From the Quasi-Total Strong Differential to Quasi-Total Italian Domination in Graphs

This paper is devoted to the study of the quasi-total strong differential of a graph, and it is a contribution to the Special Issue “Theoretical computer science and discrete mathematics” of Symmetry. Given a vertex x∈V(G) of a graph G, the neighbourhood of x is denoted by N(x). The neighbourhood of a set X⊆V(G) is defined to be N(X)=⋃x∈XN(x), while the external neighbourhood of X is defined to be Ne(X)=N(X)∖X. Now, for every set X⊆V(G) and every vertex x∈X, the external private neighbourhood of x with respect to X is defined as the set Pe(x,X)={y∈V(G)∖X:N(y)∩X={x}}. Let Xw={x∈X:Pe(x,X)≠⌀}. The strong differential of X is defined to be ∂s(X)=|Ne(X)|−|Xw|, while the quasi-total strong differential of G is defined to be ∂s*(G)=max{∂s(X):X⊆V(G)andXw⊆N(X)}. We show that the quasi-total strong differential is closely related to several graph parameters, including the domination number, the total domination number, the 2-domination number, the vertex cover number, the semitotal domination number, the strong differential, and the quasi-total Italian domination number. As a consequence of the study, we show that the problem of finding the quasi-total strong differential of a graph is NP-hard.

On Semitotal Domination in Graphs

A set $S$ of vertices of a connected graph $G$ is a semitotal dominating set if every vertex in $V(G)\setminus S$ is adjacent to a vertex in $S$, and every vertex in $S$ is of distance at most $2$ from another vertex in $S$. A semitotal dominating set $S$ in $G$ is a secure semitotal dominating set if for every $v\in V(G)\setminus S$, there is a vertex $x\in S$ such that $x$ is adjacent to $v$ and that $\left(S\setminus\{x\}\right)\cup \{v\}$ is a semitotal dominating set in $G$. In this paper, we characterize the semitotal dominating sets and the secure semitotal dominating sets in the join, corona and lexicographic product of graphs and determine their corresponding semitotal domination and secure semitotal domination numbers.