The integer $\{2\}$-domination number of grids

Abstract: For positive integers $m$ and $n$, the grid graph $G_{m,n}$ is the Cartesian product of the path graph $P_m$ on $m$ vertices and the path graph $P_n$ on $n$ vertices. An integer ${2}$-dominating function of a graph is a mapping from the vertex set to ${0,1,2}$ such that the sum of the mapped values of each vertex and its neighbors is at least $2$; the integer ${2}$-domination number of a graph is defined to be the minimum sum of mapped values of all vertices among all integer ${2}$-dominating functions. In this paper, we compute the integer ${2}$-domination numbers of $G_{1,n}$ and $G_{2,n}$, attain an upper bound to the integer ${2}$-domination numbers of $G_{3,n}$, and propose an algorithm to count the integer ${2}$-domination numbers of $G_{m,n}$ for arbitrary $m$ and $n$. As a future work, we list the integer ${2}$-domination numbers of $G_{4,n}$ for small $n$, and conjecture on its formula.