1. The Small World Problem. Psychology Today;Milgram,1967
2. https://en.wikipedia.org/wiki/Erdos_number.
3. The Structure and Dynamics of Networks;Newman,2006
4. Linked: How Everything Is Connected To Everything Else and What It Means for Business, Science, and Everyday Life;Barabasi,2014
5. Neocortex size as a constraint on group size in primates;Dunbar;J. Human Evol.,1992