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 $\mathcal{X} = \{X_1,\ldots,X_k\}$ of the vertex set $V(G)$ of $G$ such that for all $i \in [k]$, each set $X_i \in \mathcal{X}$ satisfies one of the following two conditions: (1) $X_i$ is a dominating set of $G$ with a single vertex, or (2) $X_i$ forms a coalition with some other set $X_j \in \mathcal{X}$. %The coalition number ${C}(G)$ is the maximum cardinality of a $c$-partition of $G$. Let ${\cal A} = \{A_1,\ldots,A_r\}$ and ${\cal B}= \{B_1,\ldots, B_s\}$ be two partitions of $V(G)$. Partition ${\cal B}$ is a refinement of partition ${\cal A}$ if every set $B_i \in {\cal B} $ is either equal to, or a proper subset of, some set $A_j \in {\cal A}$. 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.

翻译：设$G$是一个图，$S$是$V(G)$的子集。若$V(G) \setminus S$中的每个顶点都与$S$中的某个顶点相邻，则称$S$为$G$的一个支配集。$G$中的一个联盟由两个不相交的顶点子集$X$和$Y$组成，且$X$和$Y$均不是支配集，但它们的并集$X \cup Y$是$G$的一个支配集。这样的集合$X$和$Y$构成$G$中的一个联盟。$G$中的一个联盟划分（简称$c$-划分）是$G$的顶点集$V(G)$的一个划分$\mathcal{X} = \{X_1,\ldots,X_k\}$，使得对于所有$i \in [k]$，每个集合$X_i \in \mathcal{X}$满足以下两个条件之一：(1) $X_i$是$G$的一个单顶点支配集，或(2) $X_i$与另一个集合$X_j \in \mathcal{X}$构成一个联盟。设${\cal A} = \{A_1,\ldots,A_r\}$和${\cal B}= \{B_1,\ldots, B_s\}$是$V(G)$的两个划分。若每个集合$B_i \in {\cal B}$要么等于某个$A_j \in {\cal A}$，要么是它的真子集，则称划分${\cal B}$是划分${\cal A}$的一个细化。进一步地，如果${\cal A} \ne {\cal B}$，则${\cal B}$是${\cal A}$的一个真细化。如果划分${\cal A}$不是另一个$c$-划分的真细化，则称其为极小$c$-划分。Haynes等人 [AKCE Int. J. Graphs Combin. 17 (2020), no. 2, 653--659] 定义了图$G$的极小联盟数$c_{\min}(G)$为$G$的极小$c$-划分的最小基数。我们证明$2 \le c_{\min}(G) \le n$，并刻画了满足$c_{\min}(G) = n$的$n$阶图$G$。对于给定图$G$，我们给出了一个多项式时间算法以判断是否$c_{\min}(G)=2$。此外，我们给出了图$G$满足$c_{\min}(G) \ge 3$的充要条件，并刻画了最小度为$2$且$c_{\min}(G)= 4$的图$G$的特征。