Bisimplicial separators

A minimal separator of a graph $G$ is a set $S \subseteq V(G)$ such that there exist vertices $a,b \in V(G) \setminus S$ with the property that $S$ separates $a$ from $b$ in $G$, but no proper subset of $S$ does. For an integer $k\ge 0$, we say that a minimal separator is $k$-simplicial if it can be covered by $k$ cliques and denote by $\mathcal{G}k$ the class of all graphs in which each minimal separator is $k$-simplicial. We show that for each $k \geq 0$, the class $\mathcal{G}k$ is closed under induced minors, and we use this to show that the Maximum Weight Stable Set problem can be solved in polynomial time for $\mathcal{G}k$. We also give a complete list of minimal forbidden induced minors for $\mathcal{G}2$. Next, we show that, for $k \geq 1$, every nonnull graph in $\mathcal{G}k$ has a $k$-simplicial vertex, i.e., a vertex whose neighborhood is a union of $k$ cliques; we deduce that the Maximum Weight Clique problem can be solved in polynomial time for graphs in $\mathcal{G}2$. Further, we show that, for $k \geq 3$, it is NP-hard to recognize graphs in $\mathcal{G}k$; the time complexity of recognizing graphs in $\mathcal{G}2$ is unknown. We also show that the Maximum Clique problem is NP-hard for graphs in $\mathcal{G}3$. Finally, we prove a decomposition theorem for diamond-free graphs in $\mathcal{G}2$ (where the diamond is the graph obtained from $K4$ by deleting one edge), and we use this theorem to obtain polynomial-time algorithms for the Vertex Coloring and recognition problems for diamond-free graphs in $\mathcal{G}2$, and improved running times for the Maximum Weight Clique and Maximum Weight Stable Set problems for this class of graphs.