Onnela, J-P, Jari Saramäki, Jorkki Hyvönen, György Szab\ o, David Lazer, Kimmo Kaski, J\ anos Kert\ esz, and A-L Barab\ asi. 2007. “Structure and Tie Strengths in Mobile Communication Networks”. Proceedings of the National Academy of Sciences 104 (18): 7332–7336.