Tight bounds for parameterized complexity of Cluster Editing with a small number of clusters

In the Cluster Editing problem, also known as Correlation Clustering, we are given an undirected n-vertex graph G and a positive integer k. The task is to decide if G can be transformed into a cluster graph, i.e., a disjoint union of cliques, by changing at most k adjacencies, i.e. by adding/deleting at most k edges. We give a subexponential-time parameterized algorithm that in time View the MathML source decides whether G can be transformed into a cluster graph with exactly p cliques by changing at most k adjacencies. Our algorithmic findings are complemented by the following tight lower bound on the asymptotic behavior of our algorithm. We show that unless ETH fails, for any constant 0<σ≤1, there is p=Θ(kσ) such that there is no algorithm deciding in time View the MathML source whether G can be transformed into a cluster graph with at most p cliques by changing at most k adjacencies.