Parameterized Complexity of Safe Set

In this paper we study the problem of finding a small safe set S in a graph G, i.e. a non-empty set of vertices such that no connected component of G[S] is adjacent to a larger component in \(G - S\). We enhance our understanding of the problem from the viewpoint of parameterized complexity by showing that (1) the problem is W[2]-hard when parameterized by the pathwidth \(\mathsf {pw}\) and cannot be solved in time \(n^{o(\mathsf {pw})}\) unless the ETH is false, (2) it admits no polynomial kernel parameterized by the vertex cover number \(\mathsf {vc}\) unless \(\mathrm {PH} = \varSigma ^{\mathrm {p}}_{3}\), but (3) it is fixed-parameter tractable (FPT) when parameterized by the neighborhood diversity \(\mathsf {nd}\), and (4) it can be solved in time \(n^{f(\mathsf {cw})}\) for some double exponential function f where \(\mathsf {cw}\) is the clique-width. We also present (5) a faster FPT algorithm when parameterized by solution size.