On $k$-coalition in graphs: bounds and exact values

Given a graph G=\big{(}V(G),E(G)\big{)}, a set $S\subseteq V(G)$ is called a $k$-dominating set if every vertex in $V(G)\setminus S$ has at least $k$ neighbors in $S$. Two disjoint sets $A,B\subset V(G)$ form a $k$-coalition in $G$ if neither set is a $k$-dominating set in $G$ but their union $A\cup B$ is a $k$-dominating set. A partition $\Omega$ of $V(G)$ is a $k$-coalition partition if each set in $\Omega$ is either a $k$-dominating set of cardinality $k$ or forms a $k$-coalition with another set in $\Omega$. The $k$-coalition number $C_{k}(G)$ equals the maximum cardinality of a $k$-coalition partition of $G$. In this work, we give general upper and lower bounds on this parameter. In particular, we show that if $G$ has minimum degree $\delta \ge 2$ and maximum degree $\Delta \ge 4 \lfloor \delta/2 \rfloor$, then $C_{2}(G) \leq (\Delta-2\lfloor \delta/2 \rfloor+1)(\lfloor \delta/2 \rfloor+1) + \lceil \delta/2 \rceil+1$, and this bound is sharp. If $T$ is a tree of order~$n \ge 2$, then we prove the upper bound $C_{2}(T) \leq \big\lfloor \frac{n}{2}\big\rfloor+1$ and we characterize the extremal trees achieving equality in this bound. We determine the exact value of $C_{k}(G)$ for any cubic graph $G$ and $k\geq2$. Finally, we give the exact value of $C_{k}$ for any complete bipartite graph, which completes a partial result and resolves an issue from an earlier paper.