Regardless this ancient roots, in the last decades overlapping clustering has not attracted as much attention as nonoverlapping clustering. One close sibling is fuzzy clustering [22], where each data point has a membership value in all the clusters. In this context cluster membership is “soft”, as apposed to our paper that we are interested in “hard” cluster assignments. Obviously a hard (and overlapping) cluster assignment can be obtained by thresholding membership values. The prototypical fuzzy-clustering method is fuzzy c-means, which is essentially a soft version of k-means.