Pammolli, Fabio and Yamasaki, Kazuko and Matia, Kaushik and Buldyrev, Sergey V. and Fu, Dongfeng and Riccaboni, Massimo and Stanley, H. Eugene  Preferential attachment and growth dynamics in complex systems.  Physical Review E, 74 (3).  035103(R)8.  ISSN 1539-3755      (2006)