Three remarks on $\mathbf{W_2}$ graphs

Let $k \geq 1$. A graph $G$ is $\mathbf{Wk}$ if for any $k$ pairwise disjoint independent vertex subsets $A1, \dots, Ak$ in $G$, there exist $k$ pairwise disjoint maximum independent sets $S1, \dots, Sk$ in $G$ such that $Ai \subseteq Si$ for $i \in [k]$. Recognizing $\mathbf{W1}$ graphs is co-NP-hard, as shown by Chv\'atal and Slater (1993) and, independently, by Sankaranarayana and Stewart (1992). Extending this result and answering a recent question of Levit and Tankus, we show that recognizing $\mathbf{Wk}$ graphs is co-NP-hard for $k \geq 2$. On the positive side, we show that recognizing $\mathbf{Wk}$ graphs is, for each $k\geq 2$, FPT parameterized by clique-width and by tree-width. Finally, we construct graphs $G$ that are not $\mathbf{W_2}$ such that, for every vertex $v$ in $G$ and every maximal independent set $S$ in $G - N[v]$, the largest independent set in $N(v) \setminus S$ consists of a single vertex, thereby refuting a conjecture of Levit and Tankus.