Korean J. Math.  Vol 29, No 1 (2021)  pp.1-12
DOI: https://doi.org/10.11568/kjm.2021.29.1.1

### The forcing nonsplit domination number of a graph

John Johnson, Malchijah Raj

#### Abstract

A dominating set $S$ of a graph $G$ is said to be nonsplit dominating set if the subgraph $\langle V-S \rangle$ is connected. The minimum cardinality of a nonsplit dominating set is called the nonsplit domination number and is denoted by $\gamma_{ns}(G)$. For a minimum nonsplit dominating set $S$ of $G$, a set $T \subseteq S$ is called a forcing subset for $S$ if $S$ is the unique $\gamma_{ns}$-set containing $T$. A forcing subset for $S$ of minimum cardinality is a minimum forcing subset of $S$. The forcing nonsplit domination number of $S$, denoted by $f_{\gamma ns}(S)$, is the cardinality of a minimum forcing subset of $S$. The forcing nonsplit domination number of $G$, denoted by $f_{\gamma ns}(G)$ is defined by $f_{\gamma ns}(G)= min \{f_{\gamma ns}(S)\}$, where the minimum is taken over all $\gamma_{ns}$-sets $S$ in $G$. The forcing nonsplit domination number of certain standard graphs are determined. It is shown that, for every pair of positive integers $a$ and $b$ with $0 \leq a \leq b$ and $b \geq 1$, there exists a connected graph $G$ such that  $f_{\gamma ns}(G)=a$ and $\gamma_{ns}(G)=b$. It is shown that, for every  integer $a \geq 0$, there exists a connected graph $G$ with $f_\gamma(G)=f_{\gamma ns}(G)=a$, where $f_\gamma(G)$ is the forcing domination number of the graph. Also, it is shown that, for every pair $a, b$ of integers with $a \geq0$ and $b \geq 0$ there exists a connected graph $G$ such that $f_\gamma(G)=a$ and $f_{\gamma ns}(G)=b$.

#### Keywords

dominating set, domination number, forcing domination number, nonsplit domination number, forcing nonsplit domination number

05C69

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