Pammolli, Fabio and Buldyrev, Sergey V. and Riccaboni, Massimo and Yamasaki, Kazuko and Fu, Dongfeng and Matia, Kaushik and Stanley, H. Eugene  A generalized preferential attachment model for business firms growth rates: II. Mathematical treatment.  The European Physical Journal B, 57 (2).  pp. 131-138.  ISSN 1434-6028      (2007)