartesian product of the path graph and any pair of graphs

TL;DR

This paper improves lower bounds on the domination number of the Cartesian product of two graphs with a path graph, advancing understanding related to Vizing's conjecture.

Contribution

It introduces space projections to enhance lower bounds on domination numbers in Cartesian graph products involving paths.

Findings

Improved lower bound: /3 of the product of domination numbers.

Established a bound involving /3 for -vertex path graphs.

Derived a near /4 constant for large path graphs.

Abstract

It is known that for any graph $G,$ $\gamma (G\square P_2)\geq \gamma (G)$ where $\gamma$ stands for the domination number, $\square$ for the cartesian product and $P_2$ is the path graph on two vertices. In an attempt to prove Vizing's conjecture, Clark and Suen proved in $2000$ that $\gamma (X\square Y)\geq \frac{1}{2}\gamma (X)\gamma (Y)$ for any pair of graphs $X$ and $Y.$ Combining these two inequalities, we have $\gamma (X\square Y\square P_2)\geq \frac{1}{2}\gamma (X)\gamma (Y).$ In this paper, we use space projections to improve this lower bound and show that $\gamma (X\square Y\square P_2)\geq \frac{2}{3}\gamma (X)\gamma (Y)$ for any pair of graphs $X$ and $Y.$ In addition, we prove that $\gamma (X\square Y\square P_{n})\geq c_n\gamma (X)\gamma (Y)\gamma (P_{n}),$ where $c_n$ is almost $\frac{3}{4}$ when $n$ is big enough.