we note that Cheng made something very cited in 2015 and it continues his work also up to 2018; Arayci made somthing in 2008, but his more recent papers are not so much cited. Rezgui did something a lot interesting in 2017 but then it seems he stopped working on this topic.
- Author productivity (Fig.6): the plot is quite in-line respect theoretical shape of Lotka's Law; a possible comment is that there are some more "newcomer" writers that theorethical ones, so the topis is still debated.