We strengthen a result by Laskar and Lyle (Discrete Appl. Math. (2009), 330 338) by proving that it is NP-complete to decide whether a bipartite planar graph can be partitioned into three independent dominating sets. In contrast, we show that this is always possible for every maximal outerplanar graph with at least three vertices. Moreover, we extend their previous result by proving that deciding whether a bipartite graph can be partitioned into k independent dominating sets is NP-complete for every k ≥ 3. We also strengthen a result by Hen-ning et al. (Discrete Math. (2009), 6451-6458) by showing that it is NP-complete to determine if a graph has two disjoint independent dominating sets, even when the problem is restricted to triangle-free planar graphs. Finally, for every k ≥ 3, we show that there is some constant t depending only on k such that deciding whether a k-regular graph can be partitioned into t independent dominating sets is NP-complete. We conclude by deriving moderately exponential-time algorithms for the problem.
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