s153.
See Barabási (2002); John Scott (1991), Social Network Analysis; Mark Buchanan (2003), Nexus; Newman, Barabási and Watts (2006); and Homer-Dixon (2006), chapter 5. For a discussion of the idea of “six degrees of separation” (that you are no more than six handshakes away from any other person on the planet), see Barabási (2002), chapter 3. The two key papers on this “small world” hypothesis are Ithiel de Sola Pool and Manfred Kochen, “Contacts and Influence”, Social Networks, 1, 1978, pp 5–51 (a mathematical analysis); and Jeffrey Travers and Stanley Milgram, “An Experimental Study in the Small World Problem”, Sociometry, 43, 1969, pp 425–443. Both of these articles are reprinted in Newman, Barabási and Watts (2006), pp 83–148. For a discussion of redundancy, see Charles Perrow (2007), The Next Catastrophe, pp 258, 301 and 327–330.