The minmin coalition number in graphs

A set $S$ of vertices in a graph $G$ is a dominating set if every vertex of $V(G) \setminus S$ is adjacent to a vertex in $S$. A coalition in $G$ consists of two disjoint sets of vertices $X$ and $Y$ of $G$, neither of which is a dominating set but whose union $X \cup Y$ is a dominating set of $G$. Such sets $X$ and $Y$ form a coalition in $G$. A coalition partition, abbreviated $c$-partition, in $G$ is a partition $\mathcal{X} = {X1,\ldots,Xk}$ of the vertex set $V(G)$ of $G$ such that for all $i \in [k]$, each set $Xi \in \mathcal{X}$ satisfies one of the following two conditions: (1) $Xi$ is a dominating set of $G$ with a single vertex, or (2) $Xi$ forms a coalition with some other set $Xj \in \mathcal{X}$. %The coalition number ${C}(G)$ is the maximum cardinality of a $c$-partition of $G$. Let ${\cal A} = {A1,\ldots,Ar}$ and ${\cal B}= {B1,\ldots, Bs}$ be two partitions of $V(G)$. Partition ${\cal B}$ is a refinement of partition ${\cal A}$ if every set $Bi \in {\cal B} $ is either equal to, or a proper subset of, some set $Aj \in {\cal A}$. Further if ${\cal A} \ne {\cal B}$, then ${\cal B}$ is a proper refinement of ${\cal A}$. Partition ${\cal A}$ is a minimal $c$-partition if it is not a proper refinement of another $c$-partition. Haynes et al. [AKCE Int. J. Graphs Combin. 17 (2020), no. 2, 653--659] defined the minmin coalition number $c{\min}(G)$ of $G$ to equal the minimum order of a minimal $c$-partition of $G$. We show that $2 \le c{\min}(G) \le n$, and we characterize graphs $G$ of order $n$ satisfying $c{\min}(G) = n$. A polynomial-time algorithm is given to determine if $c{\min}(G)=2$ for a given graph $G$. A necessary and sufficient condition for a graph $G$ to satisfy $c{\min}(G) \ge 3$ is given, and a characterization of graphs $G$ with minimum degree~$2$ and $c{\min}(G)= 4$ is provided.