Episode three of A Further Five Numbers, the BBC radio series
presented by Simon Singh. Six is often treated as 2x3, but has
many characteristics of its own. Six is also the "pivot" of its
divisors (1 2 3=6=1x2x3) and also the centre of the first five
even numbers: 2, 4, 6, 8, 10. Six seems to have a pivoting
action both mathematically and socially. How is it that everyone
in the world can be linked through just six social ties? As
Simon discovers, the concept of “six degrees of separation”
emerged from a huge postal experiment conducted by the social
psychologist Stanley Milgram in 1967. Milgram asked volunteers
to send a package by mail to one of a hundred people chosen at
random. But they could only send mail to people they knew on
first name terms.