@Article{chain-letters,
  author =	 {David Liben-Nowell and Jon Kleinberg},
  title =	 {Tracing the Flow of Information on a Global Scale
                  Using {I}nternet Chain-Letter Data},
  journal = 	 {Proceedings of the National Academy of Sciences},
  year = 	 2008,
  volume =	 105,
  number =	 12,
  pages =	 {4633--4638},
  month =	 {25 } # mar 
}