“…In fact, several different PA models appear in the literature, depending on the concrete details of the attachment mechanism. For instance, in [5,9,15,19,20,22,23,25] the authors investigate a directed PA model, while in [6,8,9,14] an undirected version is considered. However, classical PA networks do not always fit real-world network data well, or in many applications it is natural to assign some kind of features to the vertices or to the edges.…”